Ruby Cheat Sheet

This cheat sheet describes Ruby features. It's not a reference to the language. You do have a reference to the language: the full text of Programming Ruby: The Pragmatic Programmer's Guide is installed with Ruby. Select start -> All Programs -> Ruby -> Ruby Documentation -> RubyBook Help.

Comments

In Ruby, any text on a single line that follows a # is a comment, and is ignored by the Ruby interpreter at run time.

# comment

Function Calls

Parentheses can sometimes be omitted. If you're not sure whether they're required, put them in. To be safe, put them in whenever the call is at all complicated. Even one as simple as this.

puts "hello"
puts("hello")
assert_equal(5, number)

Variables

Ordinary (local) variables are created through assignment:

number = 5

Now the variable number has the value 5. Ordinary variables begin with lowercase letters. After the first character, they can contain any alphabetical or numeric character. Underscores are helpful for making them readable:

this_is_my_variable = 5

A variable's value is gotten simply by using the name of the variable. The following has the value 10:

number + this_is_my_variable # returns 10

Strings, Objects and Methods

Strings are sequences of text in quotation marks. You can use single or double quotes.

name = 'simon'
name = "simon"

Strings are objects in Ruby. This means that they have methods. (In fact everything in Ruby is an object.)
A method call looks like this:

"simon".upcase # returns "SIMON"

A method is a function for a particular type of object. In this case, the thing before the period is the object, in this case a string simon. The method is upcase. It capitalizes its object.
Like functions, methods can have arguments.

"bookkeeper".include?('book') # returns true

This method is true if its argument book is a substring of bookkeeper.
You can also concatenate strings using the + operator.

"dog" + "house" # returns "doghouse"

Conditionals (if)

if number == 5
  puts "Success" # "Success" is a string. Strings can be surrounded with single or double quotes. 
else
  puts "FAILURE"
end

Put the if, else, and end on separate lines as shown. You don't have to indent, but you should.

Function Definitions

def assert_equal(expected, actual)
  if expected != actual
    puts "FAILURE!"
  end
end

Functions can return values, and those values can be assigned to variables. The return value is the last statement in the definition. Here's a simple example:

def five # note that no parentheses are required
  5
end
box = five # box's value is 5

Note that we didn't need to say five(), as is required in some languages. You can put in the parentheses if you prefer.
The value of the last statement is always the value returned by the function. Some people like to include a return statement to make this clear, but it doesn't change how the function works. This does the same thing:

def five
  return 5
end

Here's a little more complicated example:

def make_positive(number)
  if number < 0
    -number
  else
    number
  end
end
variable = make_positive(-5) # variable's value is 5
variable = make_positive(five) # variable's value is 5

Libraries

Libraries contain functions or methods that can be used in many Ruby programs. Suppose we store the make_positive function defined above in a file called mathplus.rb.
To use it in another script, we must require it:

require 'mathplus'

This will cause Ruby to search its loadpath for a file named mathplus.rb. (It will automatically add the .rb.) It will search the directories that normally contain Ruby libraries, as well as the current directory (typically the same directory as your script).
If your library is in a location that Ruby doesn't know about, you will need to change the loadpath:

$LOAD_PATH << 'c:/my_lib/'

Make sure you include this line before you require libraries in it.

Arrays

This is an array with nothing in it:

[]

This is an array with two numbers in it:

[1, 2]

This is an array with two numbers and a string in it. You can put anything into an array.

[1, 'hello!', 220]

Here's how you get something out of an array:

array = [1, 'hello', 220]
array[0] # value is 1

Here's how you get the last element out:

array[2] # value is 220

Here's another way to get the last element:

array.last # value is 220

Here's how you change an element:

array[0]= 'boo!' # value printed is 'boo!'
# array is now ['boo', 'hello', 220]

How long is an array?

array.length # value is 3

Here's how you tack something onto the end of an array:

array.push('fred') # array is now ['boo', 'hello', 220, 'fred']

Iteration

When you do something multiple times, it is called iteration. There are many ways to do this. The following will print hello five times:

5.times do
  puts 'hello'
end

Here's one way to print the numbers from one to 10:

for x in 1..10 do
  puts x
end

And here's another:

(1..10).each do |x|
  puts x
end

The part between the do and the end is called a block. You can replace the do and end with braces:

(1..10).each { |x| puts x }

The 1..10 is a range, which works like an array of the numbers from 1 to 10. The each is a method that iterates through each element of the range. It is called an iterator.
This prints each value of an array:

["a", "b", "c"].each { |x| puts x }

What if you want to transform each element of an array? The following capitalizes each element of an array.

["hi", "there"].collect { |word| word.capitalize } # The result is ["Hi", "There"].

Regular Expressions

Regular expressions are a useful feature common to many languages. They allow you to match patterns in strings.
Regular expressions are characters surrounded by // or %r{}. A regular expression is compared to a string like this:

regexp =~ string

Most characters in a regular expression match the same character in a string. So, these all match:

/a/ =~ 'a string'
/a/ =~ 'string me along'

This also matches:

/as/ =~ 'a string with astounding length'

Notice that the regular expression can match anywhere in the string. If you want it to match only the beginning of the string, start it with a caret:

/^as/ =~ 'alas, no match'

If you want it to match at the end, end with a dollar sign:

/no$/ =~ 'no match, alas'

If you want the regular expression to match any character in a string, use a period:

/^.s/ =~ "As if I didn't know better!"

There are a number of other special characters that let you amazing and wonderful things with strings. Ruby uses the standard syntax for regular expressions used in many scripting languages. See Programming Ruby for more information about regular expressions.

Truth and Falsehood

If you try the examples above, you'll see that the ones that match print a number. That's the position of the first character in the match. The first expression (/a/ =~ 'a string') returns 0. (Ruby, like most programming languages, starts counting with 0.) The second returns 10.
What happens if there's no match? Type this:

/^as/ =~ 'alas, no match'

and the result will be nil, signifying no match. You can use these results in an if, like this:

if /^as/ =~ some_string
  puts 'the string begins with "as".'
end

In Ruby, anything but the two special values false and nil are considered true for purposes of an if statement. So match results like 0 and 10 count as true.

Blocks

A block is like a function without a name. It contains a set of parameters and one or more lines of code. Blocks are used a lot in Ruby. Iterators like each use blocks.
Here's how to search an array for an element:

gems = ['emerald', 'pearl', 'ruby']
gems.detect { |gem| /^r/ =~ gem } # returns "ruby" 

The detect method takes a block as an argument. This block returns true if the first argument starts with an r. (Actually it returns 0, which counts as true.) The detect method itself returns the first element for which its block is true.
When blocks are longer than one line, they are usually written using do and end. This is another way of writing the same code:

gems.detect do |gem| 
  /^r/ =~ gem 
end 

Dictionaries

A dictionary lets you say "Give me the value corresponding to key." Dictionaries are also called hashes or associative arrays.
Here's how you create a dictionary:

dict = {}

Here's how you associate a value with a key:

dict['bret'] = 'texas' # looks a lot like an array, except that the key doesn't have to be a number.

Here's how you retrieve a value, given a key:

dict['bret'] # value is 'texas'.

Here's how you ask how many key/value pairs are in the dictionary:

dict.length # value is 1

What values does a dictionary have?

dict.values # value is the Array ['texas'].

What keys does it have?

dict.keys # value is the Array ['bret'].

Expression Substitution

Expression substitution evaluates an expression within a string:
You can do this

name = "Aidy"
puts "My name is: #{name}" 

=> My name is: Aidy

And this:

puts "The sum of four times four is: #{4*4}"

=> The sum of four times four is: 16