Ruby <- StackOverflow top 100

Question

How do I write a switch statement in Ruby?

Answer accepted (score 2579)

Ruby uses the case expression instead.

case x
when 1..5
  "It's between 1 and 5"
when 6
  "It's 6"
when "foo", "bar"
  "It's either foo or bar"
when String
  "You passed a string"
else
  "You gave me #{x} -- I have no idea what to do with that."
end

Ruby compares the object in the when clause with the object in the case clause using the === operator. For example, 1..5 === x, and not x === 1..5.

This allows for sophisticated when clauses as seen above. Ranges, classes and all sorts of things can be tested for rather than just equality.

Unlike switch statements in many other languages, Ruby’s case does not have fall-through, so there is no need to end each when with a break. You can also specify multiple matches in a single when clause like when "foo", "bar".

Answer 2 (score 429)

case...when behaves a bit unexpectedly when handling classes. This is due to the fact that it uses the === operator.

That operator works as expected with literals, but not with classes:

1 === 1           # => true
Fixnum === Fixnum # => false

This means that if you want to do a case ... when over an object’s class, this will not work:

obj = 'hello'
case obj.class
when String
  print('It is a string')
when Fixnum
  print('It is a number')
else
  print('It is not a string or number')
end

Will print “It is not a string or number”.

Fortunately, this is easily solved. The === operator has been defined so that it returns true if you use it with a class and supply an instance of that class as the second operand:

Fixnum === 1 # => true

In short, the code above can be fixed by removing the .class:

obj = 'hello'
case obj  # was case obj.class
when String
  print('It is a string')
when Fixnum
  print('It is a number')
else
  print('It is not a string or number')
end

I hit this problem today while looking for an answer, and this was the first appearing page, so I figured it would be useful to others in my same situation.

Answer 3 (score 207)

It is done by case in Ruby. Also see this article on Wikipedia.

Quoted:

case n
when 0
  puts 'You typed zero'
when 1, 9
  puts 'n is a perfect square'
when 2
  puts 'n is a prime number'
  puts 'n is an even number'
when 3, 5, 7
  puts 'n is a prime number'
when 4, 6, 8
  puts 'n is an even number'
else
  puts 'Only single-digit numbers are allowed'
end

Another example:

score = 70

result = case score
   when 0..40 then "Fail"
   when 41..60 then "Pass"
   when 61..70 then "Pass with Merit"
   when 71..100 then "Pass with Distinction"
   else "Invalid Score"
end

puts result

On around page 123 (I am using Kindle) of The Ruby Programming Lanugage (1st Edition, O’Reilly), it says the then keyword following the when clauses can be replaced with a newline or semicolon (just like in the if then else syntax). (Ruby 1.8 also allows a colon in place of then… But this syntax is no longer allowed in Ruby 1.9.)

Question

I have a value 'Dog' and an array ['Cat', 'Dog', 'Bird'].

How do I check if it exists in the array without looping through it? Is there a simple way of checking if the value exists, nothing more?

Answer accepted (score 1856)

You’re looking for include?:

>> ['Cat', 'Dog', 'Bird'].include? 'Dog'
=> true

Answer 2 (score 240)

There is an in? method in ActiveSupport (part of Rails) since v3.1, as pointed out by @campaterson. So within Rails, or if you require 'active_support', you can write:

'Unicorn'.in?(['Cat', 'Dog', 'Bird']) # => false

OTOH, there is no in operator or #in? method in Ruby itself, even though it has been proposed before, in particular by Yusuke Endoh a top notch member of ruby-core.

As pointed out by others, the reverse method include? exists, for all Enumerables including Array, Hash, Set, Range:

['Cat', 'Dog', 'Bird'].include?('Unicorn') # => false

Note that if you have many values in your array, they will all be checked one after the other (i.e. O(n)), while that lookup for a hash will be constant time (i.e O(1)). So if you array is constant, for example, it is a good idea to use a Set instead. E.g:

require 'set'
ALLOWED_METHODS = Set[:to_s, :to_i, :upcase, :downcase
                       # etc
                     ]

def foo(what)
  raise "Not allowed" unless ALLOWED_METHODS.include?(what.to_sym)
  bar.send(what)
end

A quick test reveals that calling include? on a 10 element Set is about 3.5x faster than calling it on the equivalent Array (if the element is not found).

A final closing note: be wary when using include? on a Range, there are subtleties, so refer to the doc and compare with cover?…

Answer 3 (score 164)

Try

['Cat', 'Dog', 'Bird'].include?('Dog')

Question

I have a string variable with content as follows:

varMessage =   
            "hi/thsid/sdfhsjdf/dfjsd/sdjfsdn\n"


            "/my/name/is/balaji.so\n"
            "call::myFunction(int const&)\n"
            "void::secondFunction(char const&)\n"
             .
             .
             .
            "this/is/last/line/liobrary.so"

in above string i have to find a sub string i.e. 

"hi/thsid/sdfhsjdf/dfjsd/sdjfsdn\n"


"/my/name/is/balaji.so\n"
"call::myFunction(int const&)\n"

How can I find it? I just need to determine whether the substring is present or not.

Answer accepted (score 1282)

You can use the include? method:

my_string = "abcdefg"
if my_string.include? "cde"
   puts "String includes 'cde'"
end

Answer 2 (score 82)

If case is irrelevant, then a case-insensitive regular expression is a good solution:

'aBcDe' =~ /bcd/i  # evaluates as true

This will also work for multi-line strings.

See Ruby’s Regexp class.

Answer 3 (score 41)

You can also do this…

my_string = "Hello world"

if my_string["Hello"]
  puts 'It has "Hello"'
else
  puts 'No "Hello" found'
end

# => 'It has "Hello"'

Question

How do I take a string and convert it to lower or upper case in Ruby?

Answer accepted (score 1543)

Ruby has a few methods for changing the case of strings. To convert to lowercase, use downcase:

"hello James!".downcase    #=> "hello james!"

Similarly, upcase capitalizes every letter and capitalize capitalizes the first letter of the string but lowercases the rest:

"hello James!".upcase      #=> "HELLO JAMES!"
"hello James!".capitalize  #=> "Hello james!"
"hello James!".titleize    #=> "Hello James!"

If you want to modify a string in place, you can add an exclamation point to any of those methods:

string = "hello James!"
string.downcase!
string   #=> "hello james!"

Refer to the documentation for String for more information.

Answer 2 (score 121)

You can find out all the methods available on a String by opening irb and running:

"MyString".methods.sort

And for a list of the methods available for strings in particular:

"MyString".own_methods.sort

I use this to find out new and interesting things about objects which I might not otherwise have known existed.

Answer 3 (score 39)

Like @endeR mentioned, if internationalization is a concern, the unicode_utils gem is more than adequate.

$ gem install unicode_utils
$ irb
> require 'unicode_utils'
=> true
> UnicodeUtils.downcase("FEN BİLİMLERİ", :tr)
=> "fen bilimleri"

String manipulations in Ruby 2.4 are now unicode-sensitive.

Question

I need to read the data out of database and then save it in a text file.

How can I do that in Ruby? Is there any file management system in Ruby?

Answer accepted (score 174)

The Ruby File class will give you the ins and outs of ::new and ::open but its parent, the IO class, gets into the depth of #read and #write.

Answer 2 (score 904)

Are you looking for the following?

File.open(yourfile, 'w') { |file| file.write("your text") }

Answer 3 (score 602)

You can use the short version:

File.write('/path/to/file', 'Some glorious content')

It returns the length written; see ::write for more details and options.

To append to the file, if it already exists, use:

File.write('/path/to/file', 'Some glorious content', mode: 'a')

Question

I am looking for a more elegant way of concatenating strings in Ruby.

I have the following line:

source = "#{ROOT_DIR}/" << project << "/App.config"

Is there a nicer way of doing this?

And for that matter what is the difference between << and +?

Answer accepted (score 553)

You can do that in several ways:

1. As you shown with << but that is not the usual way

2. With string interpolation

   source = "#{ROOT_DIR}/#{project}/App.config"
   ```</li>
   <li><p>with `+`</p>
   
   ```ruby
   source = "#{ROOT_DIR}/" + project + "/App.config"
   ```</li>
   </ol>
   
   The second method seems to be more efficient in term of memory/speed from what I've seen (not measured though). All three methods will throw an uninitialized constant error when ROOT_DIR is nil.  
   
   When dealing with pathnames, you may want to use `File.join` to avoid messing up with pathname separator.  
   
   In the end, it is a matter of taste.  
   
   #### Answer 2 (score 92)
   The `+` operator is the normal concatenation choice, and is probably the fastest way to concatenate strings.  
   
   The difference between `+` and `&lt;&lt;` is that `&lt;&lt;` changes the object on its left hand side, and `+` doesn't.  
   
   ```ruby
   irb(main):001:0> s = 'a'
   => "a"
   irb(main):002:0> s + 'b'
   => "ab"
   irb(main):003:0> s
   => "a"
   irb(main):004:0> s << 'b'
   => "ab"
   irb(main):005:0> s
   => "ab"

Answer 3 (score 77)

If you are just concatenating paths you can use Ruby’s own File.join method.

source = File.join(ROOT_DIR, project, 'App.config')

Question

I am in the process of porting a CLI library from Ruby over to Node.js. In my code I execute several third party binaries when necessary. I am not sure how best to accomplish this in Node.

Here’s an example in Ruby where I call PrinceXML to convert a file to a PDF:

cmd = system("prince -v builds/pdf/book.html -o builds/pdf/book.pdf")

What is the equivalent code in Node?

Answer 2 (score 965)

For even newer version of Node.js (v8.1.4), the events and calls are similar or identical to older versions, but it’s encouraged to use the standard newer language features. Examples:

For buffered, non-stream formatted output (you get it all at once), use child_process.exec:

const { exec } = require('child_process');
exec('cat *.js bad_file | wc -l', (err, stdout, stderr) => {
  if (err) {
    // node couldn't execute the command
    return;
  }

  // the *entire* stdout and stderr (buffered)
  console.log(`stdout: ${stdout}`);
  console.log(`stderr: ${stderr}`);
});

You can also use it with Promises:

const util = require('util');
const exec = util.promisify(require('child_process').exec);

async function ls() {
  const { stdout, stderr } = await exec('ls');
  console.log('stdout:', stdout);
  console.log('stderr:', stderr);
}
ls();

If you wish to receive the data gradually in chunks (output as a stream), use child_process.spawn:

const { spawn } = require('child_process');
const child = spawn('ls', ['-lh', '/usr']);

// use child.stdout.setEncoding('utf8'); if you want text chunks
child.stdout.on('data', (chunk) => {
  // data from standard output is here as buffers
});

// since these are streams, you can pipe them elsewhere
child.stderr.pipe(dest);

child.on('close', (code) => {
  console.log(`child process exited with code ${code}`);
});

Both of these functions have a synchronous counterpart. An example for child_process.execSync:

const { execSync } = require('child_process');
// stderr is sent to stderr of parent process
// you can set options.stdio if you want it to go elsewhere
let stdout = execSync('ls');

As well as child_process.spawnSync:

const { spawnSync} = require('child_process');
const child = spawnSync('ls', ['-lh', '/usr']);

console.log('error', child.error);
console.log('stdout ', child.stdout);
console.log('stderr ', child.stderr);

Note: The following code is still functional, but is primarily targeted at users of ES5 and before.

The module for spawning child processes with Node.js is well documented in the documentation (v5.0.0). To execute a command and fetch its complete output as a buffer, use child_process.exec:

var exec = require('child_process').exec;
var cmd = 'prince -v builds/pdf/book.html -o builds/pdf/book.pdf';

exec(cmd, function(error, stdout, stderr) {
  // command output is in stdout
});

If you need to use handle process I/O with streams, such as when you are expecting large amounts of output, use child_process.spawn:

var spawn = require('child_process').spawn;
var child = spawn('prince', [
  '-v', 'builds/pdf/book.html',
  '-o', 'builds/pdf/book.pdf'
]);

child.stdout.on('data', function(chunk) {
  // output will be here in chunks
});

// or if you want to send output elsewhere
child.stdout.pipe(dest);

If you are executing a file rather than a command, you might want to use child_process.execFile, which parameters which are almost identical to spawn, but has a fourth callback parameter like exec for retrieving output buffers. That might look a bit like this:

var execFile = require('child_process').execFile;
execFile(file, args, options, function(error, stdout, stderr) {
  // command output is in stdout
});

As of v0.11.12, Node now supports synchronous spawn and exec. All of the methods described above are asynchronous, and have a synchronous counterpart. Documentation for them can be found here. While they are useful for scripting, do note that unlike the methods used to spawn child processes asynchronously, the synchronous methods do not return an instance of ChildProcess.

