Methods

Default parameters

All arguments that don't have a default value are mandatory. In the following case dogs are mandatory. No arguments passed causes a compile time error.

def create_shelter(cats = 1, mice = 1, dogs)
    puts "#{cats} cats, #{mice} mice and #{dogs} dogs"
end

Give 4 as the argument and that sets the optional parameter dogs as 4

create_shelter(4) # => 1 cats, 1 mice and 4 dogs

Give 2, 5, 7 as the argument and that overrides the default parameters

create_shelter(2, 5, 7) # => 2 cats, 5 mice and 7 dogs

Mandatory parameters go at the start or the end of the argument list.

def create_big_shelter(lion, elephant = 3, tigers = 9, giraffe)
    ...
end

Once the mandatory arguments assign the rest of the arguments assign from left to write.

create_big_shelter(9, 4, 2) # => 9 lions, 4 elephants, 9 tigers and 4 giraffes

Default parameter example

$ ruby examples/default-parameters.rb
1 cats, 1 mice and 4 dogs # args 4
2 cats, 5 mice and 7 dogs # args 2, 5, 7
2 cats, 1 mice and 5 dogs # args 2, 5
9 lions, 3 elephants, 9 tigers and 4 giraffes # args 9, 4
9 lions, 4 elephants, 9 tigers and 2 giraffes # args 9, 4, 2

Variable Length Argument Parameter

A method can have 1 or more optional arguments using the splat operator *. This argument needs to go at the end of the parameter list.

def create_bookshop(*books)
    books.join(', ')
end

This will accept either an array preceded by the splat operator * or strings

create_bookshop(*['War of Worlds', 'The Hobbit'])
create_bookshop('War of Worlds', 'The Hobbit')

If one of the arguments is mandatory it has to be before the variable argument

def create_bookshop(quantity = 0, *books)
    puts "#{quantity} books: #{books.join(', ')}"
end

Variable arguments example

$ ruby examples/variable-arguments.rb
2 books: War of Worlds, The Hobbit
2 books: War of Worlds, The Hobbit

Keyword Arguments

An argument can have a keyword assigned to it, the order of the arguments don't matter. Any arguments without keywords should go at the start or the end of the argument list.

def add_book(genre, author: 'unknown', title: 'unknown')
    puts "#{genre.to_s.capitalize} book by #{author}: #{title}"
end

To make this also accept variable amount of arguments use the double splat operator **

def add_complex_book(genre, author: 'unknown', title: 'unknown', **extras)
    puts "#{genre.to_s.capitalize} book by #{author}: #{title} Extras: #{extras.map {|key, value| "#{key}: #{value}"}.join(' ')}"
end

Keyword arguments example

$ ruby examples/keyword-arguments.rb
Crime book by No Jësbo: The boy who played with a lighter
Crime book by No Jësbo: The boy who played with a lighter
Psychological book by Haula Pawkins: Man on a bus Extras: restricted: 18

Method aliasing

Create a reference to a method with a different name

class String
    alias_method "id", "object_id"
end

example = "some string"
puts example.object_id
puts example.id

Method alias example

$ ruby examples/keyword-arguments.rb
70309453787940
70309453787940

Challenge 🎠

Use Rspec to run the challenges

bin/rspec spec/challenges/methods_spec.rb --format doc

fix ../lib/challenges/methods.rb so the tests pass ✔

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Blocks