Ruby: Learn Ruby in 1 Hour

By Xah Lee. Date: . Last updated: .

This is a Ruby tutorial. I assume you already know a scripting language well. (e.g. Python, PHP, JavaScript, …) This intro is designed to quickly get you started. After reading, you should be able to use standard documentation.

Ruby has a interactive command line. In terminal, type irb.

To find your Ruby version, type ruby --version

This tutorial is based on Ruby 2. Version 2 is compatible with 1.9.x. But Ruby 1.9 is NOT compatible with Ruby 1.8.

[see Ruby Version Release Dates]

If you don't have Ruby installed, see:

Printing

Print Ruby Version

# -*- coding: utf-8 -*-
# Ruby

# print Ruby version from within Ruby script
p RUBY_VERSION # "1.9.73"

Save the above in a file test.rb. Run it in terminal like this: ruby test.rb.

For emacs users, install this function to run Ruby file in emacs: Emacs Lisp: Execute/Compile Current File.

To print, use p, puts, print.

# -*- coding: utf-8 -*-
# ruby
a = 5
b = "rabbit"
p a, b, 3 # prints each on a new line.

=begin
this is a multi-line comment

p a, b, 3

can be written as

p(a, b, 3)

parenthesis are often not necessary
=end

What is the difference between {puts, print, p}?

http://ruby-doc.org/core-1.9.3/Kernel.html#method-i-p

What does newline or indentation mean in Ruby?

Strings

Single Quote for Literal String

Use single quote to quote string exactly. (literal string). \n has no special meaning inside single quote.

# -*- coding: utf-8 -*-
# ruby

aa = 'cat' # single quote for literal string
p aa # prints 「"cat"」

bb = '4\n5'
p bb # prints 「"4\\n5"」 in one single line.

# single quoted string containing newline or tab will be printed as they are
cc = '1
2'
p cc # prints 「"1\n2"」

(Perl tip: Ruby's string quoting system is similar to Perl's. [see Perl: Quoting Strings])

Double Quote String to Evaluate Embedded Variable and Ruby Code

Use double quote for strings that contain newline escape \n, or include variable values or Ruby code.

# -*- coding: utf-8 -*-
# ruby
mm = "cat\nsnake"
puts mm # prints each word in separate line

You can use \n for newline, and \t for tab, etc.

To evaluate a variable or Ruby code within a string, use #{…}.

# -*- coding: utf-8 -*-
# ruby

# put variable value inside a string
aa = 4
bb = "there are #{aa} cats"
p bb # prints “there are 4 cats”

# eval Ruby code inside a string
p "there are #{1+2} cats" # prints “there are 3 cats”

Quoting Chunk of Text

See: Ruby: Quote Long String, Heredoc.

String Length

Use method length to get number of character in string.

# -*- coding: utf-8 -*-
# ruby

"2831".length # 4

Get Substring

Get substring.

# -*- coding: utf-8 -*-
# ruby

# get one character
p "abcdefg"[2] # c

# string can contain Unicode
p "α♥β"[1] # ♥

# get substring, start at index 2, get 3 chars
p "abcdefg"[2, 3] # cde

[see Ruby: Unicode Tutorial 💎]

String Manipulation

String join.

# -*- coding: utf-8 -*-
# ruby

# string join. Use +
p "aa" + "bb" # aabb

Replacing substring.

# -*- coding: utf-8 -*-
# ruby

# replacing string

xx = "0123456789"

xx[1]= "a"
p xx # "0a23456789"

# start at index 1, replace 8 chars
xx[1,8]= "what"
p xx # "0what9"

Simple string matching.

# -*- coding: utf-8 -*-
# ruby

# get the start index of a substring
p "in love?".index("love") # 3
p "in love?".index("456") # nil (not found)

Split a string into a array.

# -*- coding: utf-8 -*-
# ruby

p "once upon a time".split(" ") # ["once", "upon", "a", "time"]

Numbers and String Conversion

Ruby does not automatically convert between strings and integers.

