C calculate wrong number in multiplication-CodePudding

i am simply trying to multiply 2 numbers and store them on an unsigned long long variable. however the result seems to be wrong and i also made sure that the result does not cause any overflow to happen. why is it calculating the wrong number?

#include <stdio.h>

int main() {
    unsigned long long num = 0;
    
    num = 260000000 * 25;
    printf("num = %llu\n", num);
    
    num = 0;
    
    for (int i=0; i<25; i  ){
        num  = 260000000;
    }
    printf("num = %llu\n", num);
    return 0;
}

Result:
num = 18446744071619617024
num = 6500000000

notice in the first example i'm multiplying directly and it produces the wrong number while the second example by looping them manually gives the correct answer.

CodePudding user response：

260000000 * 25 is an int multiplication. If you want an unsigned long long value then you need to use the ULL (or ull) suffix:

    num = 260000000ULL * 25;

Or you write it like this:

    unsigned long long num = 260000000;
    num *= 25;

Or use a cast:

    num = (unsigned long long) 260000000 * 25;