Answer 3 (score 234)

Node JS v12.9.1, LTS v10.16.3, and v8.16.1 — Aug 2019

Async method (Unix):

'use strict';

const { spawn } = require( 'child_process' );
const ls = spawn( 'ls', [ '-lh', '/usr' ] );

ls.stdout.on( 'data', data => {
    console.log( `stdout: ${data}` );
} );

ls.stderr.on( 'data', data => {
    console.log( `stderr: ${data}` );
} );

ls.on( 'close', code => {
    console.log( `child process exited with code ${code}` );
} );

Async method (Windows):

'use strict';

const { spawn } = require( 'child_process' );
const dir = spawn( 'dir', [ '.' ] );

dir.stdout.on( 'data', data => console.log( `stdout: ${data}` ) );
dir.stderr.on( 'data', data => console.log( `stderr: ${data}` ) );
dir.on( 'close', code => console.log( `child process exited with code ${code}` ) );

Sync:

'use strict';

const { spawnSync } = require( 'child_process' );
const ls = spawnSync( 'ls', [ '-lh', '/usr' ] );

console.log( `stderr: ${ls.stderr.toString()}` );
console.log( `stdout: ${ls.stdout.toString()}` );

From Node.js v12.9.1 Documentation

The same goes for Node.js v10.16.3 Documentation and Node.js v8.16.1 Documentation

Question

How do I generate a random number between 0 and n?

Answer accepted (score 941)

Use rand(range)

From Ruby Random Numbers:

If you needed a random integer to simulate a roll of a six-sided die, you’d use: 1 + rand(6). A roll in craps could be simulated with 2 + rand(6) + rand(6).

Finally, if you just need a random float, just call rand with no arguments.

As Marc-André Lafortune mentions in his answer below (go upvote it), Ruby 1.9.2 has its own Random class (that Marc-André himself helped to debug, hence the 1.9.2 target for that feature).

For instance, in this game where you need to guess 10 numbers, you can initialize them with:

10.times.map{ 20 + Random.rand(11) } 
#=> [26, 26, 22, 20, 30, 26, 23, 23, 25, 22]

Note:

• Using Random.new.rand(20..30) (using Random.new) generally would not be a good idea, as explained in detail (again) by Marc-André Lafortune, in his answer (again).

• But if you don’t use Random.new, then the class method rand only takes a max value, not a Range, as banister (energetically) points out in the comment (and as documented in the docs for Random). Only the instance method can take a Range, as illustrated by generate a random number with 7 digits.

This is why the equivalent of Random.new.rand(20..30) would be 20 + Random.rand(11), since Random.rand(int) returns “a random integer greater than or equal to zero and less than the argument.” 20..30 includes 30, I need to come up with a random number between 0 and 11, excluding 11.

Answer 2 (score 588)

While you can use rand(42-10) + 10 to get a random number between 10 and 42 (where 10 is inclusive and 42 exclusive), there’s a better way since Ruby 1.9.3, where you are able to call:

rand(10...42) # => 13

Available for all versions of Ruby by requiring my backports gem.

Ruby 1.9.2 also introduced the Random class so you can create your own random number generator objects and has a nice API:

r = Random.new
r.rand(10...42) # => 22
r.bytes(3) # => "rnd"

The Random class itself acts as a random generator, so you call directly:

Random.rand(10...42) # => same as rand(10...42)

Notes on Random.new

In most cases, the simplest is to use rand or Random.rand. Creating a new random generator each time you want a random number is a really bad idea. If you do this, you will get the random properties of the initial seeding algorithm which are atrocious compared to the properties of the random generator itself.

If you use Random.new, you should thus call it as rarely as possible, for example once as MyApp::Random = Random.new and use it everywhere else.

The cases where Random.new is helpful are the following:

• you are writing a gem and don’t want to interfere with the sequence of rand/Random.rand that the main programs might be relying on
• you want separate reproducible sequences of random numbers (say one per thread)
• you want to be able to save and resume a reproducible sequence of random numbers (easy as Random objects can marshalled)

Answer 3 (score 46)

If you’re not only seeking for a number but also hex or uuid it’s worth mentioning that the SecureRandom module found its way from ActiveSupport to the ruby core in 1.9.2+. So without the need for a full blown framework:

require 'securerandom'

p SecureRandom.random_number(100) #=> 15
p SecureRandom.random_number(100) #=> 88

p SecureRandom.random_number #=> 0.596506046187744
p SecureRandom.random_number #=> 0.350621695741409

p SecureRandom.hex #=> "eb693ec8252cd630102fd0d0fb7c3485"

It’s documented here: Ruby 1.9.3 - Module: SecureRandom (lib/securerandom.rb)

Question

How do I call shell commands from inside of a Ruby program? How do I then get output from these commands back into Ruby?

Answer accepted (score 1269)

This explanation is based on a commented Ruby script from a friend of mine. If you want to improve the script, feel free to update it at the link.

First, note that when Ruby calls out to a shell, it typically calls /bin/sh, not Bash. Some Bash syntax is not supported by /bin/sh on all systems.

Here are ways to execute a shell script:

cmd = "echo 'hi'" # Sample string that can be used

1. `Kernel#, commonly called backticks –cmd``

   This is like many other languages, including Bash, PHP, and Perl.

   Returns the result (i.e. standard output) of the shell command.

   Docs: http://ruby-doc.org/core/Kernel.html#method-i-60

   value = `echo 'hi'`
   value = `#{cmd}`
   ```</li>
   <li><p>Built-in syntax, `%x( cmd )`</p>
   
   <p>Following the `x` character is a delimiter, which can be any character.
   If the delimiter is one of the characters `(`, `[`, `{`, or `&lt;`,
   the literal consists of the characters up to the matching closing delimiter,
   taking account of nested delimiter pairs. For all other delimiters, the
   literal comprises the characters up to the next occurrence of the
   delimiter character.  String interpolation `#{ ... }` is allowed.</p>
   
   Returns the result (i.e. standard output) of the shell command, just like the backticks.  
   
   Docs: <a href="http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html" rel="noreferrer">http://www.ruby-doc.org/docs/ProgrammingRuby/html/language.html</a>  
   
   ```ruby
   value = %x( echo 'hi' )
   value = %x[ #{cmd} ]
   ```</li>
   <li><p>`Kernel#system`</p>
   
   Executes the given command in a subshell.   
   
   Returns `true` if the command was found and run successfully, `false` otherwise.  
   
   Docs: <a href="http://ruby-doc.org/core/Kernel.html#method-i-system" rel="noreferrer">http://ruby-doc.org/core/Kernel.html#method-i-system</a>  
   
   ```ruby
   wasGood = system( "echo 'hi'" )
   wasGood = system( cmd )
   ```</li>
   <li><p>`Kernel#exec`</p>
   
   Replaces the current process by running the given external command.  
   
   Returns none, the current process is replaced and never continues.  
   
   Docs: <a href="http://ruby-doc.org/core/Kernel.html#method-i-exec" rel="noreferrer">http://ruby-doc.org/core/Kernel.html#method-i-exec</a>  
   
   ```ruby
   exec( "echo 'hi'" )
   exec( cmd ) # Note: this will never be reached because of the line above
   ```</li>
   </ol>
   
   <p>Here's some extra advice:
   `$?`, which is the same as `$CHILD_STATUS`, accesses the status of the last system executed command if you use the backticks, `system()` or `%x{}`.
   You can then access the `exitstatus` and `pid` properties:</p>
   
   ```ruby
   $?.exitstatus

   For more reading see:

   • http://www.elctech.com/blog/i-m-in-ur-commandline-executin-ma-commands
   • http://blog.jayfields.com/2006/06/ruby-kernel-system-exec-and-x.html
   • http://tech.natemurray.com/2007/03/ruby-shell-commands.html

Answer 2 (score 234)

Here’s a flowchart based on this answer. See also, using script to emulate a terminal.

Answer 3 (score 158)

The way I like to do this is using the %x literal, which makes it easy (and readable!) to use quotes in a command, like so:

directorylist = %x[find . -name '*test.rb' | sort]

Which, in this case, will populate file list with all test files under the current directory, which you can process as expected:

directorylist.each do |filename|
  filename.chomp!
  # work with file
end

Question

What is the Ruby function to remove all white space? Kind of like php’s trim()?

Answer 2 (score 818)

If you want to remove only leading and trailing whitespace (like PHP’s trim) you can use .strip, but if you want to remove all whitespace, you can use .gsub(/\s+/, "") instead .

Answer 3 (score 478)

s = "I have white space".delete(' ')

And to emulate PHP’s trim() function:

s = "   I have leading and trailing white space   ".strip

Question

PHP, for all its warts, is pretty good on this count. There’s no difference between an array and a hash (maybe I’m naive, but this seems obviously right to me), and to iterate through either you just do

foreach (array/hash as $key => $value)

In Ruby there are a bunch of ways to do this sort of thing:

array.length.times do |i|
end

array.each

array.each_index

for i in array

Hashes make more sense, since I just always use

hash.each do |key, value|

Why can’t I do this for arrays? If I want to remember just one method, I guess I can use each_index (since it makes both the index and value available), but it’s annoying to have to do array[index] instead of just value.

Oh right, I forgot about array.each_with_index. However, this one sucks because it goes |value, key| and hash.each goes |key, value|! Is this not insane?

Answer accepted (score 540)

This will iterate through all the elements:

array = [1, 2, 3, 4, 5, 6]
array.each { |x| puts x }

Prints:

1
2
3
4
5
6

This will iterate through all the elements giving you the value and the index:

array = ["A", "B", "C"]
array.each_with_index {|val, index| puts "#{val} => #{index}" }

Prints:

A => 0
B => 1
C => 2

I’m not quite sure from your question which one you are looking for.

Answer 2 (score 81)

I think there is no one right way. There are a lot of different ways to iterate, and each has its own niche.

• each is sufficient for many usages, since I don’t often care about the indexes.
• each_ with _index acts like Hash#each - you get the value and the index.
• each_index - just the indexes. I don’t use this one often. Equivalent to “length.times”.
• map is another way to iterate, useful when you want to transform one array into another.
• select is the iterator to use when you want to choose a subset.
• inject is useful for generating sums or products, or collecting a single result.

It may seem like a lot to remember, but don’t worry, you can get by without knowing all of them. But as you start to learn and use the different methods, your code will become cleaner and clearer, and you’ll be on your way to Ruby mastery.

Answer 3 (score 59)

I’m not saying that Array -> |value,index| and Hash -> |key,value| is not insane (see Horace Loeb’s comment), but I am saying that there is a sane way to expect this arrangement.

When I am dealing with arrays, I am focused on the elements in the array (not the index because the index is transitory). The method is each with index, i.e. each+index, or |each,index|, or |value,index|. This is also consistent with the index being viewed as an optional argument, e.g. |value| is equivalent to |value,index=nil| which is consistent with |value,index|.

When I am dealing with hashes, I am often more focused on the keys than the values, and I am usually dealing with keys and values in that order, either key => value or hash[key] = value.

If you want duck-typing, then either explicitly use a defined method as Brent Longborough showed, or an implicit method as maxhawkins showed.

Ruby is all about accommodating the language to suit the programmer, not about the programmer accommodating to suit the language. This is why there are so many ways. There are so many ways to think about something. In Ruby, you choose the closest and the rest of the code usually falls out extremely neatly and concisely.

As for the original question, “What is the “right” way to iterate through an array in Ruby?", well, I think the core way (i.e. without powerful syntactic sugar or object oriented power) is to do:

for index in 0 ... array.size
  puts "array[#{index}] = #{array[index].inspect}"
end

But Ruby is all about powerful syntactic sugar and object oriented power, but anyway here is the equivalent for hashes, and the keys can be ordered or not:

for key in hash.keys.sort
  puts "hash[#{key.inspect}] = #{hash[key].inspect}"
end

So, my answer is, “The “right” way to iterate through an array in Ruby depends on you (i.e. the programmer or the programming team) and the project.". The better Ruby programmer makes the better choice (of which syntactic power and/or which object oriented approach). The better Ruby programmer continues to look for more ways.

Now, I want to ask another question, “What is the “right” way to iterate through a Range in Ruby backwards?"! (This question is how I came to this page.)

It is nice to do (for the forwards):

(1..10).each{|i| puts "i=#{i}" }

but I don’t like to do (for the backwards):

(1..10).to_a.reverse.each{|i| puts "i=#{i}" }

Well, I don’t actually mind doing that too much, but when I am teaching going backwards, I want to show my students a nice symmetry (i.e. with minimal difference, e.g. only adding a reverse, or a step -1, but without modifying anything else). You can do (for symmetry):

(a=*1..10).each{|i| puts "i=#{i}" }

and

(a=*1..10).reverse.each{|i| puts "i=#{i}" }

which I don’t like much, but you can’t do

(*1..10).each{|i| puts "i=#{i}" }
(*1..10).reverse.each{|i| puts "i=#{i}" }
#
(1..10).step(1){|i| puts "i=#{i}" }
(1..10).step(-1){|i| puts "i=#{i}" }
#
(1..10).each{|i| puts "i=#{i}" }
(10..1).each{|i| puts "i=#{i}" }   # I don't want this though.  It's dangerous

You could ultimately do

class Range

  def each_reverse(&block)
    self.to_a.reverse.each(&block)
  end

end

but I want to teach pure Ruby rather than object oriented approaches (just yet). I would like to iterate backwards:

• without creating an array (consider 0..1000000000)
• working for any Range (e.g. Strings, not just Integers)
• without using any extra object oriented power (i.e. no class modification)

I believe this is impossible without defining a pred method, which means modifying the Range class to use it. If you can do this please let me know, otherwise confirmation of impossibility would be appreciated though it would be disappointing. Perhaps Ruby 1.9 addresses this.