Each string or number object has methods to convert to other types.

# -*- coding: utf-8 -*-
# ruby

# string to int
p "3".to_i # 3

# string to float
p "3".to_f # 3.0
# -*- coding: utf-8 -*-
# ruby

# int to string
p 3.to_s # "3"

# int to float
p 3.to_f # 3.0
# -*- coding: utf-8 -*-
# ruby

# float to string
p 3.0.to_s # "3.0"

# float to int
p 3.0.to_i # 3

WARNING: decimal number such as 3. must be written with 0 at end, like this 3.0, because dot is used for method call.

# -*- coding: utf-8 -*-
# ruby

# decimal number must end with dot and digit

p 5/2 # 2

p 5/2.0 # 2.5

p 5/2. # syntax error, unexpected end-of-input, expecting '('

Everything is a Object; Finding Object's Methods

In Ruby, practically everything is a “Object” in the sense of Object Oriented programing (OOP). Even numbers and strings are objects. What this effective means is that for any “thing” in Ruby, you can append it with .name where the name is a method (function).

For example, 3 is a thing in Ruby. So, it has many methods, for example: to_i (convert to integer), to_s (convert to string), and you can call it like this: 3.to_i.

To find the methods of a thing, use the method methods. Example:

# -*- coding: utf-8 -*-
# ruby

# methods for a integer object
p 5.methods # [:to_s, :-@, :+, :-, :*, :/, :div, :%, :modulo, :divmod, :fdiv, :**, :abs, :magnitude, :==, :===, :<=>, :>, :>=, :<, :<=, :~, :&, :|, :^, :[], :<<, :>>, :to_f, :size, :zero?, :odd?, :even?, :succ, :integer?, :upto, :downto, :times, :next, :pred, :chr, :ord, :to_i, :to_int, :floor, :ceil, :truncate, :round, :gcd, :lcm, :gcdlcm, :numerator, :denominator, :to_r, :rationalize, :singleton_method_added, :coerce, :i, :+@, :eql?, :quo, :remainder, :real?, :nonzero?, :step, :to_c, :real, :imaginary, :imag, :abs2, :arg, :angle, :phase, :rectangular, :rect, :polar, :conjugate, :conj, :between?, :nil?, :=~, :!~, :hash, :class, :singleton_class, :clone, :dup, :initialize_dup, :initialize_clone, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :frozen?, :inspect, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :respond_to_missing?, :extend, :display, :method, :public_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]
# -*- coding: utf-8 -*-
# ruby

# methods for a string object
p "rabbit".methods # [:<=>, :==, :===, :eql?, :hash, :casecmp, :+, :*, :%, :[], :[]=, :insert, :length, :size, :bytesize, :empty?, :=~, :match, :succ, :succ!, :next, :next!, :upto, :index, :rindex, :replace, :clear, :chr, :getbyte, :setbyte, :byteslice, :to_i, :to_f, :to_s, :to_str, :inspect, :dump, :upcase, :downcase, :capitalize, :swapcase, :upcase!, :downcase!, :capitalize!, :swapcase!, :hex, :oct, :split, :lines, :bytes, :chars, :codepoints, :reverse, :reverse!, :concat, :<<, :prepend, :crypt, :intern, :to_sym, :ord, :include?, :start_with?, :end_with?, :scan, :ljust, :rjust, :center, :sub, :gsub, :chop, :chomp, :strip, :lstrip, :rstrip, :sub!, :gsub!, :chop!, :chomp!, :strip!, :lstrip!, :rstrip!, :tr, :tr_s, :delete, :squeeze, :count, :tr!, :tr_s!, :delete!, :squeeze!, :each_line, :each_byte, :each_char, :each_codepoint, :sum, :slice, :slice!, :partition, :rpartition, :encoding, :force_encoding, :valid_encoding?, :ascii_only?, :unpack, :encode, :encode!, :to_r, :to_c, :>, :>=, :<, :<=, :between?, :nil?, :!~, :class, :singleton_class, :clone, :dup, :initialize_dup, :initialize_clone, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :frozen?, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :respond_to_missing?, :extend, :display, :method, :public_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]
# -*- coding: utf-8 -*-
# ruby

