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Ruby

Ruby is an interpreted language with dynamic typing used for scripting mostly.

puts "hello there"

is the hello world

puts "hello, #{1 + 1}"
# 2

You can put expressions inside the `#{}` and they will be evaluated.

days = "Mon Tue Wed Thu Fri Sat Sun"
puts "Here are the days: #{days}"

Use triple quotes to write multiline strings.

fat_cat = """
I'll do a list:
\t* Cat food
\t* Fishies
\t* Catnip\n\t* Grass
"""
# this will print
# I'll do a list:
#        * Cat food
#        * Fishies
#        * Catnip
#        * Grass

puts is simply print with a newline

Get user input:

print "how old are you"
age = gets.chomp
puts "okay so you are #{age} years old"

You can convert the string to integer using age = gets.chomp.to_i, similarly, to_f makes it a float

You can accept the arguments from the user

first, second, third = ARGV

puts "Your first variable is: #{first}" # here, first has the 1st argument for eg

To accept input, one should use $stdin.gets.chomp

We can read a file using:

filename = ARGV.first # or file_again = $stdin.gets.chomp
txt = open(filename)
puts txt.read

Methods

def foo(x)
  x*2
end

The last statement is the return type. Calling is foo 2, or foo(2)

Recall what blocks are, they are a block of code, that can be executed. All methods accept an implicit block. You can insert it whereever needed with the yield keyword

def surround
  puts '{'
  yield
  puts '}'
end

puts surround { puts 'hello world' }
#=> {
#=> hello world
#=> }

The block can also be converted to a proc object, which can be called with .call invocation.

def guests(&block)
  block.class #=> Proc
  block.call(4)
end

The arguments passed to call are given to the block

def guests(&block)
  block.class #=> Proc
  block.call(4)
end

guests { |n| "You have #{n} guests." }
# => "You have 4 guests."

By convention, all methods that return booleans end with a question mark.

By convention, if a method name ends with an exclamation mark, it does something destructive like mutate the receiver. Many methods have a ! version to make a change, and a non-! version to just return a new changed version. Like we saw in reverse!

Classes

Classes can be defined with a class keyword. There are:

• class variables @@foo, which are shared by all instances of this class.
• instance variables @foo, which can be initialized anywhere, belong to the particular instance of the class

Basic setter:

def name=(name)
    @name = name
  end

Basic initializer:

def initialize(name, age = 0)
   # Assign the argument to the 'name' instance variable for the instance.
   @name = name
   # If no age given, we will fall back to the default in the arguments list.
   @age = age
 end

Basic getter method

def name
  @name
end

Example:

class Foo
  def initialize(name, age=0)
    @name = name
  end

  def getName
    @name
  end
end

f = Foo.new "Foobar"
puts f.getName
# Foobar

The getter/setter is such a pattern that there are shortcuts to auto create them:

attr_reader :name

So, this works now:

class Foo
  attr_reader :name
  def initialize(name, age=0)
    @name = name
  end

  def getName
    @name
  end
end

f = Foo.new "Foobar"
puts f.getName
puts f.name
# Foobar
# Foobar

Note, when I call f.name = "bar", I am actually doing f.name=("bar") aka f.name= "bar" So I better have a method called name=

Another example:

class Person
  attr_reader :name

  def initialize(name)
    @name = name
  end

  def name=(name)
    @name = name
  end
end

john = Person.new("John")
john.name = "Jim"
puts john.name # => Jim

It can also be a single line attr_accessor :name

A class method uses self to distinguish from instance methods. It can only be called on the class, not an instance.

class Human
 def self.say(msg)
   puts msg
 end
end

puts Human.say "hello"
# hello

Variable’s scopes are defined by the way we name them. Variables that start with $ have global scope.

$var = "I'm a global var"
defined? $var #=> "global-variable"

Variables that start with @ have instance scope.

@var = "I'm an instance var"
defined? @var #=> "instance-variable"

Variables that start with @@ have class scope.

@@var = "I'm a class var"
defined? @@var #=> "class variable"

Variables that start with a capital letter are constants.

Var = "I'm a constant"
defined? Var #=> "constant"

Modules

Including modules binds their methods to the class instances.

class Person
  include ModuleExample
end

Extending modules binds their methods to the class itself.

class Book
  extend ModuleExample
end

Ruby misc

2+2 The + symbol is just syntactic sugar for the calling the + function on the number So, these are equivalent

puts 2+2
puts 2.+ 2

Note, in ruby, the syntax to call functions is: fn_name [<arg1> <arg2> ... ]

For loop are like this elsewhere,

for counter in 1..5
  puts "iteration #{counter}"
end

In ruby, they look like this:

(1..5).each do |counter|
  puts "iteration #{counter}"
end

This is a block. The ‘each’ method of a range runs the block once for each element of the range. The block is passed a counter as a parameter.

The lambda nature of blocks is more easily visible by alternative equivalent form:

(1..5).each { |counter| puts "iteration #{counter}" }

But the general syntax to follow is:

array.each do |foo|
  puts foo
end