(Thanks for your time in reading this.)

Question

How can I get the current absolute URL in my Ruby on Rails view?

The request.request_uri only returns the relative URL.

Answer accepted (score 1412)

For Rails 3.2 or Rails 4+

You should use request.original_url to get the current URL.

This method is documented at original_url method, but if you’re curious, the implementation is:

def original_url
  base_url + original_fullpath
end

For Rails 3:

You can write "#{request.protocol}#{request.host_with_port}#{request.fullpath}", since request.url is now deprecated.

For Rails 2:

You can write request.url instead of request.request_uri. This combines the protocol (usually http://) with the host, and request_uri to give you the full address.

Answer 2 (score 133)

I think that the Ruby on Rails 3.0 method is now request.fullpath.

Answer 3 (score 117)

You could use url_for(only_path: false)

Question

I’m trying to install a gem using gem install mygem or update RubyGems using gem update --system, and it fails with this error:

ERROR:  While executing gem ... (Gem::FilePermissionError)
You don't have write permissions for the /Library/Ruby/Gems/2.0.0 directory.

Does anyone have an idea how to solve this?

Answer accepted (score 310)

You don’t have write permissions into the /Library/Ruby/Gems/1.8 directory.

means exactly that, you don’t have permission to write there.

That is the version of Ruby installed by Apple, for their own use. While it’s OK to make minor modifications to that if you know what you’re doing, because you are not sure about the permissions problem, I’d say it’s not a good idea to continue along that track.

Instead, I’ll strongly suggest you look into using either rbenv or RVM to manage a separate Ruby, installed into a sandbox in your home directory, that you can modify/fold/spindle/change without worrying about messing up the system Ruby.

Between the two, I use rbenv, though I used RVM a lot in the past. rbenv takes a more “hands-off” approach to managing your Ruby installation. RVM has a lot of features and is very powerful, but, as a result is more intrusive. In either case, READ the installation documentation for them a couple times before starting to install whichever you pick.

Answer 2 (score 394)

Try adding --user-install instead of using sudo:

gem install mygem --user-install

Answer 3 (score 89)

You really should be using a Ruby version manager.

Using one properly would prevent and can resolve your permission problem when executing a gem update command.

I recommend rbenv.

However, even when you use a Ruby version manager, you may still get that same error message.

If you do, and you are using rbenv, just verify that the ~/.rbenv/shims directory is before the path for the system Ruby.

$ echo $PATH will show you the order of your load path.

If you find that your shims directory comes after your system Ruby bin directory, then edit your ~/.bashrc file and put this as your last export PATH command: export PATH=$HOME/.rbenv/shims:$PATH

$ ruby -v shows you what version of Ruby you are using

This shows that I’m currently using the system version of Ruby (usually not good)

$ ruby -v
ruby 1.8.7 (2012-02-08 patchlevel 358) [universal-darwin12.0]

$ rbenv global 1.9.3-p448 switches me to a newer, pre-installed version (see references below).

This shows that I’m using a newer version of Ruby (that likely won’t cause the Gem::FilePermissionError)

$ ruby -v
ruby 1.9.3p448 (2013-06-27 revision 41675) [x86_64-darwin12.4.0]

You typically should not need to preface a gem command with sudo. If you feel the need to do so, something is probably misconfigured.

For details about rbenv see the following:

• https://github.com/sstephenson/rbenv
• http://robots.thoughtbot.com/post/47273164981/using-rbenv-to-manage-rubies-and-gems

Question

I am having a hard time understanding attr_accessor in Ruby. Can someone explain this to me?

Answer accepted (score 2266)

Let’s say you have a class Person.

class Person
end

person = Person.new
person.name # => no method error

Obviously we never defined method name. Let’s do that.

class Person
  def name
    @name # simply returning an instance variable @name
  end
end

person = Person.new
person.name # => nil
person.name = "Dennis" # => no method error

Aha, we can read the name, but that doesn’t mean we can assign the name. Those are two different methods. The former is called reader and latter is called writer. We didn’t create the writer yet so let’s do that.

class Person
  def name
    @name
  end

  def name=(str)
    @name = str
  end
end

person = Person.new
person.name = 'Dennis'
person.name # => "Dennis"

Awesome. Now we can write and read instance variable @name using reader and writer methods. Except, this is done so frequently, why waste time writing these methods every time? We can do it easier.

class Person
  attr_reader :name
  attr_writer :name
end

Even this can get repetitive. When you want both reader and writer just use accessor!

class Person
  attr_accessor :name
end

person = Person.new
person.name = "Dennis"
person.name # => "Dennis"

Works the same way! And guess what: the instance variable @name in our person object will be set just like when we did it manually, so you can use it in other methods.

class Person
  attr_accessor :name

  def greeting
    "Hello #{@name}"
  end
end

person = Person.new
person.name = "Dennis"
person.greeting # => "Hello Dennis"

That’s it. In order to understand how attr_reader, attr_writer, and attr_accessor methods actually generate methods for you, read other answers, books, ruby docs.

Answer 2 (score 125)

attr_accessor is just a method. (The link should provide more insight with how it works - look at the pairs of methods generated, and a tutorial should show you how to use it.)

The trick is that class is not a definition in Ruby (it is “just a definition” in languages like C++ and Java), but it is an expression that evaluates. It is during this evaluation when the attr_accessor method is invoked which in turn modifies the current class - remember the implicit receiver: self.attr_accessor, where self is the “open” class object at this point.

The need for attr_accessor and friends, is, well:

1. Ruby, like Smalltalk, does not allow instance variables to be accessed outside of methods¹ for that object. That is, instance variables cannot be accessed in the x.y form as is common in say, Java or even Python. In Ruby y is always taken as a message to send (or “method to call”). Thus the attr_* methods create wrappers which proxy the instance @variable access through dynamically created methods.

2. Boilerplate sucks

Hope this clarifies some of the little details. Happy coding.

¹ This isn’t strictly true and there are some “techniques” around this, but there is no syntax support for “public instance variable” access.

Answer 3 (score 67)

attr_accessor is (as @pst stated) just a method. What it does is create more methods for you.

So this code here:

class Foo
  attr_accessor :bar
end

is equivalent to this code:

class Foo
  def bar
    @bar
  end
  def bar=( new_value )
    @bar = new_value
  end
end

You can write this sort of method yourself in Ruby:

class Module
  def var( method_name )
    inst_variable_name = "@#{method_name}".to_sym
    define_method method_name do
      instance_variable_get inst_variable_name
    end
    define_method "#{method_name}=" do |new_value|
      instance_variable_set inst_variable_name, new_value
    end
  end
end

class Foo
  var :bar
end

f = Foo.new
p f.bar     #=> nil
f.bar = 42
p f.bar     #=> 42

Question

I have a string that I want to parse in Ruby:

string = '{"desc":{"someKey":"someValue","anotherKey":"value"},"main_item":{"stats":{"a":8,"b":12,"c":10}}}'

Is there an easy way to extract the data?

Answer accepted (score 523)

This looks like JavaScript Object Notation (JSON). You can parse JSON that resides in some variable, e.g. json_string, like so:

require 'json'
JSON.parse(json_string)

If you’re using an older Ruby, you may need to install the json gem.

There are also other implementations of JSON for Ruby that may fit some use-cases better:

• YAJL C Bindings for Ruby
• JSON::Stream

Answer 2 (score 200)

Just to extend the answers a bit with what to do with the parsed object:

# JSON Parsing example
require "rubygems"
require "json"

string = '{"desc":{"someKey":"someValue","anotherKey":"value"},"main_item":{"stats":{"a":8,"b":12,"c":10}}}'
parsed = JSON.parse(string) # returns a hash

p parsed["desc"]["someKey"]
p parsed["main_item"]["stats"]["a"]

# Read JSON from a file, iterate over objects
file = open("shops.json")
json = file.read

parsed = JSON.parse(json)

parsed["shop"].each do |shop|
  p shop["id"]
end

Answer 3 (score 37)

As of Ruby v1.9.3 you don’t need to install any Gems in order to parse JSON, simply use require 'json':

require 'json'

json = JSON.parse '{"foo":"bar", "ping":"pong"}'
puts json['foo'] # prints "bar"

See JSON at Ruby-Doc.

Question

I’ve been using Eclipse with RDT (not RadRails) a lot lately, and I’m quite happy with it, but I’m wondering if you guys know any decent alternatives. I know NetBeans also supports Ruby these days, but I’m not sure what it has to offer over Eclipse.

Please, list any features you think are brilliant or useful when suggesting an IDE, makes it easier to compare.

Also, I said Ruby, not Rails. While Rails support is a plus, I prefer things to be none Rails-centric. It should also be available on Linux and optionally Solaris.

Answer accepted (score 28)

Have you tried Aptana? It’s based on Eclipse and they have a sweet Rails plugin.

Answer 2 (score 76)

RubyMine from JetBrains. (Also available as a plugin to IntelliJ IDEA)

Answer 3 (score 26)

Redcar has been getting some attention lately, as well. Still early in its life, but it shows promise.

Question

I am using the following code to check if a variable is not nil and not zero

if(discount != nil && discount != 0) 
  ...
end

Is there a better way to do this?

Answer accepted (score 397)

unless discount.nil? || discount == 0
  # ...
end

Answer 2 (score 40)

class Object
  def nil_zero?
    self.nil? || self == 0
  end
end

# which lets you do
nil.nil_zero? # returns true
0.nil_zero?   # returns true
1.nil_zero?   # returns false
"a".nil_zero? # returns false

unless discount.nil_zero?
  # do stuff...
end

Beware of the usual disclaimers… great power/responsibility, monkey patching leading to the dark side etc.

Answer 3 (score 28)

unless [nil, 0].include?(discount) 
  # ...
end

Question

I have an array of integers.

For example:

array = [123,321,12389]

Is there any nice way to get the sum of them?

I know, that

sum = 0
array.each { |a| sum+=a }

would work.

Answer accepted (score 598)

Try this:

array.inject(0){|sum,x| sum + x }

See Ruby’s Enumerable Documentation

(note: the 0 base case is needed so that 0 will be returned on an empty array instead of nil)

Answer 2 (score 795)

Or try the Ruby 1.9 way:

array.inject(0, :+)

Note: the 0 base case is needed otherwise nil will be returned on empty arrays:

> [].inject(:+)
nil
> [].inject(0, :+)
0

Answer 3 (score 280)

array.reduce(0, :+)

While equivalent to array.inject(0, :+), the term reduce is entering a more common vernacular with the rise of MapReduce programming models.

inject, reduce, fold, accumulate, and compress are all synonymous as a class of folding functions. I find consistency across your code base most important, but since various communities tend to prefer one word over another, it’s nonetheless useful to know the alternatives.

To emphasize the map-reduce verbiage, here’s a version that is a little bit more forgiving on what ends up in that array.

array.map(&:to_i).reduce(0, :+)

Some additional relevant reading:

• http://ruby-doc.org/core-1.9.3/Enumerable.html#method-i-inject
• http://en.wikipedia.org/wiki/MapReduce
• http://en.wikipedia.org/wiki/Fold_(higher-order_function)

Question

I’ll use python as an example of what I’m looking for (you can think of it as pseudocode if you don’t know Python):

>>> a = 1
>>> type(a)
<type 'int'>

I know in ruby I can do :

1.9.3p194 :002 > 1.class
 => Fixnum 

But is this the proper way to determine the type of the object?

Answer accepted (score 550)

The proper way to determine the “type” of an object, which is a wobbly term in the Ruby world, is to call object.class.

Since classes can inherit from other classes, if you want to determine if an object is “of a particular type” you might call object.is_a?(ClassName) to see if object is of type ClassName or derived from it.

Normally type checking is not done in Ruby, but instead objects are assessed based on their ability to respond to particular methods, commonly called “Duck typing”. In other words, if it responds to the methods you want, there’s no reason to be particular about the type.

For example, object.is_a?(String) is too rigid since another class might implement methods that convert it into a string, or make it behave identically to how String behaves. object.respond_to?(:to_s) would be a better way to test that the object in question does what you want.

Answer 2 (score 71)

you could also try: instance_of?

p 1.instance_of? Fixnum    #=> True
p "1".instance_of? String  #=> True
p [1,2].instance_of? Array #=> True

Answer 3 (score 40)

Oftentimes in Ruby, you don’t actually care what the object’s class is, per se, you just care that it responds to a certain method. This is known as Duck Typing and you’ll see it in all sorts of Ruby codebases.