xx = [3,4,5]  #  array

# methods for a array object
p xx.methods # [:inspect, :to_s, :to_a, :to_ary, :frozen?, :==, :eql?, :hash, :[], :[]=, :at, :fetch, :first, :last, :concat, :<<, :push, :pop, :shift, :unshift, :insert, :each, :each_index, :reverse_each, :length, :size, :empty?, :find_index, :index, :rindex, :join, :reverse, :reverse!, :rotate, :rotate!, :sort, :sort!, :sort_by!, :collect, :collect!, :map, :map!, :select, :select!, :keep_if, :values_at, :delete, :delete_at, :delete_if, :reject, :reject!, :zip, :transpose, :replace, :clear, :fill, :include?, :<=>, :slice, :slice!, :assoc, :rassoc, :+, :*, :-, :&, :|, :uniq, :uniq!, :compact, :compact!, :flatten, :flatten!, :count, :shuffle!, :shuffle, :sample, :cycle, :permutation, :combination, :repeated_permutation, :repeated_combination, :product, :take, :take_while, :drop, :drop_while, :pack, :entries, :sort_by, :grep, :find, :detect, :find_all, :flat_map, :collect_concat, :inject, :reduce, :partition, :group_by, :all?, :any?, :one?, :none?, :min, :max, :minmax, :min_by, :max_by, :minmax_by, :member?, :each_with_index, :each_entry, :each_slice, :each_cons, :each_with_object, :chunk, :slice_before, :nil?, :===, :=~, :!~, :class, :singleton_class, :clone, :dup, :initialize_dup, :initialize_clone, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :respond_to_missing?, :extend, :display, :method, :public_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]

Method name ending in “!” means it changes the variable's value. For example, myArray.reverse returns a reversed array, myArray.reverse! also changes the variable myArray.

Methods ending in “?” means it returns true or false. (aka “predicate function”.)

Variables

In Ruby, variables do not need declaration. Variable does not have a type. Values do have type. The type is the class they belong to.

Finding the type of a value.

# -*- coding: utf-8 -*-
# ruby

p 5.kind_of?(Integer) # true. The “kind_of?” is a method.

p 5.class # Fixnum
p "5".class # String
p [3,4,5].class # Array
p true.class # TrueClass
p false.class # FalseClass
p nil.class # NilClass

Variable Name Prefix and Variable Scope

Starting SymbolExplanation
a…z or _lower case = local variable
A…Zupper case = Constant
$global variable
@instance variable (of a class in OOP)
@@class variable (of a class in OOP)
# -*- coding: utf-8 -*-
# ruby

a = 3
$a = 4
@a = 5
@@a = 6

p defined?(a) # "local-variable"
p defined?($a) # "global-variable"
p defined?(@a) # "instance-variable"
p defined?(@@a) # "class variable"

Constants starts with a capital letter. This is enforced by Ruby interpreter. However, Ruby actually allows you to redefine a constant. When you do so, it gives a warning.

# -*- coding: utf-8 -*-
# ruby

C2 = 5 # constant
C2 = 6 # warning: already initialized constant C2
p C2 # prints 6

Predefined Global Variables

Variable NameVariable Value
$0name of the Ruby script file currently executing
$*command line arguments used to invoke the script. (a array)
$$Ruby process ID
$?exit status of last executed child process

There are many more. For complete list, see: Ruby: List of Predefined Global Variables

True, False

true and false are builtin objects.

The following evaluates to false:

Everything else evaluates to true (including zero, empty string, empty array.).

Conditional: if then else

Simple “if then”.

