Illustrated Ruby: Methods

Methods are central in Ruby. We’ll learn about methods with a few different examples.

Here’s a method that takes two Integers and returns their sum as an Integer. In Ruby the return value of a method is the value of its last expression.

def plus(a, b)
  a + b
end

A method can be redefined. The previous method is replaced. Here we make arguments optional by providing default values.

def plus(a = 0, b = 0)
  a + b
end

Call a method as you’d expect, with name(args).

res = plus(1, 2)
p res
res = plus()
p res

In Ruby, parentheses can be omitted.

res = plus 1, 2
p res
res = plus
p res

Thanks to the arguments, Ruby can disambiguate between a variable and a method call.

plus = '+'
res = plus 4, 5
p res

When there’s no argument to a method, Ruby resolves to the local variable.

res = plus
p res

Disambiguation can be achieved by using parentheses.

res = plus()
p res

There is no type attached to variables: as long as the arguments can be added with + the method works.

res = plus "2", "3"
p res

Methods don’t close over outside references.

def print_res
  puts res
end


begin
  print_res
rescue NameError => e
  puts e
end

Method names can end with a question mark. By convention these return booleans, are called “predicates”, and a is_ or has_ prefix is omitted.

def even?(i)
  i % 2 == 0
end
p even?(4)

Method names can end with an exclamation mark. By convention this is done to to draw attention, evoking “danger”, especially in contrast with a non-suffixed method of the same name.

This can mean the method performs a mutation.

def plus!(str, tail)
  str << tail
end

The “danger” is to unsuspectedly mutate when calling the method.

s = "hello"
plus!(s, " world")
p s

This can mean the method raises an exception.

def plus!(a, b)
  fail "no odds!" if odd?(a) || odd?(b)
  a + b
end

The “danger” is to have the uncaught exception crash the runtime.

begin
  plus!(1, 2)
rescue => e
  puts e
end


$ ruby examples/methods/methods.rb
3
0
3
0
9
"+"
0
"23"
undefined local variable or method `res' for main
true
"hello world"
undefined method `odd?' for main

Next example: Constants

by Loic Nageleisen | source | license