Ruby Cheatsheet - Ruby Syntax & Methods Reference

This reference is for developers picking up Ruby (including Rails) or recalling its idioms. It starts with variables, symbols, and common built-in methods, then arrays/hashes, control flow (including postfix if and unless), methods and blocks/Proc/Lambda, classes & modules with attr_accessor, include/extend, and inheritance, and ends with begin-rescue-ensure exception handling. Unlike many languages, almost everything in Ruby is an object and methods return the value of their last expression, and blocks elegantly wrap iteration. After reading you should be able to write idiomatic, concise Ruby: iterate with blocks instead of hand loops, use symbols as hash keys, and organize reusable behavior into modules.

Languages·69 commands·Last updated 2026-07-21

Basics 7

name = "Ruby"

Variable assignment, dynamic typing needs no declaration

name = "Hello, #{name}"

String interpolation, works in double quotes

:symbol

Symbol, immutable and unique, good as hash key

# single-line comment

Single-line comment starts with #

=begin
comment block
=end

Multi-line comment block

nil

Nil value, nil is also an object

true / false

Boolean value, also an object

Array & Hash 9

arr = [1, 2, 3]

Create an array

arr << 4

Append an element

arr[0]

Access element at index 0

arr.first / arr.last

First / last element

hash = { a: 1, b: 2 }

Create a hash (symbol-key shorthand)

hash[:a]

Access a hash value by key

hash[:c] = 3

Set a hash value by key

hash.key?(:a)

Check if a key exists

hash.keys / hash.values

Get all keys / values

Control Flow 10

if x > 0\n puts "positive"\nend

if conditional

puts "positive" if x > 0

Trailing if, concise form

unless x > 0\n puts "not positive"\nend

unless conditional, equivalent to if !

case x\nwhen 1 then "one"\nelse "other"\nend

case multi-branch match

3.times { puts "hi" }

Repeat a fixed number of times

1.upto(5) { |n| puts n }

Increment from 1 to 5

5.downto(1) { |n| puts n }

Decrement from 5 to 1

loop { break if done }

Infinite loop, needs manual break

while x > 0\n x -= 1\nend

while loop

until x == 0\n x -= 1\nend

until loop, equivalent to while !

Methods 9

def greet(name)\n "Hello, #{name}"\nend

Define a method, last line auto-returns

def greet(name = "World")

Method parameter with default value

def sum(*args)

splat operator, accepts variable args

def method(a:, b:)

Keyword arguments, order-independent

return value

Explicit return value (optional)

def method\n yield if block_given?\nend

yield invokes the passed block

my_proc = Proc.new { |x| x*2 }

Create a Proc object

my_lambda = ->(x) { x*2 }

Create a Lambda (arrow syntax)

proc.call(5)

Call a Proc / Lambda

Class & Module 10

class MyClass\nend

Define a class

attr_accessor :name

Auto-generate getter and setter

attr_reader :name

Read-only getter

attr_writer :name

Write-only setter

class Child < Parent

Class inheritance

module MyModule\nend

Define a module

include MyModule

Mixin module methods (instance methods)

extend MyModule

Mixin module methods (class methods)

self.class_method

self refers to current object, define class method

class << self\nend

Singleton class, define class methods

Exception 7

begin\n risky_call\nrescue\n "error"\nend

begin-rescue catches exceptions

rescue SpecificError => e

Catch a specific exception type

ensure\n cleanup\nend

ensure guarantees cleanup runs

raise "error message"

Manually raise an exception

raise ArgumentError, "msg"

Raise a specific exception type

retry

retry inside rescue repeats the begin block

catch(:done) { throw(:done) }

catch/throw non-local jump

Built-in Methods 10

arr.map { |x| x*2 }

Array#map transforms an array

arr.select { |x| x.even? }

Array#select filters evens

arr.reject { |x| x.even? }

Array#reject excludes evens

arr.reduce(0) { |s, x| s + x }

Array#reduce sums

hash.merge({ c: 3 })

Hash#merge merges hashes

"hello".upcase

String#upcase uppercases

"hello".split("")

String#split splits a string

"hello".gsub(/l/, "L")

String#gsub global replace

arr.flatten

Array#flatten flattens nested arrays

arr.uniq

Array#uniq deduplicates

Blocks & Iterators 7

[1, 2].each { |n| puts n }

each iterates over elements

[1, 2].each do |n|\n puts n\nend

do...end block, equivalent to {}

(1..5).map { |n| n**2 }

map transforms (returns new array)

[1, 2, 3].select(&:even?)

select with &: shorthand

arr.partition { |x| x.even? }

partition into two groups [[evens], [odds]]

arr.chunk { |x| x > 0 }

chunk groups by condition

arr.group_by { |x| x % 3 }

group_by groups into a hash by condition

Tips

Everything in Ruby is an object, including nil and numbers.
each is the most common iterator; map transforms, select filters.
Symbol :symbol is immutable and unique, good as a hash key.
attr_accessor auto-generates getter and setter.
Blocks are Ruby's most powerful feature; do...end and {} are equivalent.

Official References

Each command links to its official documentation below, so you can verify the latest usage and read deeper.

Official Docs

Maintained by LaoHand

Publicly updated on Jul 21, 2026, continuously proofread against official docs.

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