modelo of square is zero, factorial modelo problem

Question

i was doing a CS homework, the question is to get n!, where n <=10^6, and to fit it, i need n! % m,

i wrote the following code:

#include <bits/stdc++.h>
using namespace std;

int main ()
{
long long n, m = 7+10e9, fact = 1;//value of m is given
cin>>n;
n++;

while (--n)
{
       fact = ((fact%m)*(n%m))%m;
       cout<<fact<<endl;//added this of debugging 
       if (fact == 0) break;//and this also

}
cout <<fact;
return 0;
}

the code is fine with small numbers (i.e < 10^5) however when the numbers get bigger the fact output is 0, after printing the fact in each step, i realized that when for some reason the fact(x) == fact (x-1) (x is the step), the new fact is zero

for example when i entered 10^6, the fact got 8478216162 two times successively, and then got zero.

any help is thanked in advance

`7+10^9` does not do what you think it does. `^` is bitwise logical exclusive-or. `10^9` evaluates to `3`, if my mental math is correct. — alter igel, Dec 29 '19 at 06:24
have you assumed that 7+10^9 = 1000000007 ? because it is not. ^ is xor operator — Breakpoint, Dec 29 '19 at 06:25
you are right, the original code is correct, these were typos during copying — Allaw Hussein, Dec 29 '19 at 12:24
no it can not, unless you mean n, anyways, my problem is still unaddressed — Allaw Hussein, Dec 29 '19 at 16:44
Note that 7+10e9 is 10,000,000,007, which is not a prime number. You want 7+1e9, which is. (`AeB` is `A` times the `B`th power of 10, not the `B`th power of `A`.) — molbdnilo, Dec 29 '19 at 16:50
man non of you are helping, that "m" is a given, i can not modify it for any reason — Allaw Hussein, Dec 29 '19 at 16:54
@AllawHussein Unless your `m` is a prime number, you always risk ending up with zero. — molbdnilo, Dec 29 '19 at 16:57

score 2 · Answer 1 · answered Dec 29 '19 at 06:40

2

There are two mistakes in your code.

m = 7+10^9 . Here ^means bitwise xor operator, not power. You should write it either m = 7 + 10e9 or m = 1000000007
if (fact = 0) . When you give single equal then it is not checking. Rather than it is assigning fact value to 0. So write double equal. if (fact == 0)

Your full code should be like this -

#include <bits/stdc++.h>
using namespace std;

int main () {
    long long n, m = 7+10e9, fact = 1;//value of m is given
    cin>>n;
    n++;
    while (--n) {
        fact = ((fact%m)*(n%m))%m;
        //cout<<fact<<endl;//added this of debugging
        if (fact == 0)
            break;//and this also

    }
    cout <<fact;
    return 0;
}

answered Dec 29 '19 at 06:40

Faruk Hossain

1,135
3
11

4

I think you should add that use of [`#include `](https://stackoverflow.com/Questions/31816095/Why-Should-I-Not-Include-Bits-Stdc-H) and [`using namespace std;`](https://stackoverflow.com/questions/1452721/why-is-using-namespace-std-considered-bad-practice) are discouraged. – Jaideep Shekhar Dec 29 '19 at 07:43
what you addressed are just typos, thanks. But the problem is not solved not even addressed – Allaw Hussein Dec 29 '19 at 12:25

modelo of square is zero, factorial modelo problem

1 Answers1