So in many (if not most) cases, its best to use Duck Typing using #respond_to?(method):

object.respond_to?(:to_i)

Question

I’m currently generating an 8-character pseudo-random uppercase string for “A” .. “Z”:

value = ""; 8.times{value  << (65 + rand(25)).chr}

but it doesn’t look clean, and it can’t be passed as an argument since it isn’t a single statement. To get a mixed-case string “a” .. “z” plus “A” .. “Z”, I changed it to:

value = ""; 8.times{value << ((rand(2)==1?65:97) + rand(25)).chr}

but it looks like trash.

Does anyone have a better method?

Answer accepted (score 941)

(0...8).map { (65 + rand(26)).chr }.join

I spend too much time golfing.

(0...50).map { ('a'..'z').to_a[rand(26)] }.join

And a last one that’s even more confusing, but more flexible and wastes fewer cycles:

o = [('a'..'z'), ('A'..'Z')].map(&:to_a).flatten
string = (0...50).map { o[rand(o.length)] }.join

Answer 2 (score 778)

Why not use SecureRandom?

require 'securerandom'
random_string = SecureRandom.hex

# outputs: 5b5cd0da3121fc53b4bc84d0c8af2e81 (i.e. 32 chars of 0..9, a..f)

SecureRandom also has methods for:

• base64
• random_bytes
• random_number

see: http://ruby-doc.org/stdlib-1.9.2/libdoc/securerandom/rdoc/SecureRandom.html

Answer 3 (score 245)

I use this for generating random URL friendly strings with a guaranteed maximum length:

rand(36**length).to_s(36)

It generates random strings of lowercase a-z and 0-9. It’s not very customizable but it’s short and clean.

Question

I’ve recently started programming in Ruby, and I am looking at exception handling.

I was wondering if ensure was the Ruby equivalent of finally in C#? Should I have:

file = File.open("myFile.txt", "w")

begin
  file << "#{content} \n"
rescue
  #handle the error here
ensure
  file.close unless file.nil?
end

or should I do this?

#store the file
file = File.open("myFile.txt", "w")

begin
  file << "#{content} \n"
  file.close
rescue
  #handle the error here
ensure
  file.close unless file.nil?
end

Does ensure get called no matter what, even if an exception isn’t raised?

Answer accepted (score 1142)

Yes, ensure ensures that the code is always evaluated. That’s why it’s called ensure. So, it is equivalent to Java’s and C#’s finally.

The general flow of begin/rescue/else/ensure/end looks like this:

begin
  # something which might raise an exception
rescue SomeExceptionClass => some_variable
  # code that deals with some exception
rescue SomeOtherException => some_other_variable
  # code that deals with some other exception
else
  # code that runs only if *no* exception was raised
ensure
  # ensure that this code always runs, no matter what
  # does not change the final value of the block
end

You can leave out rescue, ensure or else. You can also leave out the variables in which case you won’t be able to inspect the exception in your exception handling code. (Well, you can always use the global exception variable to access the last exception that was raised, but that’s a little bit hacky.) And you can leave out the exception class, in which case all exceptions that inherit from StandardError will be caught. (Please note that this does not mean that all exceptions are caught, because there are exceptions which are instances of Exception but not StandardError. Mostly very severe exceptions that compromise the integrity of the program such as SystemStackError, NoMemoryError, SecurityError, NotImplementedError, LoadError, SyntaxError, ScriptError, Interrupt, SignalException or SystemExit.)

Some blocks form implicit exception blocks. For example, method definitions are implicitly also exception blocks, so instead of writing

def foo
  begin
    # ...
  rescue
    # ...
  end
end

you write just

def foo
  # ...
rescue
  # ...
end

or

def foo
  # ...
ensure
  # ...
end

The same applies to class definitions and module definitions.

However, in the specific case you are asking about, there is actually a much better idiom. In general, when you work with some resource which you need to clean up at the end, you do that by passing a block to a method which does all the cleanup for you. It’s similar to a using block in C#, except that Ruby is actually powerful enough that you don’t have to wait for the high priests of Microsoft to come down from the mountain and graciously change their compiler for you. In Ruby, you can just implement it yourself:

# This is what you want to do:
File.open('myFile.txt', 'w') do |file|
  file.puts content
end

# And this is how you might implement it:
def File.open(filename, mode='r', perm=nil, opt=nil)
  yield filehandle = new(filename, mode, perm, opt)
ensure
  filehandle&.close
end

And what do you know: this is already available in the core library as File.open. But it is a general pattern that you can use in your own code as well, for implementing any kind of resource cleanup (à la using in C#) or transactions or whatever else you might think of.

The only case where this doesn’t work, if acquiring and releasing the resource are distributed over different parts of the program. But if it is localized, as in your example, then you can easily use these resource blocks.

BTW: in modern C#, using is actually superfluous, because you can implement Ruby-style resource blocks yourself:

class File
{
    static T open<T>(string filename, string mode, Func<File, T> block)
    {
        var handle = new File(filename, mode);
        try
        {
            return block(handle);
        }
        finally
        {
            handle.Dispose();
        }
    }
}

// Usage:

File.open("myFile.txt", "w", (file) =>
{
    file.WriteLine(contents);
});

Answer 2 (score 34)

FYI, even if an exception is re-raised in the rescue section, the ensure block will be executed before the code execution continues to the next exception handler. For instance:

begin
  raise "Error!!"
rescue
  puts "test1"
  raise # Reraise exception
ensure
  puts "Ensure block"
end

Answer 3 (score 14)

If you want to ensure a file is closed you should use the block form of File.open:

File.open("myFile.txt", "w") do |file|
  begin
    file << "#{content} \n"
  rescue
  #handle the error here
  end
end

Question

How can I comment multiple lines in Ruby?

Answer accepted (score 1311)

#!/usr/bin/env ruby

=begin
Every body mentioned this way
to have multiline comments.

The =begin and =end must be at the beginning of the line or
it will be a syntax error.
=end

puts "Hello world!"

<<-DOC
Also, you could create a docstring.
which...
DOC

puts "Hello world!"

"..is kinda ugly and creates
a String instance, but I know one guy
with a Smalltalk background, who
does this."

puts "Hello world!"

##
# most
# people
# do
# this


__END__

But all forgot there is another option.
Only at the end of a file, of course.

• This is how it looks (via screenshot) - otherwise it’s hard to interpret how the above comments will look. Click to Zoom-in:

Answer 2 (score 122)

=begin
My 
multiline
comment
here
=end

Answer 3 (score 54)

Despite the existence of =begin and =end, the normal and a more correct way to comment is to use #’s on each line. If you read the source of any ruby library, you will see that this is the way multi-line comments are done in almost all cases.

Question

Using the command-line gem tool, how can I install a specific version of a gem?

Answer accepted (score 1085)

Use the -v flag:

$ gem install fog -v 1.8

Answer 2 (score 184)

Use the --version parameter (shortcut -v):

$ gem install rails -v 0.14.1

You can also use version comparators like >= or ~>

$ gem install rails -v '~> 0.14.0'

Or with newer versions of gem even:

$ gem install rails:0.14.4 rubyzip:'< 1'
…
Successfully installed rails-0.14.4
Successfully installed rubyzip-0.9.9

Answer 3 (score 78)

For installing gem install gemname -v versionnumber

For uninstall gem uninstall gemname -v versionnumber

Question

I want to check whether the “user” key is present or not in the session hash. How can I do this?

Note that I don’t want to check whether the key’s value is nil or not. I just want to check whether the “user” key is present.

Answer accepted (score 893)

Hash’s key? method tells you whether a given key is present or not.

session.key?("user")

Answer 2 (score 283)

While Hash#has_key? gets the job done, as Matz notes here, it has been deprecated in favour of Hash#key?.

hash.key?(some_key)

Answer 3 (score 37)

In latest Ruby versions Hash instance has a key? method:

{a: 1}.key?(:a)
=> true

Be sure to use the symbol key or a string key depending on what you have in your hash:

{'a' => 2}.key?(:a)
=> false

Question

To add a new pair to Hash I do:

{:a => 1, :b => 2}.merge!({:c => 3})   #=> {:a => 1, :b => 2, :c => 3}

Is there a similar way to delete a key from Hash ?

This works:

{:a => 1, :b => 2}.reject! { |k| k == :a }   #=> {:b => 2}

but I would expect to have something like:

{:a => 1, :b => 2}.delete!(:a)   #=> {:b => 2}

It is important that the returning value will be the remaining hash, so I could do things like:

foo(my_hash.reject! { |k| k == my_key })

in one line.

Answer accepted (score 701)

Rails has an except/except! method that returns the hash with those keys removed. If you’re already using Rails, there’s no sense in creating your own version of this.

class Hash
  # Returns a hash that includes everything but the given keys.
  #   hash = { a: true, b: false, c: nil}
  #   hash.except(:c) # => { a: true, b: false}
  #   hash # => { a: true, b: false, c: nil}
  #
  # This is useful for limiting a set of parameters to everything but a few known toggles:
  #   @person.update(params[:person].except(:admin))
  def except(*keys)
    dup.except!(*keys)
  end

  # Replaces the hash without the given keys.
  #   hash = { a: true, b: false, c: nil}
  #   hash.except!(:c) # => { a: true, b: false}
  #   hash # => { a: true, b: false }
  def except!(*keys)
    keys.each { |key| delete(key) }
    self
  end
end

Answer 2 (score 196)

Oneliner plain ruby, it works only with ruby > 1.9.x:

1.9.3p0 :002 > h = {:a => 1, :b => 2}
 => {:a=>1, :b=>2} 
1.9.3p0 :003 > h.tap { |hs| hs.delete(:a) }
 => {:b=>2} 

Tap method always return the object on which is invoked…

Otherwise if you have required active_support/core_ext/hash (which is automatically required in every Rails application) you can use one of the following methods depending on your needs:

➜  ~  irb
1.9.3p125 :001 > require 'active_support/core_ext/hash' => true 
1.9.3p125 :002 > h = {:a => 1, :b => 2, :c => 3}
 => {:a=>1, :b=>2, :c=>3} 
1.9.3p125 :003 > h.except(:a)
 => {:b=>2, :c=>3} 
1.9.3p125 :004 > h.slice(:a)
 => {:a=>1} 

except uses a blacklist approach, so it removes all the keys listed as args, while slice uses a whitelist approach, so it removes all keys that aren’t listed as arguments. There also exist the bang version of those method (except! and slice!) which modify the given hash but their return value is different both of them return an hash. It represents the removed keys for slice! and the keys that are kept for the except!:

1.9.3p125 :011 > {:a => 1, :b => 2, :c => 3}.except!(:a)
 => {:b=>2, :c=>3} 
1.9.3p125 :012 > {:a => 1, :b => 2, :c => 3}.slice!(:a)
 => {:b=>2, :c=>3} 

Answer 3 (score 169)

Why not just use:

hash.delete(key)

Question

somearray = ["some", "thing"]

anotherarray = ["another", "thing"]

somearray.push(anotherarray.flatten!)

I expected

["some","thing","another","thing"]

Answer 2 (score 674)

You’ve got a workable idea, but the #flatten! is in the wrong place – it flattens its receiver, so you could use it to turn [1, 2, ['foo', 'bar']] into [1,2,'foo','bar'].

I’m doubtless forgetting some approaches, but you can concatenate:

a1.concat a2
a1 + a2              # creates a new array, as does a1 += a2

or prepend/append:

a1.push(*a2)         # note the asterisk
a2.unshift(*a1)      # note the asterisk, and that a2 is the receiver

or splice:

a1[a1.length, 0] = a2
a1[a1.length..0] = a2
a1.insert(a1.length, *a2)

or append and flatten:

(a1 << a2).flatten!  # a call to #flatten instead would return a new array

Answer 3 (score 199)

You can just use the + operator!

irb(main):001:0> a = [1,2]
=> [1, 2]
irb(main):002:0> b = [3,4]
=> [3, 4]
irb(main):003:0> a + b
=> [1, 2, 3, 4]

You can read all about the array class here: http://ruby-doc.org/core/classes/Array.html

Question

What’s the syntax for dropping a database table column through a Rails migration?

Answer accepted (score 868)

remove_column :table_name, :column_name

For instance:

remove_column :users, :hobby

would remove the hobby Column from the users table.

Answer 2 (score 359)

For older versions of Rails

ruby script/generate migration RemoveFieldNameFromTableName field_name:datatype

For Rails 3 and up

rails generate migration RemoveFieldNameFromTableName field_name:datatype

Answer 3 (score 115)

Rails 4 has been updated, so the change method can be used in the migration to drop a column and the migration will successfully rollback. Please read the following warning for Rails 3 applications:

Rails 3 Warning

Please note that when you use this command:

rails generate migration RemoveFieldNameFromTableName field_name:datatype

The generated migration will look something like this:

  def up
    remove_column :table_name, :field_name
  end

  def down
    add_column :table_name, :field_name, :datatype
  end

Make sure to not use the change method when removing columns from a database table (example of what you don’t want in the migration file in Rails 3 apps):

  def change
    remove_column :table_name, :field_name
  end

The change method in Rails 3 is not smart when it comes to remove_column, so you will not be able to rollback this migration.