# -*- coding: utf-8 -*-
# ruby

xx = 4

if xx > 0 then p 1 end # prints 1

# formatted in multiple lines
if xx > 0 then
  p 1
end # prints 1

“if then else”.

# -*- coding: utf-8 -*-
# ruby

xx = 4

if xx > 0 then p 1 else p 0 end # prints 1

Multilpe “else”.

# -*- coding: utf-8 -*-
# ruby

xx = 4

# multiple “else if”
if xx > 4 then
  p 1
elsif xx < 4 then
  p 0
elsif xx == 4 then
  p "yy"
end # prints "yy"

# formatted in another way
if xx > 4 then p 1
elsif xx < 4 then p 0
elsif xx == 4 then p "yy"
end # prints "yy"

# formatted in one line
if xx > 4 then p 1 elsif xx < 4 then p 0 elsif xx == 4 then p "yy" end # prints "yy"

“if” construct is also a expression. It returns the value of the last line of the block executed.

# -*- coding: utf-8 -*-
# ruby

xx = 4

p(if xx > 0 then 1 end) # prints 1

p(if xx > 0 then 1 else 0 end) # prints 1

p(
if xx > 4 then 1
elsif xx < 4 then 0
elsif xx == 4 then "yes"
end
) # prints "yes"

# short form
p( xx > 2 ? 1 : 0) # prints 1

“everything” in Ruby is a expression. They return a value.

Using “case”. (in other languages, it's also known as {which, switch, cond})

# -*- coding: utf-8 -*-
# ruby

xx = 3

myResult =
case xx
  when 1 then "one"
  when 2 then "two"
  when 3 then "three"
  when 3 then "four"
  else "cat"
end

p myResult # “three”

“case” construct is also a expression. It returns a value.

Loop, Iteration

“for” loop.

# -*- coding: utf-8 -*-
# ruby

for ii in 0..4 do p ii end # prints 0 to 4

Nested “for” loop.

# -*- coding: utf-8 -*-
# ruby

for ii in 1..2 do
  for jj in 1..3 do
    puts "#{ii}, #{jj}"
  end
end

# prints
# 1, 1
# 1, 2
# 1, 3
# 2, 1
# 2, 2
# 2, 3

# can also be written in one line.
for ii in 1..2 do for jj in 1..3 do puts "#{ii}, #{jj}" end end

“while” loop.

# -*- coding: utf-8 -*-
# ruby

ii = 1

while ii < 9 do
puts ii;
if ii == 5 then break end
ii += 1
end

# prints 1 to 5

Semicolon can be replaced by newline.

Creating a range. Use (1..5).to_a. This creates a array with elements 1 to 5.

# -*- coding: utf-8 -*-
# ruby
a = (1..5).to_a # “to_a” converts to array
p a # [1, 2, 3, 4, 5]
# -*- coding: utf-8 -*-
# ruby

p (1..5).class # Range

# methods for the range
p (1..5).methods # [:==, :===, :eql?, :hash, :each, :step, :begin, :end, :first, :last, :min, :max, :to_s, :inspect, :exclude_end?, :member?, :include?, :cover?, :to_a, :entries, :sort, :sort_by, :grep, :count, :find, :detect, :find_index, :find_all, :select, :reject, :collect, :map, :flat_map, :collect_concat, :inject, :reduce, :partition, :group_by, :all?, :any?, :one?, :none?, :minmax, :min_by, :max_by, :minmax_by, :each_with_index, :reverse_each, :each_entry, :each_slice, :each_cons, :each_with_object, :zip, :take, :take_while, :drop, :drop_while, :cycle, :chunk, :slice_before, :nil?, :=~, :!~, :<=>, :class, :singleton_class, :clone, :dup, :initialize_dup, :initialize_clone, :taint, :tainted?, :untaint, :untrust, :untrusted?, :trust, :freeze, :frozen?, :methods, :singleton_methods, :protected_methods, :private_methods, :public_methods, :instance_variables, :instance_variable_get, :instance_variable_set, :instance_variable_defined?, :instance_of?, :kind_of?, :is_a?, :tap, :send, :public_send, :respond_to?, :respond_to_missing?, :extend, :display, :method, :public_method, :define_singleton_method, :object_id, :to_enum, :enum_for, :equal?, :!, :!=, :instance_eval, :instance_exec, :__send__, :__id__]

List/Array

Creating a array.

