How to write a recursive factorial function in Ruby?

I just want some help with how to write a recursive factorial function in Ruby. I have the following code which is lisp, but I want to do the same thing in Ruby.

(defun factorial (N)
    (if (= N 1) 1
        (* N (factorial (- N 1)))))

Solution

Here's how to write the your code in ruby:

def factorial(n)
  return 1 if n == 1
  n * factorial(n - 1)
end

factorial(5)
#=> 120
factorial(7)
#=> 5040

Edit for Stefan's comment:

To avoid a SystemStackError error with large values of n, use the tail-recursive method. Also Ruby's tailcall optimization must be enabled.

# before edit
factorial(100_000).to_s.size
#=> stack level too deep (SystemStackError)

To avoid SystemStackError

RubyVM::InstructionSequence.compile_option = {
  tailcall_optimization: true,
  trace_instruction: false
}

RubyVM::InstructionSequence.new(<<-CODE).eval
  def factorial(n, acc = 1)
    return acc if n == 1
    factorial(n - 1, n * acc)
  end
CODE

puts factorial(100_000).to_s.size
#=> 456574

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