Problems with the challenge

Question

Can anyone explain why in this exercise the result is 4? Why fact(1) is 1, but not 0?

int fact(int n) {
if (!n)
return 1;
return n * fact(n-1);
}
int main () {
cout << fact(3) - fact(2);
}

Answer 1

tippfehler, in your code "fact(-1)" returns -1.
In C/C++ a negation of a non-zero number returns 0 (false) and a negation of 0 returns 1 (true). The compiler does an implicit cast of 0/1 integers to their boolean equivalents false/true and vice-versa.

Answer 2

In the if statement, when n is false, !n is true. When !n is true, the function returns 1 (what's inside the if statement), else it does n * fact(n-1).

fact(1) = 1
fact(0) = 1
fact(2) = 2 * 1
fact(3) = 3* fact(2) * fact(1) = 6
fact(3) - fact(2) = 6 - 2 = 4.

Answer 3

Okay, but why 1 and 0 in fact(1) and fact(0) is "!n"? And what if I write fact(-1)?