# -*- coding: utf-8 -*-
# ruby

# creating a array
aa = [3, "four",5]
p aa # prints [3, "four", 5]

# another way
bb = Array.new(3)
p bb # prints [nil, nil, nil]

# create array of 3 elements, with value of 1
cc = Array.new(3,1)
p cc # prints [1, 1, 1]

# array can be nested
aa = [0, 1, 2, ["8", 9], 3]

Counting elements:

# -*- coding: utf-8 -*-
# ruby

aa = [3, "four",5]
p aa.length # 3

Getting a element.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2, 3, "four", 5, [6, 8], 7]
p aa[0] # 0
p aa[-1] # 7

p aa[6] # [6, 8]
p aa[6][0] # 6

Finding the index of a element.

# -*- coding: utf-8 -*-
# ruby

aa = [3, "four",5]
p aa.index("four") # 1
p aa.index("7") # nil

Extracting a sequences of elements.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2, 3, "four", 5, 6, 7]

# starting at index 3, get 2 elements
p aa[3, 2] # [3, "four"]

Modify a element.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2, 3, "four"]
aa[1] = "one"
p aa # [0, "one", 2, 3, "four"]

Adding element into array.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2]

p aa.insert(1,"b") # [0, "b", 1, 2]
p aa # [0, "b", 1, 2]

Appending a element.

# -*- coding: utf-8 -*-
# ruby

# appending to array

aa = [0, 1]
aa << 2
p aa # [0, 1, 2]

aa << [5, 6]
p aa # [0, 1, 2, [5, 6]]

Removing Element.

# -*- coding: utf-8 -*-
# ruby

aa = [0, "b", 2]

aa.delete_at(1) # "b"
p aa # [0, 2]

push and pop. (append or remove a element from the end.)

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2]

# add a element to the end
p aa.push("a") # [0, 1, 2, "a"]

# original array also changed
p aa # [0, 1, 2, "a"]

# pop removes the last element, and returns that
p aa.pop # "a"

p aa # [0, 1, 2]

Join 2 arrays.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2, 3]
bb = ["four", 5]

p aa + bb # [0, 1, 2, 3, "four", 5]

Array Union, Intersection, Difference

# -*- coding: utf-8 -*-
# ruby

aa = [0, 8, 7, 3, 1]
bb = [3, 4]

# difference
p aa - bb # [0, 8, 7, 1]

# intersection
p aa & bb # [3]

# union
p aa | bb # [0, 8, 7, 3, 1, 4]

Get uniq elements.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 7, 3, 7]

# uniq
p aa.uniq # [0, 7, 3]

# original array not modified
p aa # [0, 7, 3, 7]

# uniq, modify original array
aa.uniq!
p aa # [0, 7, 3]

Sorting array.

# -*- coding: utf-8 -*-
# ruby

aa = [0, 4, 2]

aa.sort # [0, 2, 4]

# original not changed
p aa # [0, 4, 2]

# change original
aa.sort! # [0, 2, 4]
p aa # [0, 2, 4]

Array Comparison

# -*- coding: utf-8 -*-
# ruby

aa = [0, 1, 2]
bb = [0, 1, 2.0]
cc = [0, 1, "2"]

# check if 2 arrays are the same (ignores type.)
p aa == bb # true
p aa == cc # false

# check if 2 arrays are the same, and each element has the same type
p aa.eql?(bb) # false

Hash Table

(aka key/value list, Dictionary)

#-*- coding: utf-8 -*-
# ruby

# define a keyed list
hh = {:john => 3, :mary => 4, :jane => 5, :vicky => 7}
p hh # {:john=>3, :mary=>4, :jane=>5, :vicky=>7}

