Ruby Cheatsheet

Below are notes I took while learning Ruby, written from the point of view of a Python programmer.

• To play with the interactive Ruby shell:

$ irb
irb(main):001:0> x = 10
irb(main):002:0> x
=> 10

• To run a Ruby file:

$ ruby foobar.rb

• To lookup language and API documentation:

$ sudo apt install ruby2.7-doc

$ ri <whatever you want>
$ ri print

• To print to standard output:

print("Hello!\n")
print("Value of x is: ", x)

Note that unlike Python, print does not add spaces or newlines automatically. Spaces and newlines have to be explicitly specified. WYSIWYG strictly.

• To import a module, provide its path, but drop the file extension:

require "foo/bar"
require_relative "where/is/it"

• A module is a .rb Ruby file with its own named module block inside it, inside whose namespace we can have variables, methods or classes.

# foobar.rb
module FooBar
   # Vars, methods, classes go here
   def foobar(x, y)
   end
end

# Accessed from another file as ...
require_relative "foobar"
FooBar.foobar(10, 20)

• Classes are defined similar to Python and initialize is the ctor. Instance variables have the @ prefix and cannot be accessed outside the class.

class Foobar
  def initialize(x, y)
    @x = x
    @y = y
  end

  def another_func()
  end
end

• Create getter for class instance variables using attr_reader definitions in the class:

class Foobar
  attr_reader :x
  attr_reader :y
  def initialize(x, y)
    @x = x
    @y = y
  end
end

f = Foobar(10, 20)
f.x  # Works!

• Use self to refer one’s own methods in the class:

class Foobar
  def func1()
  end
  def func2()
    self.func1()
  end
end

• List is called Array, but rest of usage is similar to Python:

x = []
y = ["foo", 1, nil]
x[0] = y[0]
x.append(y[2])

• To iterate items of list:

alist = [1, 2, 3]
for x in alist
  print(x, "\n")
end

• Dict is called hash in Ruby.

empty_hash = {}
ahash = {"x": 1, "y": 2}

• To iterate key-value pairs of hash:

ahash.each do |k, v|
  print(k, v)
end

• To check key in hash:

if ahash.key?("foobar")
  print("Foobar present in hash")
end

• To convert anything (string or int) to float:

"123".to_f
123.to_f

• To truncate float to int:

> x = 3.14
> x.to_i
=> 3

• To round float to nearest int:

> x = 3.99
> x.round
=> 4

• Difference between comparing by value and checking if they are the same object:

"x" == "x"       # Always true
"x".equal?("x")  # false

• Use the : colon operator to create a symbol, a trick to have a single copy of literal strings:

:foo.equal?(:foo)  # true

References

• Ruby API Documentation
  • Keywords
  • Kernel module: Its methods are included by Object, so available in every Ruby object.
• Ruby Core Reference