Question

I need to update my ruby version from 2.0.0 to the latest version, I can not use some gems because my version is not updated. I had used Homebrew to install Ruby some time ago, How can i update my Ruby version?

Answer accepted (score 635)

Open your terminal and run

curl -sSL https://raw.githubusercontent.com/rvm/rvm/master/binscripts/rvm-installer | bash -s stable

When this is complete, you need to restart your terminal for the rvm command to work.

Now, run rvm list known

This shows the list of versions of the ruby.

Now, run rvm install ruby-2.4.2

If you type ruby -v in the terminal, you should see ruby 2.4.2.

If it still shows you ruby 2.0., run rvm use ruby-2.4.2 --default.

Answer 2 (score 195)

Brew only solution

Update:

From the comments (kudos to Maksim Luzik), I haven’t tested but seems like a more elegant solution:

After installing ruby through brew, run following command to update the links to the latest ruby installation: brew link --overwrite ruby

Original answer:

Late to the party, but using brew is enough. It’s not necessary to install rvm and for me it just complicated things.

By brew install ruby you’re actually installing the latest (currently v2.4.0). However, your path finds 2.0.0 first. To avoid this just change precedence (source). I did this by changing ~/.profile and setting:

export PATH=/usr/local/bin:$PATH

After this I found that bundler gem was still using version 2.0.0, just install it again: gem install bundler

Answer 3 (score 95)

I recommend rbenv* https://github.com/rbenv/rbenv

• If this meets your criteria: https://github.com/rbenv/rbenv/wiki/Why-rbenv?:

rbenv does…

• Provide support for specifying application-specific Ruby versions.
• Let you change the global Ruby version on a per-user basis.
• Allow you to override the Ruby version with an environment variable.

In contrast with RVM, rbenv does not…

• Need to be loaded into your shell. Instead, rbenv’s shim approach works by adding a directory to your $PATH.
• Override shell commands like cd or require prompt hacks. That’s dangerous and error-prone.
• Have a configuration file. There’s nothing to configure except which version of Ruby you want to use.
• Install Ruby. You can build and install Ruby yourself, or use ruby-build to automate the process.
• Manage gemsets. Bundler is a better way to manage application dependencies. If you have projects that are not yet using Bundler you can install the rbenv-gemset plugin.
• Require changes to Ruby libraries for compatibility. The simplicity of rbenv means as long as it’s in your $PATH, nothing else needs to know about it.

INSTALLATION

Install Homebrew http://brew.sh

Then:

$ brew update
$ brew install rbenv 
$ brew install rbenv ruby-build

# Add rbenv to bash so that it loads every time you open a terminal
echo 'if which rbenv > /dev/null; then eval "$(rbenv init -)"; fi' >> ~/.bash_profile
source ~/.bash_profile

UPDATE
There’s one additional step after brew install rbenv Run rbenv init and add one line to .bash_profile as it states. After that reopen your terminal window […] SGI Sep 30 at 12:01 —https://stackoverflow.com/users/119770

$ rbenv install --list
Available versions:
 1.8.5-p113
 1.8.5-p114
 […]
 2.3.1
 2.4.0-dev
 jruby-1.5.6
 […]
$ rbenv install 2.3.1
[…]

Set the global version:

$ rbenv global 2.3.1
$ ruby -v
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-darwin15]

Set the local version of your repo by adding .ruby-version to your repo’s root dir:

$ cd ~/whatevs/projects/new_repo
$ echo "2.3.1" > .ruby-version

For MacOS visit this link

Question

What are all the common ways to read a file in Ruby?

For instance, here is one method:

fileObj = File.new($fileName, "r")
while (line = fileObj.gets)
  puts(line)
end
fileObj.close

I know Ruby is extremely flexible. What are the benefits/drawbacks of each approach?

Answer accepted (score 250)

File.open("my/file/path", "r") do |f|
  f.each_line do |line|
    puts line
  end
end
# File is closed automatically at end of block

It is also possible to explicitly close file after as above (pass a block to open closes it for you):

f = File.open("my/file/path", "r")
f.each_line do |line|
  puts line
end
f.close

Answer 2 (score 316)

The easiest way if the file isn’t too long is:

puts File.read(file_name)

Indeed, IO.read or File.read automatically close the file, so there is no need to use File.open with a block.

Answer 3 (score 221)

Be wary of “slurping” files. That’s when you read the entire file into memory at once.

The problem is that it doesn’t scale well. You could be developing code with a reasonably sized file, then put it into production and suddenly find you’re trying to read files measuring in gigabytes, and your host is freezing up as it tries to read and allocate memory.

Line-by-line I/O is very fast, and almost always as effective as slurping. It’s surprisingly fast actually.

I like to use:

IO.foreach("testfile") {|x| print "GOT ", x }

or

File.foreach('testfile') {|x| print "GOT", x }

File inherits from IO, and foreach is in IO, so you can use either.

I have some benchmarks showing the impact of trying to read big files via read vs. line-by-line I/O at “Why is”slurping" a file not a good practice?".

Question

The difference between rake db:migrate and rake db:reset is pretty clear in my head. The thing which I don’t understand is how rake db:schema:load different from the former two.

Just to be sure that I am on the same page:

• rake db:migrate - Runs the migrations which haven’t been run yet.
• rake db:reset - Clears the database (presumably does a rake db:drop + rake db:create + rake db:migrate) and runs migration on a fresh database.

Please help to clarify, if my understanding has gone wrong.

Answer accepted (score 1248)

• db:migrate runs (single) migrations that have not run yet.
• db:create creates the database
• db:drop deletes the database

• db:schema:load creates tables and columns within the (existing) database following schema.rb

• db:setup does db:create, db:schema:load, db:seed

• db:reset does db:drop, db:setup

Typically, you would use db:migrate after having made changes to the schema via new migration files (this makes sense only if there is already data in the database). db:schema:load is used when you setup a new instance of your app.

I hope that helps.

UPDATE for rails 3.2.12:

I just checked the source and the dependencies are like this now:

• db:create creates the database for the current env
• db:create:all creates the databases for all envs
• db:drop drops the database for the current env
• db:drop:all drops the databases for all envs
• db:migrate runs migrations for the current env that have not run yet
• db:migrate:up runs one specific migration
• db:migrate:down rolls back one specific migration
• db:migrate:status shows current migration status
• db:rollback rolls back the last migration
• db:forward advances the current schema version to the next one
• db:seed (only) runs the db/seed.rb file
• db:schema:load loads the schema into the current env’s database

• db:schema:dump dumps the current env’s schema (and seems to create the db as well)

• db:setup runs db:schema:load, db:seed

• db:reset runs db:drop db:setup
• db:migrate:redo runs (db:migrate:down db:migrate:up) or (db:rollback db:migrate) depending on the specified migration
• db:migrate:reset runs db:drop db:create db:migrate

For further information please have a look at https://github.com/rails/rails/blob/v3.2.12/activerecord/lib/active_record/railties/databases.rake (for Rails 3.2.x) and https://github.com/rails/rails/blob/v4.0.5/activerecord/lib/active_record/railties/databases.rake (for Rails 4.0.x)

Answer 2 (score 21)

TLDR

Use

• rake db:migrate If you wanna make changes to the schema
• rake db:reset If you wanna drop the database, reload the schema from schema.rb, and reseed the database
• rake db:schema:load If you wanna reset database to schema as provided in schema.rb (This will delete all data)

Explanations

rake db:schema:load will set up the schema as provided in schema.rb file. This is useful for a fresh install of app as it doesn’t take as much time as db:migrate

Important note, db:schema:load will delete data on server.

rake db:migrate makes changes to the existing schema. Its like creating versions of schema. db:migrate will look in db/migrate/ for any ruby files and execute the migrations that aren’t run yet starting with the oldest. Rails knows which file is the oldest by looking at the timestamp at the beginning of the migration filename. db:migrate comes with a benefit that data can also be put in the database. This is actually not a good practice. Its better to use rake db:seed to add data.

rake db:migrate provides tasks up, down etc which enables commands like rake db:rollback and makes it the most useful command.

rake db:reset does a db:drop and db:setup
It drops the database, create it again, loads the schema, and initializes with the seed data

Relevant part of the commands from databases.rake

namespace :schema do
  desc 'Creates a db/schema.rb file that is portable against any DB supported by Active Record'
  task :dump => [:environment, :load_config] do
    require 'active_record/schema_dumper'
    filename = ENV['SCHEMA'] || File.join(ActiveRecord::Tasks::DatabaseTasks.db_dir, 'schema.rb')
    File.open(filename, "w:utf-8") do |file|
      ActiveRecord::SchemaDumper.dump(ActiveRecord::Base.connection, file)
    end
    db_namespace['schema:dump'].reenable
  end

  desc 'Loads a schema.rb file into the database'
  task :load => [:environment, :load_config, :check_protected_environments] do
    ActiveRecord::Tasks::DatabaseTasks.load_schema_current(:ruby, ENV['SCHEMA'])
  end

  # desc 'Drops and recreates the database from db/schema.rb for the current environment and loads the seeds.'
  task :reset => [ 'db:drop', 'db:setup' ]

namespace :migrate do
  # desc  'Rollbacks the database one migration and re migrate up (options: STEP=x, VERSION=x).'
  task :redo => [:environment, :load_config] do
    if ENV['VERSION']
      db_namespace['migrate:down'].invoke
      db_namespace['migrate:up'].invoke
    else
      db_namespace['rollback'].invoke
      db_namespace['migrate'].invoke
    end
  end

Answer 3 (score 2)

As far as I understand, it is going to drop your database and re-create it based on your db/schema.rb file. That is why you need to make sure that your schema.rb file is always up to date and under version control.

Question

How is the conditional operator (? :) used in Ruby?

For example, is this correct?

<% question = question.size > 20 ? question.question.slice(0, 20)+"..." : question.question %>

Answer accepted (score 480)

It is the ternary operator, and it works like in C (the parenthesis are not required). It’s an expression that works like:

if_this_is_a_true_value ? then_the_result_is_this : else_it_is_this

However, in Ruby, if is also an expression so: if a then b else c end === a ? b : c, except for precedence issues. Both are expressions.

Examples:

puts (if 1 then 2 else 3 end) # => 2

puts 1 ? 2 : 3                # => 2

x = if 1 then 2 else 3 end
puts x                        # => 2

Note that in the first case parenthesis are required (otherwise Ruby is confused because it thinks it is puts if 1 with some extra junk after it), but they are not required in the last case as said issue does not arise.

You can use the “long-if” form for readability on multiple lines:

question = if question.size > 20 then
  question.slice(0, 20) + "..."
else 
  question
end

Answer 2 (score 34)

puts true ? "true" : "false"
=> "true"


puts false ? "true" : "false"
=> "false"

Answer 3 (score 26)

Your use of ERB suggests that you are in Rails. If so, then consider truncate, a built-in helper which will do the job for you:

<% question = truncate(question, :length=>30) %>

Question

I’m looking at the documentation for FileUtils. I’m confused by the following line:

FileUtils.cp %w(cgi.rb complex.rb date.rb), '/usr/lib/ruby/1.6'

What does the %w mean? Can you point me to the documentation?

Answer accepted (score 1185)

%w(foo bar) is a shortcut for ["foo", "bar"]. Meaning it’s a notation to write an array of strings separated by spaces instead of commas and without quotes around them. You can find a list of ways of writing literals in zenspider’s quickref.

Answer 2 (score 448)

I think of %w() as a “word array” - the elements are delimited by spaces and it returns an array of strings.

There are other % literals:

• %r() is another way to write a regular expression.
• %q() is another way to write a single-quoted string (and can be multi-line, which is useful)
• %Q() gives a double-quoted string
• %x() is a shell command
• %i() gives an array of symbols (Ruby >= 2.0.0)
• %s() turns foo into a symbol (:foo)

I don’t know any others, but there may be some lurking around in there…

Answer 3 (score 58)

There is also %s that allows you to create any symbols, for example:

%s|some words|          #Same as :'some words'
%s[other words]         #Same as :'other words'
%s_last example_        #Same as :'last example'

Since Ruby 2.0.0 you also have:

%i( a b c )   # => [ :a, :b, :c ]
%i[ a b c ]   # => [ :a, :b, :c ]
%i_ a b c _   # => [ :a, :b, :c ]
# etc...

Question

How can I check whether a variable is defined in Ruby? Is there an isset-type method available?

Answer accepted (score 778)

Use the defined? keyword (documentation). It will return a String with the kind of the item, or nil if it doesn’t exist.

>> a = 1
 => 1
>> defined? a
 => "local-variable"
>> defined? b
 => nil
>> defined? nil
 => "nil"
>> defined? String
 => "constant"
>> defined? 1
 => "expression"

As skalee commented: “It is worth noting that variable which is set to nil is initialized.”

