calculation timer problem

[handramw]

Hello,
I am trying to write a program that determines how many calculations it would take to find the determinant of a matrix, and how long it would take to do those calculations. I keep getting 0 seconds for an output. Can anybody see where I may have gone wrong?

double mults_for_det(int n)
{
    if(n == 1)
        return 0;
    else
        return n + n * mults_for_det(n - 1);
}
int main(void)
{
    int n;
    double mults;
    clock_t start, end;
    
    printf("Enter a number for an nxn matrix.\n");
    scanf("%d",&n);
    start = clock();
    mults = mults_for_det(n);
    end = clock();
    printf("For n = %d, the %g multiplications take %g seconds.\n",n,
    mults,(double)(end - start) / (double)CLOCKS_PER_SEC);
    return 0;
}

[oogabooga]

Print out CLOCKS_PER_SEC. If it's only 1000 (or even 1000000) it's probably not enough precision to measure one execution of your subroutine. Put it in a loop and execute it a million times. Then divide the clock difference by CLOCKS_PER_SEC and 1000000.

Also, you need to change the input of your subroutine to double.

[memcpy]

Oogabooga's solution works, in addition, if you're using x86, you could always use the timestamp counter:

#define rdtscl(val) asm volatile ("rdtsc" : "=A" (val) : : );
/* ... */
unsigned long t1, t2;
rdtscl(t1);
mults = mults_for_det(n);
rdtscl(t2);

printf("Cycles = %lu\n", t2 - t1);

But note that the count is not representative of any accurate real-time measure, it's mainly useful for comparing functions in optimisation. And don't expect accuracy over ~20 cycles, because so many factors can influence the time readings.

You can *estimate* how long it takes by assuming that 1 cycle is approximately (1/c) nanoseconds, where c is the CPU frequency of your computer, in gHz. For example, running a 3.06gHz machine, a cycle is approximately 0.32 nanoseconds.