[SOLVED] Operation on a variable and constant giving different result in c

Operation on a variable and constant giving different result in c

A simple calculation: 3^20%15.
The answer, according to a calculator, is 6.

The following code generates answer 7.

#include <stdio.h>
#include <math.h>
int main() {
        int i = 20;
        printf("%d\n", ((int)pow(3,20)%15));
        return 0;
}

If I replace 20 in the printf statement with the variable i, it gives -8 as output.

Assuming that the calculator is correct (or not?), what is the problem in the program?

Solution

The result of pow(3,20) can't fit in an int on your platform (or mine for that matter). Because of that, you're experiencing unexpected results.

Changing to a larger integer type such as long long will do the job.

Moreover, pow works with floating-point numbers which aren't represented exactly in memory (look that up). During conversion to an integer, that can cause certain errors. For example, for printf("%fl\n", (pow(3,20))); I get 3486784401.000000l which is not a precise integer value.