Ruby Cheat Sheet

Learn Ruby from the basics →

Variables & Types

name = "Ada": Assign a variable (no keyword, no type)
MAX_USERS = 100: Constant (starts with a capital letter)
:active: Symbol — a lightweight, immutable name
nil: The absence of a value
42.class # => Integer: Every value is an object with a class
"42".to_i / Integer("42"): Lenient vs strict string-to-integer conversion

Strings

"Hello, #{name}!": Interpolation (double quotes only)
s.upcase / s.downcase / s.capitalize: Change case
s.strip / s.chomp: Trim whitespace / remove trailing newline
s.split(","): Split into an array
s.include?("Ru"): Substring check
s.gsub(/\d/, "#"): Replace every regex match

Control Flow

if x > 0 ... elsif x < 0 ... else ... end: Conditional
puts "hi" if ready: Modifier if
unless done ... end: Inverse of if
case grade when (90..) then "A" else "B" end: case/when with ranges
3.times { |i| puts i }: Repeat a fixed number of times
while n > 0 ... end: While loop

Arrays & Hashes

nums = [1, 2, 3]: Array literal
nums << 4: Append to an array
nums.map { |n| n * 2 }: Transform every element
nums.select(&:even?): Keep matching elements
nums.sum / nums.reduce(:+): Combine into one value
user = { name: "Ada", age: 36 }: Hash with symbol keys
user.fetch(:email, "none"): Look up with a default
user.each { |key, value| ... }: Iterate a hash

Methods & Blocks

def greet(name, greeting: "Hi") = "#{greeting}, #{name}": Endless method with a keyword argument
def total(*nums) nums.sum end: Any number of positional arguments
yield value if block_given?: Call the block passed to a method
square = ->(x) { x * x }; square.(4): Lambda
empty? / sort!: Naming: ? returns a boolean, ! is the "dangerous" version

Classes & Modules

class User; attr_reader :name; def initialize(name); @name = name; end; end: Class with a constructor and reader
class Admin < User: Inheritance
module Greetable; def greet = "Hi, #{name}"; end: Module of reusable methods
include Comparable: Mix a module into a class
Point = Data.define(:x, :y): Immutable value object (Ruby 3.2+)

Errors

begin ... rescue ZeroDivisionError => e ... ensure ... end: Handle an exception
raise ArgumentError, "age must be positive": Raise an exception
class InsufficientFunds < StandardError; end: Custom exception class
user&.name: Safe navigation — nil instead of NoMethodError

Tooling

ruby app.rb: Run a script
irb: Interactive Ruby console
bundle init / bundle add rspec: Create a Gemfile / add a gem
bundle exec rspec: Run a command with the project's gem versions
bundle exec rubocop -a: Lint and auto-correct style