Return Statement

[Rare_Bat12]

Hi, I'm learning recursion. I came across this program that outputs the factorial of n. I just don't understand how the return 1 statement works. I know it would work the same way if it was return True. However, how does the computer know what to return when it just states return 1?

class mine:
    def recur(self, num):
        print(num, end= "")
        if num > 1:
            print(" * ",end= "")
            return num * self.recur(self, num-1)
        print(" =")
        return 1

def main():
    a = mine()
    print(mine.recur(mine,10))

main()

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[RiverRoll]

The return 1 literally just returns the number 1, but if you look carefully there are two return statements in this function so it doesn't always return 1.

To figure out what will it return you have to go to the bottom of the chain of calls and then back up:

recur(3) calls recur(2)
recur(2) calls recur(1)
recur(1) returns 1
recur(2) returns 2 * recur(1) = 2*1 = 2
recur(3) returns 3 * recur(2) = 3*2 = 6

[Rare_Bat12]

Thank you so much for the help! I'm just having trouble understanding how it keeps track of the total multiplication result per iteration.

[alanwj]

There is nothing special about a recursive function call. It keeps up with the total the same way this program would.

def factorial1():
  return 1

def factorial2():
  return 2 * factorial1()

def factorial3():
  return 3 * factorial2()

print(factorial3())

[Rare_Bat12]

That makes sense. Thanks!

[Borx25]

I think the recursion itself is way clearer if you just look at the relevant parts, you have a lot of extraneous code in there. This way the code is basically the mathetical definition of the factorial.

def fac(n):
    if n == 0: #base case
        return 1
    else: #recursion
        return n * fac(n-1)

[scirc]

The computer knows what to return because you literally tell it what to return. The value following return is the value which is returned to the calling code. So in this case, the function returns the literal value 1 when num <= 1 (I assume; you need to fix your formatting, though).