Valid recursion question

Question

In the c# callenges, one question asks for filling a valid recursion, and the final answer is as follows:

public static int fact (int num) {
if (num ==1) return 1;
return num*fact (num -1);
}

Now, if this is the case, if my input is -1, for example, don't i get an infinite loop? Shouldn't it be ' if (num <= 1) '?

Answer 1

That function represents factorial in math. Factorial is valid only for positive numbers, special case 0! = 1. So calculation of factorial with negative numbers doesn't make any sense.

Answer 2

Yes, condition should be changed if you want to calculate with negative numbers. But there's no need to check for 0 (for normal factorial in math) because it is known that 0! = 1. That's why the base case is num = 1.

Answer 3

German Pais
one of the laws of recursion : A recursive algorithm must have a base case.
If your input breaks this, you've simply made a
while(true) loop

Answer 4

I understand, and that's totally true. Nevertheless, I still can pass a negative number as a parameter and get an infinite loop? For that case, perhaps there should be another conditional for 0 and lesser numbers, right?