How is this method solving my factorial?

Question

I'm working on a ruby challenge and had to write a method that would calculate the Factorial of a number. I came across a solution below, but I don't understand how it works, specifically the section in the else statement:

def factorial(number)
  if number <= 1
    1
  else
    number * factorial(number - 1)
  end
end

Lets say I run factorial(5) How is the else statement iterating through 5*4*3*2*1 in the number * factorial(number - 1) statement? I know this probably seems like it should be obvious, but it's not for me. Appreciate the help in advance.

Answer 1

This concept is known as recursion.

factorial(5) evaluates to

5 * factorial(4)

factorial(4) evaluates to

4 * factorial(3)

factorial(3) evaluates to

3 * factorial(2)

factorial(2) evaluates to

2 * factorial(1)

factorial(1) evaluates to 1 because 1 <= 1

Substituting values appropriately results in

5 * factorial(4)
5 * 4 * factorial(3)
5 * 4 * 3 * factorial(2)
5 * 4 * 3 * 2 * factorial(1)
5 * 4 * 3 * 2 * 1