Ruby factorial function explained? [duplicate]

Question

#This function should return n!
def factorial(n)
  return nil if n < 0
  n == 0 ? 1 : n*factorial(n-1)
end

Just starting out and this function is blowing my mind I would write this function like this:

def factorial(n)
  result = 1

  if n == 0 
    return 1
  end

  while n > 0
    result *= n
    n -= 1
  end
  return result
end

I understande the short hand for the if/else statement. What I dont understand is using n*factorial(n-1) inside the function itself.

It looks like the factorial function is called inside the factorial function, but that can't be whats going on right?

That is in fact what is going on here. This is a recursive approach to computing factorials. — Chris Heald
– Chris Heald, Commented Feb 20, 2015 at 22:11
that is exactly whats going on. the factorial function is being called inside itself. — ptierno
– ptierno, Commented Feb 20, 2015 at 22:12
"To understand recursion, you must first understand recursion." — Ajedi32
– Ajedi32, Commented Feb 20, 2015 at 22:17
Thanks guys just the answer I was looking for. Now that I know what its called i'll be sure to look into recursion further. — RaSedg
– RaSedg, Commented Feb 20, 2015 at 22:24

hummmingbear · Accepted Answer · 2015-02-20 22:24:58Z

2

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

It's called recursion, the method is calling itself each time it evaluates until it meets the base case of 1 which is what this statement is doing: n == 0 ? 1 : n*factorial(n-1) :)

edited Feb 20, 2015 at 22:24

answered Feb 20, 2015 at 22:19

hummmingbear

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2 Comments

RaSedg Over a year ago

Clear and to the point, Thanks!

hummmingbear Over a year ago

@RaSedg - Would appreciate if you mark it as the answer if you have a moment.

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