# getting value from a key
p hh[:mary] # 4

# add a entry
hh[:pretty] = 99
p hh # {:john=>3, :mary=>4, :jane=>5, :vicky=>7, :pretty=>99}

# delete a entry
hh.delete :vicky
p hh # {:john=>3, :mary=>4, :jane=>5, :pretty=>99}

# get all keys
p hh.keys # [:john, :mary, :jane, :pretty]

# get all values
p hh.values # [3, 4, 5, 99]

# check if a key exists
p hh.has_key? :mary # true

p hh.has_value? :jenny # false

In Ruby, :something is a “symbol”. It's similar to lisp's symbol. For practical purposes, you can think of it as a static string. Whenever you need to use a string as a label that doesn't change, you should use symbol instead, because it's more efficient.

In the above example, you could replace all symbols by string. example

#-*- coding: utf-8 -*-
# ruby

# hash, with keys as string
aa = {'john' => 3, 'mary' => 4, 'jane' => 5}

# hash, with keys as symbol.
bb = {:john => 3, :mary => 4, :jane => 5}

# using symbol is more efficient, because each string is a full object

In Ruby, “everything” is a object. So, creating a string creates a object, which often means it's slow. So, the “symbol” datatype is a solution for that.

Looping thru Array/Hash

…

Calling Unix Shell Command

Use backticks `…` to call shell commands.

# -*- coding: utf-8 -*-
# ruby

# call unix commands
puts `ls -Al ~`
# list home dir

=begin
prints the following

total 4786
-rw-------+ 1 h3     None     27264 Nov 30 15:48 .bash_history
-rwx------+ 1 h3     None       403 Nov 30 15:48 .bashrc
-rwx------+ 1 h3     None       285 Nov 18 22:57 .emacs
…
=end

Using Library

…

Defining a Function

The following is a example of defining a function.

# -*- coding: utf-8 -*-
# ruby

def f
3
end

puts f

The keyword “return” can be used to return value and exit code. Else, the last expression of a function definition is its returned value.

# -*- coding: utf-8 -*-
# ruby

def f(x) x+1 end

puts f(4) # prints 5
# -*- coding: utf-8 -*-
# ruby

# function with default value
def f(x = 3) x+1 end

puts f # prints 4
# -*- coding: utf-8 -*-
# ruby

# example of defining a function with unspecified number of parameters
def ff(*xx)
 xx.each {|string| puts string}
end

ff(3,4,5)

Classes and Objects

Defining a class. Class name should start with a capital letter.

# -*- coding: utf-8 -*-
# ruby

# Object example

class Xyz

 # initializer
  def initialize(ii)
    @xx = ii # @xx is a instance variable
  end

 # a method. Return the instance variable @xx
  def mm
    @xx
  end

 # a another method.
  def nn(aa)
    @xx + aa
  end

end

# create a object.
myobj = Xyz.new(3)

# call a method
p myobj.mm # 3

# call another method with argument
p myobj.nn(2) # 5

Instance variable must be accessed by methods only. You must define methods to get instance variable's value, or change it. (this is different from Java or Python.)

Writing a Module

# -*- coding: utf-8 -*-
# ruby

# Module name must start with Capital letter
module Xyz

 # …

end

If you have a question, put $5 at patreon and message me.

  1. Ruby Basics
  2. Doc Lookup
  3. Print Version String
  4. What's RVM, Gem, Rake, Bundler, RDoc, ri, irb?
  1. Unicode 💎
  2. Quote String
  3. Quote Long String
  4. Format String
  5. String Operations
  6. True, False
  7. “if then else”
  8. for, while, Loop
  9. List
  10. Loop Thru List
  11. Hash Table
  12. Function Optional Parameter
  13. Map f to List
  14. Complex Numbers
  1. Intro to Reading Ruby Doc: What's M, C, ::, # ?
  2. Predefined Global Variables