Method Arguments

Ruby methods can take arguments in several ways. You'll see all of these in real code.

Default values

def greet(name, greeting = "Hello")
  "#{greeting}, #{name}!"
end

puts greet("Ada")
puts greet("Ada", "Welcome")

Hello, Ada!
Welcome, Ada!

Keyword arguments

Keyword arguments are passed by name, which makes calls easier to read and lets callers give them in any order:

def create_user(name:, role: "member", active: true)
  "#{name} (#{role}, active: #{active})"
end

puts create_user(name: "Ada")
puts create_user(role: "admin", name: "Linus")

Ada (member, active: true)
Linus (admin, active: true)

name: with no default is required — leaving it out raises missing keyword: :name (ArgumentError).

Compare create_user("Ada", "admin", false) — what does false mean? Keyword arguments answer that at the call site, so prefer them for optional settings and for methods with more than two or three arguments.

Any number of arguments

A * before a parameter collects any number of positional arguments into an array (this is called a splat):

def total(*prices)
  prices.sum
end

p total(1, 2, 3)
p total

6
0

**options does the same for keyword arguments, collecting them into a hash.

Common mistakes

• Putting a default positional parameter before a required one. Keep required parameters first.
• Forgetting the colon: name is positional, name: is a keyword.

Practice

Write format_price(amount, currency: "USD", decimals: 2) that returns a string like "12.50 USD". Call it once with only an amount and once with currency: "EUR".

Hint: format("%.2f", 12.5) returns "12.50". Use decimals to build the format string.

def format_price(amount, currency: "USD", decimals: 2)
  "#{format("%.#{decimals}f", amount)} #{currency}"
end

puts format_price(12.5)
puts format_price(12.5, currency: "EUR")

12.50 USD
12.50 EUR