CODEWITHSUNDEEP
c++runtime
jameselmore
jameselmore

Reputation: 442

Interesting processing time results

I've made a small application that averages the numbers between 1 and 1000000. It's not hard to see (using a very basic algebraic formula) that the average is 500000.5 but this was more of a project in learning C++ than anything else.

Anyway, I made clock variables that were designed to find the amount of clock steps required for the application to run. When I first ran the script, it said that it took 3770000 clock steps, but every time that I've run it since then, it's taken "0.0" seconds...

I've attached my code at the bottom.

Either a.) It's saved the variables from the first time I ran it, and it's just running quickly to the answer... or b.) something is wrong with how I'm declaring the time variables.

Regardless... it doesn't make sense.

Any help would be appreciated.

FYI (I'm running this through a Linux computer, not sure if that matters)

double avg (int arr[], int beg, int end)
{
    int nums = end - beg + 1;

    double sum = 0.0;

    for(int i = beg; i <= end; i++)
    {
        sum += arr[i];
    }   

    //for(int p = 0; p < nums*10000; p ++){}

    return sum/nums;

}

int main (int argc, char *argv[]) 
{ 
    int nums = 1000000;//atoi(argv[0]);
    int myarray[nums];

    double timediff;

    //printf("Arg is: %d\n",argv[0]);
    printf("Nums is: %d\n",nums);

    clock_t begin_time = clock();

    for(int i = 0; i < nums; i++)
    {
        myarray[i] = i+1;
    }

    double average = avg(myarray, 0, nums - 1);

    printf("%f\n",average); 

    clock_t end_time = clock();

    timediff = (double) difftime(end_time, begin_time);

    printf("Time to Average: %f\n", timediff);

    return 0;

}

Upvotes: 1

Views: 107

Answers (2)

Mooing Duck
Mooing Duck

Reputation: 66922

On the machine I tested, Linux's clock call was only accurate to 1/100th of a second. If your code runs in less than 0.01 seconds, it will usually say zero seconds have passed. Also, I ran your program a total of 50 times in .13 seconds, so I find it suspicous that you claim it takes 2 seconds to run it once on your computer.

Your code incorrectly uses the difftime, which may display incorrect output as well if clock says time did pass.

I'd guess that the first timing you got was with different code than that posted in this question, becase I can't think of any way the code in this question could produce a time of 3770000.

Finally, benchmarking is hard, and your code has several benchmarking mistakes:

  • You're timing how long it takes to (1) fill an array, (2) calculate an average, (3) format the result string (4) make an OS call (slow) that prints said string in the right language/font/colo/etc, which is especially slow.
  • You're attempting to time a task which takes less than a hundredth of a second, which is WAY too small for any accurate measurement.

Here is my take on your code, measuring that the average takes ~0.001968 seconds on this machine.

Upvotes: 0

wendelbsilva
wendelbsilva

Reputation: 772

You are measuring the I/O operation too (printf), that depends on external factors and might be affecting the run time. Also, clock() might not be as precise as needed to measure such a small task - look into higher resolution functions such as clock_get_time(). Even then, other processes might affect the run time by generating page fault interrupts and occupying the memory BUS, etc. So this kind of fluctuation is not abnormal at all.

Upvotes: 1

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