Ruby Functions

This section covers defining functions, passing arguments to them, and the different between parameters and arguments.

Ref: WGR Chapter 2. Objects, methods, and local variables

Ref: WGR Chapter 2, Section 2.4, "A close look at method arguments"

Expression values

In Ruby, every expression evaluates to some value

@@@ ruby
>> 2 + 2
=> 4
>> (2+2).zero?
=> false
>> "zero" if (2+2).zero?
=> nil

Parameters and return values

@@@ ruby
def to_fahrenheit(celcius)
  celcius * 9.0 / 5 + 32
end

• celcius is a parameter
• the value of a function is the value of the final statement
  • in this case, the only statement
• the keyword return is available, but usually unnecessary

Arguments vs. Parameters

@@@ ruby
def thing.to_fahrenheit(celcius)
  celcius * 9.0 / 5 + 32
end
boiling = 100
thing.to_fahrenheit(boiling)

• Technically speaking, arguments are passed and parameters are declared
• Note that the variable names don't have to match!
• In this code, boiling is an argument and celcius is a parameter
  • In practice, the two terms are interchangeable

Splat arguments

@@@ ruby
def thing.greet(greeting, *names)
  names.each do |name|
    puts "#{greeting}, #{name}!"
  end
end

>> thing.greet("Hello", "Alice", "Bob", "Charlie")
Hello, Alice!
Hello, Bob!
Hello, Charlie!

Default values

@@@ ruby
def thing.eat(food = "chicken")
  puts "Yum, #{food}!"
end

>> thing.eat
Yum, chicken!

>> thing.eat "arugula"
Yum, arugula!

The default hash parameter

When calling a method, if the final argument is a hash, you can leave off the curly braces

@@@ ruby
def print_value_plus(amount, hash)
  hash.each_pair {|k,v| puts v + amount }
end

print_value_plus 2, :x => 1, :y => 2
# same as...
print_value_plus(2, {:x => 1, :y => 2})