>> n = nil  
>> defined? n
 => "local-variable"

Answer 2 (score 91)

This is useful if you want to do nothing if it does exist but create it if it doesn’t exist.

def get_var
  @var ||= SomeClass.new()
end

This only creates the new instance once. After that it just keeps returning the var.

Answer 3 (score 70)

The correct syntax for the above statement is:

if (defined?(var)).nil? # will now return true or false
 print "var is not defined\n".color(:red)
else
 print "var is defined\n".color(:green)
end

substituting (var) with your variable. This syntax will return a true/false value for evaluation in the if statement.

Question

I have a rake task that needs to insert a value into multiple databases.

I’d like to pass this value into the rake task from the command line, or from another rake task.

How can I do this?

Answer accepted (score 334)

options and dependencies need to be inside arrays:

namespace :thing do
  desc "it does a thing"
  task :work, [:option, :foo, :bar] do |task, args|
    puts "work", args
  end

  task :another, [:option, :foo, :bar] do |task, args|
    puts "another #{args}"
    Rake::Task["thing:work"].invoke(args[:option], args[:foo], args[:bar])
    # or splat the args
    # Rake::Task["thing:work"].invoke(*args)
  end

end

Then

rake thing:work[1,2,3]
=> work: {:option=>"1", :foo=>"2", :bar=>"3"}

rake thing:another[1,2,3]
=> another {:option=>"1", :foo=>"2", :bar=>"3"}
=> work: {:option=>"1", :foo=>"2", :bar=>"3"}

NOTE: variable task is the the task object, not very helpful unless you know/care about Rake internals.

RAILS NOTE:

If running the task from rails, its best to preload the environment by adding => [:environment] which is a way to setup dependent tasks.

  task :work, [:option, :foo, :bar] => [:environment] do |task, args|
    puts "work", args
  end

Answer 2 (score 1117)

You can specify formal arguments in rake by adding symbol arguments to the task call. For example:

require 'rake'

task :my_task, [:arg1, :arg2] do |t, args|
  puts "Args were: #{args}"
end

task :invoke_my_task do
  Rake.application.invoke_task("my_task[1, 2]")
end

# or if you prefer this syntax...
task :invoke_my_task_2 do
  Rake::Task[:my_task].invoke(3, 4)
end

# a task with prerequisites passes its 
# arguments to it prerequisites
task :with_prerequisite, [:arg1, :arg2] => :my_task #<- name of prerequisite task

# to specify default values, 
# we take advantage of args being a Rake::TaskArguments object
task :with_defaults, :arg1, :arg2 do |t, args|
  args.with_defaults(:arg1 => :default_1, :arg2 => :default_2)
  puts "Args with defaults were: #{args}"
end

Then, from the command line:

> rake my_task[1,2]
Args were: {:arg1=>"1", :arg2=>"2"}

> rake "my_task[1, 2]"
Args were: {:arg1=>"1", :arg2=>"2"}

> rake invoke_my_task
Args were: {:arg1=>"1", :arg2=>"2"}

> rake invoke_my_task_2
Args were: {:arg1=>3, :arg2=>4}

> rake with_prerequisite[5,6]
Args were: {:arg1=>"5", :arg2=>"6"}

> rake with_defaults
Args with defaults were: {:arg1=>:default_1, :arg2=>:default_2}

> rake with_defaults['x','y']
Args with defaults were: {:arg1=>"x", :arg2=>"y"}

As demonstrated in the second example, if you want to use spaces, the quotes around the target name are necessary to keep the shell from splitting up the arguments at the space.

Looking at the code in rake.rb, it appears that rake does not parse task strings to extract arguments for prerequisites, so you can’t do task :t1 => "dep[1,2]". The only way to specify different arguments for a prerequisite would be to invoke it explicitly within the dependent task action, as in :invoke_my_task and :invoke_my_task_2.

Note that some shells (like zsh) require you to escape the brackets: rake my_task\['arg1'\]

Answer 3 (score 327)

In addition to answer by kch (I didn’t find how to leave a comment to that, sorry):

You don’t have to specify variables as ENV variables before the rake command. You can just set them as usual command line parameters like that:

rake mytask var=foo

and access those from your rake file as ENV variables like such:

p ENV['var'] # => "foo"

Question

I have a string in Ruby:

sentence = "My name is Robert"

How can I replace any one word in this sentence easily without using complex code or a loop?

Answer accepted (score 51)

You can try using this way :

sentence ["Robert"] = "Roger"

Then the sentence will become :

sentence = "My name is Roger" # Robert is replaced with Roger

Answer 2 (score 467)

sentence.sub! 'Robert', 'Joe'

Won’t cause an exception if the replaced word isn’t in the sentence (the []= variant will).

How to replace all instances?

The above replaces only the first instance of “Robert”.

To replace all instances use gsub/gsub! (ie. “global substitution”):

sentence.gsub! 'Robert', 'Joe'

The above will replace all instances of Robert with Joe.

Answer 3 (score 120)

If you’re dealing with natural language text and need to replace a word, not just part of a string, you have to add a pinch of regular expressions to your gsub as a plain text substitution can lead to disastrous results:

'mislocated cat, vindicating'.gsub('cat', 'dog')
=> "mislodoged dog, vindidoging"

Regular expressions have word boundaries, such as \b which matches start or end of a word. Thus,

'mislocated cat, vindicating'.gsub(/\bcat\b/, 'dog')
=> "mislocated dog, vindicating"

In Ruby, unlike some other languages like Javascript, word boundaries are UTF-8-compatible, so you can use it for languages with non-Latin or extended Latin alphabets:

'сіль у кисіль, для весіль'.gsub(/\bсіль\b/, 'цукор')
=> "цукор у кисіль, для весіль"

Question

I have an array of elements in Ruby

[2,4,6,3,8]

I need to remove elements with value 3 for example

How do I do that?

Answer accepted (score 446)

I think I’ve figured it out:

a = [2,4,6,3,8]
a.delete(3)

Answer 2 (score 211)

Borrowing from Travis in the comments, this is a better answer:

I personally like [1, 2, 7, 4, 5] - [7] which results in => [1, 2, 4, 5] from irb

I modified his answer seeing that 3 was the third element in his example array. This could lead to some confusion for those who don’t realize that 3 is in position 2 in the array.

Answer 3 (score 64)

Another option:

a = [2,4,6,3,8]

a -= [3]

which results in

=> [2, 4, 6, 8] 

Question

Is there a way to make this look a little better?

conn.exec 'select attr1, attr2, attr3, attr4, attr5, attr6, attr7 ' +
          'from table1, table2, table3, etc, etc, etc, etc, etc, ' +
          'where etc etc etc etc etc etc etc etc etc etc etc etc etc'

Like, is there a way to imply concatenation?

Answer accepted (score 552)

There are pieces to this answer that helped me get what I needed (easy multi-line concatenation WITHOUT extra whitespace), but since none of the actual answers had it, I’m compiling them here:

str = 'this is a multi-line string'\
  ' using implicit concatenation'\
  ' to prevent spare \n\'s'

=> "this is a multi-line string using implicit concatenation to eliminate spare
\\n's"

As a bonus, here’s a version using funny HEREDOC syntax (via this link):

p <<END_SQL.gsub(/\s+/, " ").strip
SELECT * FROM     users
         ORDER BY users.id DESC
END_SQL
# >> "SELECT * FROM users ORDER BY users.id DESC"

The latter would mostly be for situations that required more flexibility in the processing. I personally don’t like it, it puts the processing in a weird place w.r.t. the string (i.e., in front of it, but using instance methods that usually come afterward), but it’s there. Note that if you are indenting the last END_SQL identifier (which is common, since this is probably inside a function or module), you will need to use the hyphenated syntax (that is, p <<-END_SQL instead of p <<END_SQL). Otherwise, the indenting whitespace causes the identifier to be interpreted as a continuation of the string.

This doesn’t save much typing, but it looks nicer than using + signs, to me.

EDIT: Adding one more:

p %{
SELECT * FROM     users
         ORDER BY users.id DESC
}.gsub(/\s+/, " ").strip
# >> "SELECT * FROM users ORDER BY users.id DESC"

Answer 2 (score 165)

Yes, if you don’t mind the extra newlines being inserted:

 conn.exec 'select attr1, attr2, attr3, attr4, attr5, attr6, attr7
            from table1, table2, table3, etc, etc, etc, etc, etc,
            where etc etc etc etc etc etc etc etc etc etc etc etc etc'

Alternatively you can use a heredoc:

conn.exec <<-eos
   select attr1, attr2, attr3, attr4, attr5, attr6, attr7
   from table1, table2, table3, etc, etc, etc, etc, etc,
   where etc etc etc etc etc etc etc etc etc etc etc etc etc
eos

Answer 3 (score 163)

In ruby 2.0 you can now just use %

For example:

SQL = %{
SELECT user, name
FROM users
WHERE users.id = #{var}
LIMIT #{var2}
}

Question

I’ve set everything up that I need on my Mac (Ruby, Rails, Homebrew, Git, etc), and I’ve even written a small program. Now, how do I execute it in Terminal? I wrote the program in Redcar and saved it as a .rb, but I don’t know how to execute it through Terminal. I want to run the program and see if it actually works. How do I do this?

Answer 2 (score 229)

Just call: ruby your_program.rb

or

• start your program with #!/usr/bin/env ruby,
• make your file executable by running chmod +x your_program.rb
• and do ./your_program.rb some_param

Answer 3 (score 32)

Open your terminal and open folder where file is saved.
Ex /home/User1/program/test.rb

1. Open terminal
2. cd /home/User1/program
3. ruby test.rb

format or test.rb

class Test 
  def initialize
   puts "I love India"
  end
end

# initialize object
Test.new

output

I love India

Question

If I download a .gem file to a folder in my computer, can I install it later using gem install?

Answer accepted (score 281)

Yup, when you do gem install, it will search the current directory first, so if your .gem file is there, it will pick it up. I found it on the gem reference, which you may find handy as well:

gem install will install the named gem. It will attempt a local installation (i.e. a .gem file in the current directory), and if that fails, it will attempt to download and install the most recent version of the gem you want.

Answer 2 (score 323)

Also, you can use gem install --local path_to_gem/filename.gem

This will skip the usual gem repository scan that happens when you leave off --local.

You can find other magic with gem install --help.

Answer 3 (score 60)

you can also use the full filename to your gem file:

gem install /full/path/to/your.gem

this works as well – it’s probably the easiest way

Question

I’m new to programming. Can someone explain what .map would do in:

params = (0...param_count).map

Answer accepted (score 420)

The map method takes an enumerable object and a block, and runs the block for each element, outputting each returned value from the block (the original object is unchanged unless you use map!):

[1, 2, 3].map { |n| n * n } #=> [1, 4, 9]

Array and Range are enumerable types. map with a block returns an Array. map! mutates the original array.

Where is this helpful, and what is the difference between map! and each? Here is an example:

names = ['danil', 'edmund']

# here we map one array to another, convert each element by some rule
names.map! {|name| name.capitalize } # now names contains ['Danil', 'Edmund']

names.each { |name| puts name + ' is a programmer' } # here we just do something with each element

The output:

Danil is a programmer
Edmund is a programmer

Answer 2 (score 64)

map, along with select and each is one of Ruby’s workhorses in my code.

It allows you to run an operation on each of your array’s objects and return them all in the same place. An example would be to increment an array of numbers by one:

[1,2,3].map {|x| x + 1 }
#=> [2,3,4]

If you can run a single method on your array’s elements you can do it in a shorthand-style like so:

1. To do this with the above example you’d have to do something like this

   class Numeric
     def plusone
       self + 1
     end
   end
   [1,2,3].map(&:plusone)
   #=> [2,3,4]
   ```</li>
   <li><p>To more simply use the ampersand shortcut technique, let's use a different example:</p>
   
   ```ruby
   ["vanessa", "david", "thomas"].map(&:upcase)
   #=> ["VANESSA", "DAVID", "THOMAS"]
   ```</li>
   </ol>
   
   Transforming data in Ruby often involves a cascade of `map` operations. Study `map` &amp; `select`, they are some of the most useful Ruby methods in the primary library. They're just as important as `each`.  
   
   (`map` is also an alias for `collect`. Use whatever works best for you conceptually.)  
   
   <strong>More helpful information:</strong>  
   
   If the <a href="http://ruby-doc.org/core-2.2.3/Enumerable.html" rel="noreferrer">Enumerable</a> object you're running `each` or `map` on contains a set of Enumerable elements (hashes, arrays), you can declare each of those elements inside your block pipes like so:  
   
   ```ruby
   [["audi", "black", 2008], ["bmw", "red", 2014]].each do |make, color, year|
     puts "make: #{make}, color: #{color}, year: #{year}"
   end
   # Output:
   # make: audi, color: black, year: 2008
   # make: bmw, color: red, year: 2014

   In the case of a Hash (also an Enumerable object, a Hash is simply an array of tuples with special instructions for the interpreter). The first “pipe parameter” is the key, the second is the value.

   {:make => "audi", :color => "black", :year => 2008}.each do |k,v|
       puts "#{k} is #{v}"
   end
   #make is audi
   #color is black
   #year is 2008

   To answer the actual question:

   Assuming that params is a hash, this would be the best way to map through it: Use two block parameters instead of one to capture the key & value pair for each interpreted tuple in the hash.

   params = {"one" => 1, "two" => 2, "three" => 3}
   params.each do |k,v|
     puts "#{k}=#{v}"
   end
   # one=1
   # two=2
   # three=3

Answer 3 (score 6)

Using ruby 2.4 you can do the same thing using transform_values, this feature extracted from rails to ruby.

h = {a: 1, b: 2, c: 3}

h.transform_values { |v| v * 10 }
 #=> {a: 10, b: 20, c: 30}

Question

I have a string

"1,2,3,4"

and I’d like to convert it into an array:

[1,2,3,4]

How?

Answer accepted (score 401)

>> "1,2,3,4".split(",")
=> ["1", "2", "3", "4"]

Or for integers:

>> "1,2,3,4".split(",").map { |s| s.to_i }
=> [1, 2, 3, 4]

Or for later versions of ruby (>= 1.9 - as pointed out by Alex):

>> "1,2,3,4".split(",").map(&:to_i)
=> [1, 2, 3, 4]

Answer 2 (score 31)

"1,2,3,4".split(",") as strings

"1,2,3,4".split(",").map { |s| s.to_i } as integers

Answer 3 (score 17)

For String Integer without space as String

arr = "12345"

arr.split('')

output: ["1","2","3","4","5"]

For String Integer with space as String

arr = "1 2 3 4 5"

arr.split(' ')

output: ["1","2","3","4","5"]

For String Integer without space as Integer

arr = "12345"

arr.split('').map(&:to_i)

output: [1,2,3,4,5]

For String

arr = "abc"

arr.split('')

output: ["a","b","c"]

Explanation:

1. arr -> string which you’re going to perform any action.
2. split() -> is an method, which split the input and store it as array.
3. '' or ' ' or ',' -> is an value, which is needed to be removed from given string.

Question

Here is Bar#do_things:

class Bar   
  def do_things
    Foo.some_method(x) do |x|
      y = x.do_something
      return y_is_bad if y.bad? # how do i tell it to stop and return do_things? 
      y.do_something_else
    end
    keep_doing_more_things
  end
end

And here is Foo#some_method:

class Foo
  def self.some_method(targets, &block)
    targets.each do |target|
      begin
        r = yield(target)
      rescue 
        failed << target
      end
    end
  end
end

I thought about using raise, but I am trying to make it generic, so I don’t want to put anything any specific in Foo.

Answer accepted (score 723)

Use the keyword next. If you do not want to continue to the next item, use break.

When next is used within a block, it causes the block to exit immediately, returning control to the iterator method, which may then begin a new iteration by invoking the block again:

f.each do |line|              # Iterate over the lines in file f
  next if line[0,1] == "#"    # If this line is a comment, go to the next
  puts eval(line)
end

When used in a block, break transfers control out of the block, out of the iterator that invoked the block, and to the first expression following the invocation of the iterator:

f.each do |line|             # Iterate over the lines in file f
  break if line == "quit\n"  # If this break statement is executed...
  puts eval(line)
end
puts "Good bye"              # ...then control is transferred here

And finally, the usage of return in a block:

return always causes the enclosing method to return, regardless of how deeply nested within blocks it is (except in the case of lambdas):

def find(array, target)
  array.each_with_index do |element,index|
    return index if (element == target)  # return from find
  end
  nil  # If we didn't find the element, return nil
end

Answer 2 (score 55)

I wanted to just be able to break out of a block - sort of like a forward goto, not really related to a loop. In fact, I want to break of of a block that is in a loop without terminating the loop. To do that, I made the block a one-iteration loop:

for b in 1..2 do
    puts b
    begin
        puts 'want this to run'
        break
        puts 'but not this'
    end while false
    puts 'also want this to run'
end

Hope this helps the next googler that lands here based on the subject line.

Answer 3 (score 38)

If you want your block to return a useful value (e.g. when using #map, #inject, etc.), next and break also accept an argument.

Consider the following:

def contrived_example(numbers)
  numbers.inject(0) do |count, x|
    if x % 3 == 0
      count + 2
    elsif x.odd?
      count + 1
    else 
      count
    end
  end
end

The equivalent using next:

def contrived_example(numbers)
  numbers.inject(0) do |count, x|
    next count if x.even?
    next (count + 2) if x % 3 == 0
    count + 1
  end
end

Of course, you could always extract the logic needed into a method and call that from inside your block:

def contrived_example(numbers)
  numbers.inject(0) { |count, x| count + extracted_logic(x) }
end

def extracted_logic(x)
  return 0 if x.even?
  return 2 if x % 3 == 0
  1
end

Question

I find myself repeatedly looking for a clear definition of the differences of nil?, blank?, and empty? in Ruby on Rails. Here’s the closest I’ve come:

• blank? objects are false, empty, or a whitespace string. For example, "", " ", nil, [], and {} are blank.

• nil? objects are instances of NilClass.

• empty? objects are class-specific, and the definition varies from class to class. A string is empty if it has no characters, and an array is empty if it contains no items.

Is there anything missing, or a tighter comparison that can be made?

Answer accepted (score 1394)

.nil? can be used on any object and is true if the object is nil.

.empty? can be used on strings, arrays and hashes and returns true if:

• String length == 0
• Array length == 0
• Hash length == 0

Running .empty? on something that is nil will throw a NoMethodError.

That is where .blank? comes in. It is implemented by Rails and will operate on any object as well as work like .empty? on strings, arrays and hashes.

nil.blank? == true
false.blank? == true
[].blank? == true
{}.blank? == true
"".blank? == true
5.blank? == false
0.blank? == false

.blank? also evaluates true on strings which are non-empty but contain only whitespace:

"  ".blank? == true
"  ".empty? == false

Rails also provides .present?, which returns the negation of .blank?.

Array gotcha: blank? will return false even if all elements of an array are blank. To determine blankness in this case, use all? with blank?, for example:

[ nil, '' ].blank? == false
[ nil, '' ].all? &:blank? == true 

Answer 2 (score 657)

I made this useful table with all the cases:

blank?, present? are provided by Rails.

Answer 3 (score 209)

Just extend Julian’s table:

Ref: empty?blank?nil?傻傻分不清楚

Question

Ruby has this handy and convenient way to share instance variables by using keys like

attr_accessor :var
attr_reader :var
attr_writer :var

Why would I choose attr_reader or attr_writer if I could simply use attr_accessor? Is there something like performance (which I doubt)? I guess there is a reason, otherwise they wouldn’t have made such keys.

Answer accepted (score 734)

You may use the different accessors to communicate your intent to someone reading your code, and make it easier to write classes which will work correctly no matter how their public API is called.

class Person
  attr_accessor :age
  ...
end

Here, I can see that I may both read and write the age.

class Person
  attr_reader :age
  ...
end

Here, I can see that I may only read the age. Imagine that it is set by the constructor of this class and after that remains constant. If there were a mutator (writer) for age and the class were written assuming that age, once set, does not change, then a bug could result from code calling that mutator.

But what is happening behind the scenes?

If you write:

attr_writer :age

That gets translated into:

def age=(value)
  @age = value
end

If you write:

attr_reader :age

That gets translated into:

def age
  @age
end

If you write:

attr_accessor :age

That gets translated into:

def age=(value)
  @age = value
end

def age
  @age
end

Knowing that, here’s another way to think about it: If you did not have the attr_… helpers, and had to write the accessors yourself, would you write any more accessors than your class needed? For example, if age only needed to be read, would you also write a method allowing it to be written?

Answer 2 (score 23)

All of the answers above are correct; attr_reader and attr_writer are more convenient to write than manually typing the methods they are shorthands for. Apart from that they offer much better performance than writing the method definition yourself. For more info see slide 152 onwards from this talk (PDF) by Aaron Patterson.

Answer 3 (score 15)

Not all attributes of an object are meant to be directly set from outside the class. Having writers for all your instance variables is generally a sign of weak encapsulation and a warning that you’re introducing too much coupling between your classes.

As a practical example: I wrote a design program where you put items inside containers. The item had attr_reader :container, but it didn’t make sense to offer a writer, since the only time the item’s container should change is when it’s placed in a new one, which also requires positioning information.

Question

How do you tell a Ruby program to wait an arbitrary amount of time before moving on to the next line of code?

Answer accepted (score 627)

Like this:

sleep(num_secs)

The num_secs value can be an integer or float.

Also, if you’re writing this within a Rails app, or have included the ActiveSupport library in your project, you can construct longer intervals using the following convenience syntax:

sleep(4.minutes)
# or, even longer...
sleep(2.hours); sleep(3.days) # etc., etc.
# or shorter
sleep(0.5) # half a second

Answer 2 (score 112)

Use sleep like so:

sleep 2

That’ll sleep for 2 seconds.

Be careful to give an argument. If you just run sleep, the process will sleep forever. (This is useful when you want a thread to sleep until it’s woken.)

Answer 3 (score 2)

I find until very useful with sleep. example:

> time = Time.now
> sleep 2.seconds until Time.now > time + 10.seconds # breaks when true

Question

I was trying to use the following code to read lines from a file. But when reading a file, the contents are all in one line:

line_num=0
File.open('xxx.txt').each do |line|
  print "#{line_num += 1} #{line}"
end

But this file prints each line separately.

I have to use stdin, like ruby my_prog.rb < file.txt, where I can’t assume what the line-ending character is that the file uses. How can I handle it?

Answer accepted (score 144)

I believe my answer covers your new concerns about handling any type of line endings since both "\r\n" and "\r" are converted to Linux standard "\n" before parsing the lines.

To support the "\r" EOL character along with the regular "\n", and "\r\n" from Windows, here’s what I would do:

line_num=0
text=File.open('xxx.txt').read
text.gsub!(/\r\n?/, "\n")
text.each_line do |line|
  print "#{line_num += 1} #{line}"
end

Of course this could be a bad idea on very large files since it means loading the whole file into memory.

Answer 2 (score 503)

Ruby does have a method for this:

File.readlines('foo').each do |line|

http://ruby-doc.org/core-1.9.3/IO.html#method-c-readlines

Answer 3 (score 372)

File.foreach(filename).with_index do |line, line_num|
   puts "#{line_num}: #{line}"
end

This will execute the given block for each line in the file without slurping the entire file into memory. See: IO::foreach.

Question

Where can I find a list of data types that can be used in Ruby on Rails 4? Such as

• text
• string
• integer
• float
• date

I keep learning about new ones and I’d love to have a list I could easily refer to.

Answer accepted (score 641)

Here are all the Rails 4 (ActiveRecord migration) datatypes:

• :binary
• :boolean
• :date
• :datetime
• :decimal
• :float
• :integer
• :bigint
• :primary_key
• :references
• :string
• :text
• :time
• :timestamp

Source: http://api.rubyonrails.org/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_column
These are the same as with Rails 3.

If you use PostgreSQL, you can also take advantage of these:

• :hstore
• :json
• :jsonb
• :array
• :cidr_address
• :ip_address
• :mac_address

They are stored as strings if you run your app with a not-PostgreSQL database.

Edit, 2016-Sep-19:

There’s a lot more postgres specific datatypes in Rails 4 and even more in Rails 5.

Answer 2 (score 240)

You might also find it useful to know generally what these data types are used for:

• :string - is for small data types such as a title. (Should you choose string or text?)
• :text - is for longer pieces of textual data, such as a paragraph of information
• :binary - is for storing data such as images, audio, or movies.
• :boolean - is for storing true or false values.
• :date - store only the date
• :datetime - store the date and time into a column.
• :time - is for time only
• :timestamp - for storing date and time into a column.(What’s the difference between datetime and timestamp?)
• :decimal - is for decimals (example of how to use decimals).
• :float - is for decimals. (What’s the difference between decimal and float?)
• :integer - is for whole numbers.
• :primary_key - unique key that can uniquely identify each row in a table

There’s also references used to create associations. But, I’m not sure this is an actual data type.

New Rails 4 datatypes available in PostgreSQL:

• :hstore - storing key/value pairs within a single value (learn more about this new data type)
• :array - an arrangement of numbers or strings in a particular row (learn more about it and see examples)
• :cidr_address - used for IPv4 or IPv6 host addresses
• :inet_address - used for IPv4 or IPv6 host addresses, same as cidr_address but it also accepts values with nonzero bits to the right of the netmask
• :mac_address - used for MAC host addresses

Learn more about the address datatypes here and here.

Also, here’s the official guide on migrations: http://edgeguides.rubyonrails.org/migrations.html

Answer 3 (score 155)

It is important to know not only the types but the mapping of these types to the database types, too:

Source added - Agile Web Development with Rails 4

Question

Possible Duplicate:
How to remove rvm (ruby version manager) from my system?

How can I uninstall (or reinstall) RVM on Ubuntu 9.10? I messed up my current installation.

Answer accepted (score 835)

It’s easy; just do the following:

rvm implode

or

rm -rf ~/.rvm

And don’t forget to remove the script calls in the following files:

• ~/.bashrc
• ~/.bash_profile
• ~/.profile

And maybe others depending on whatever shell you’re using.

Question

Ryan Davis’s Ruby QuickRef says (without explanation):

Don’t rescue Exception. EVER. or I will stab you.

Why not? What’s the right thing to do?

Answer accepted (score 1342)

TL;DR: Use StandardError instead for general exception catching. When the original exception is re-raised (e.g. when rescuing to log the exception only), rescuing Exception is probably okay.

Exception is the root of Ruby’s exception hierarchy, so when you rescue Exception you rescue from everything, including subclasses such as SyntaxError, LoadError, and Interrupt.

Rescuing Interrupt prevents the user from using CTRLC to exit the program.

Rescuing SignalException prevents the program from responding correctly to signals. It will be unkillable except by kill -9.

Rescuing SyntaxError means that evals that fail will do so silently.

All of these can be shown by running this program, and trying to CTRLC or kill it:

loop do
  begin
    sleep 1
    eval "djsakru3924r9eiuorwju3498 += 5u84fior8u8t4ruyf8ihiure"
  rescue Exception
    puts "I refuse to fail or be stopped!"
  end
end

Rescuing from Exception isn’t even the default. Doing

begin
  # iceberg!
rescue
  # lifeboats
end

does not rescue from Exception, it rescues from StandardError. You should generally specify something more specific than the default StandardError, but rescuing from Exception broadens the scope rather than narrowing it, and can have catastrophic results and make bug-hunting extremely difficult.

If you have a situation where you do want to rescue from StandardError and you need a variable with the exception, you can use this form:

begin
  # iceberg!
rescue => e
  # lifeboats
end

which is equivalent to:

begin
  # iceberg!
rescue StandardError => e
  # lifeboats
end

One of the few common cases where it’s sane to rescue from Exception is for logging/reporting purposes, in which case you should immediately re-raise the exception:

begin
  # iceberg?
rescue Exception => e
  # do some logging
  raise e  # not enough lifeboats ;)
end

Answer 2 (score 81)

The real rule is: Don’t throw away exceptions. The objectivity of the author of your quote is questionable, as evidenced by the fact that it ends with

or I will stab you

Of course, be aware that signals (by default) throw exceptions, and normally long-running processes are terminated through a signal, so catching Exception and not terminating on signal exceptions will make your program very hard to stop. So don’t do this:

#! /usr/bin/ruby

while true do
  begin
    line = STDIN.gets
    # heavy processing
  rescue Exception => e
    puts "caught exception #{e}! ohnoes!"
  end
end

No, really, don’t do it. Don’t even run that to see if it works.

However, say you have a threaded server and you want all exceptions to not:

1. be ignored (the default)
2. stop the server (which happens if you say thread.abort_on_exception = true).

Then this is perfectly acceptable in your connection handling thread:

begin
  # do stuff
rescue Exception => e
  myLogger.error("uncaught #{e} exception while handling connection: #{e.message}")
    myLogger.error("Stack trace: #{backtrace.map {|l| "  #{l}\n"}.join}")
end

The above works out to a variation of Ruby’s default exception handler, with the advantage that it doesn’t also kill your program. Rails does this in its request handler.

Signal exceptions are raised in the main thread. Background threads won’t get them, so there is no point in trying to catch them there.

This is particularly useful in a production environment, where you do not want your program to simply stop whenever something goes wrong. Then you can take the stack dumps in your logs and add to your code to deal with specific exception further down the call chain and in a more graceful manner.

Note also that there is another Ruby idiom which has much the same effect:

a = do_something rescue "something else"

In this line, if do_something raises an exception, it is caught by Ruby, thrown away, and a is assigned "something else".

Generally, don’t do that, except in special cases where you know you don’t need to worry. One example:

debugger rescue nil

The debugger function is a rather nice way to set a breakpoint in your code, but if running outside a debugger, and Rails, it raises an exception. Now theoretically you shouldn’t be leaving debug code lying around in your program (pff! nobody does that!) but you might want to keep it there for a while for some reason, but not continually run your debugger.

Note:

1. If you’ve run someone else’s program that catches signal exceptions and ignores them, (say the code above) then:

   • in Linux, in a shell, type pgrep ruby, or ps | grep ruby, look for your offending program’s PID, and then run kill -9 <PID>.
   • in Windows, use the Task Manager (CTRL-SHIFT-ESC), go to the “processes” tab, find your process, right click it and select “End process”.

2. If you are working with someone else’s program which is, for whatever reason, peppered with these ignore-exception blocks, then putting this at the top of the mainline is one possible cop-out:

   %W/INT QUIT TERM/.each { |sig| trap sig,"SYSTEM_DEFAULT" }

   This causes the program to respond to the normal termination signals by immediately terminating, bypassing exception handlers, with no cleanup. So it could cause data loss or similar. Be careful!

3. If you need to do this:

   begin
     do_something
   rescue Exception => e
     critical_cleanup
     raise
   end

   you can actually do this:

   begin
     do_something
   ensure
     critical_cleanup
   end

   In the second case, critical cleanup will be called every time, whether or not an exception is thrown.

Answer 3 (score 63)

Let’s say you are in a car (running Ruby). You recently installed a new steering wheel with the over-the-air upgrade system (which uses eval), but you didn’t know one of the programmers messed up on syntax.

You are on a bridge, and realize you are going a bit towards the railing, so you turn left.

def turn_left
  self.turn left:
end

oops! That’s probably Not Good™, luckily, Ruby raises a SyntaxError.

The car should stop immediately - right?

Nope.

begin
  #...
  eval self.steering_wheel
  #...
rescue Exception => e
  self.beep
  self.log "Caught #{e}.", :warn
  self.log "Logged Error - Continuing Process.", :info
end

beep beep

Warning: Caught SyntaxError Exception.

Info: Logged Error - Continuing Process.

You notice something is wrong, and you slam on the emergency breaks (^C: Interrupt)

beep beep

Warning: Caught Interrupt Exception.

Info: Logged Error - Continuing Process.

Yeah - that didn’t help much. You’re pretty close to the rail, so you put the car in park (killing: SignalException).

beep beep

Warning: Caught SignalException Exception.

Info: Logged Error - Continuing Process.

At the last second, you pull out the keys (kill -9), and the car stops, you slam forward into the steering wheel (the airbag can’t inflate because you didn’t gracefully stop the program - you terminated it), and the computer in the back of your car slams into the seat in front of it. A half-full can of Coke spills over the papers. The groceries in the back are crushed, and most are covered in egg yolk and milk. The car needs serious repair and cleaning. (Data Loss)

Hopefully you have insurance (Backups). Oh yeah - because the airbag didn’t inflate, you’re probably hurt (getting fired, etc).

But wait! There’s more reasons why you might want to use rescue Exception => e!

Let’s say you’re that car, and you want to make sure the airbag inflates if the car is exceeding its safe stopping momentum.

 begin 
    # do driving stuff
 rescue Exception => e
    self.airbags.inflate if self.exceeding_safe_stopping_momentum?
    raise
 end

Here’s the exception to the rule: You can catch Exception only if you re-raise the exception. So, a better rule is to never swallow Exception, and always re-raise the error.

But adding rescue is both easy to forget in a language like Ruby, and putting a rescue statement right before re-raising an issue feels a little non-DRY. And you do not want to forget the raise statement. And if you do, good luck trying to find that error.

Thankfully, Ruby is awesome, you can just use the ensure keyword, which makes sure the code runs. The ensure keyword will run the code no matter what - if an exception is thrown, if one isn’t, the only exception being if the world ends (or other unlikely events).

 begin 
    # do driving stuff
 ensure
    self.airbags.inflate if self.exceeding_safe_stopping_momentum?
 end

Boom! And that code should run anyways. The only reason you should use rescue Exception => e is if you need access to the exception, or if you only want code to run on an exception. And remember to re-raise the error. Every time.

Note: As @Niall pointed out, ensure always runs. This is good because sometimes your program can lie to you and not throw exceptions, even when issues occur. With critical tasks, like inflating airbags, you need to make sure it happens no matter what. Because of this, checking every time the car stops, whether an exception is thrown or not, is a good idea. Even though inflating airbags is a bit of an uncommon task in most programming contexts, this is actually pretty common with most cleanup tasks.

TL;DR

Don’t rescue Exception => e (and not re-raise the exception) - or you might drive off a bridge.

Question

When I attempt to install the latest version of compass (https://rubygems.org/gems/compass/versions/1.0.0.alpha.17), I get the following error.

ERROR:  Error installing compass:
ERROR: Failed to build gem native extension.

 ERROR:  Error installing compass:
  ERROR: Failed to build gem native extension.

    /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby extconf.rb
checking for ffi.h... no
checking for ffi.h in /usr/local/include,/usr/include/ffi... yes
checking for ffi_call() in -lffi... yes
checking for ffi_prep_closure()... yes
checking for ffi_raw_call()... no
checking for rb_thread_blocking_region()... yes
checking for rb_thread_call_with_gvl()... yes
checking for rb_thread_call_without_gvl()... yes
checking for ffi_prep_cif_var()... no
creating extconf.h
creating Makefile

make "DESTDIR=" clean

make "DESTDIR="
compiling AbstractMemory.c
compiling ArrayType.c
compiling Buffer.c
compiling Call.c
Call.c:303:5: warning: implicit declaration of function 'rb_thread_call_without_gvl' is invalid in C99 [-Wimplicit-function-declaration]
    rbffi_thread_blocking_region(call_blocking_function, data, (void *) -1, NULL);
    ^
./Thread.h:78:39: note: expanded from macro 'rbffi_thread_blocking_region'
# define rbffi_thread_blocking_region rb_thread_call_without_gvl
                                      ^
1 warning generated.
compiling ClosurePool.c
compiling DataConverter.c
DataConverter.c:43:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
1 warning generated.
compiling DynamicLibrary.c
compiling ffi.c
compiling Function.c
Function.c:479:33: warning: incompatible pointer types passing 'VALUE (void *)' to parameter of type 'void *(*)(void *)' [-Wincompatible-pointer-types]
        rb_thread_call_with_gvl(callback_with_gvl, &cb);
                                ^~~~~~~~~~~~~~~~~
Function.c:102:46: note: passing argument to parameter 'func' here
extern void *rb_thread_call_with_gvl(void *(*func)(void *), void *data1);
                                             ^
Function.c:563:9: warning: implicit declaration of function 'rb_thread_call_without_gvl' is invalid in C99 [-Wimplicit-function-declaration]
        rb_thread_call_without_gvl(async_cb_wait, &w, async_cb_stop, &w);
        ^
Function.c:738:1: warning: control reaches end of non-void function [-Wreturn-type]
}
^
3 warnings generated.
compiling FunctionInfo.c
compiling LastError.c
compiling LongDouble.c
compiling MappedType.c
compiling MemoryPointer.c
compiling MethodHandle.c
compiling Platform.c
compiling Pointer.c
compiling Struct.c
compiling StructByReference.c
compiling StructByValue.c
compiling StructLayout.c
compiling Thread.c
compiling Type.c
compiling Types.c
compiling Variadic.c
linking shared-object ffi_c.bundle
clang: error: unknown argument: '-multiply_definedsuppress' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
make: *** [ffi_c.bundle] Error 1

make failed, exit code 2

Gem files will remain installed in /Library/Ruby/Gems/2.0.0/gems/ffi-1.9.3 for inspection.
Results logged to /Library/Ruby/Gems/2.0.0/extensions/universal-darwin-13/2.0.0/ffi-1.9.3/gem_make.out

What’s going on here? How do I install the latest compass without error?

Answer 2 (score 555)

Try this, then try to install compass again

apt-get install ruby-dev

Answer 3 (score 119)

In order to install compass On Mac OS X 10.10 (Yosemite)had to perform the following:

1. Set Up Ruby Environment

• Ensure ruby is installed and up to date: ruby -v
• Update gem’s sudo gem update --system

2. Set Up MAC Environment

Install the Xcode Command Line Tools this is the key to install Compass.

xcode-select --install

Installing the Xcode Command Line Tools are the key to getting Compass working on OS X

3. Install Compass

sudo gem install compass

Question

I want to get a specific output iterating a Ruby Hash.

This is the Hash I want to iterate over:

hash = {
  1 => ['a', 'b'], 
  2 => ['c'], 
  3 => ['d', 'e', 'f', 'g'], 
  4 => ['h']
}

This is the output I would like to get:

1-----

a

b

2-----

c

3-----

d 

e

f

g

4-----

h