Question

I have written this function to calculate the average of some floats.But it has a runtime error at the last line of the while in “average” function.What’s the problem with it?

#include <stdarg.h>
#include <stdio.h>



float average(float first , ...)
{
    int count = 0;
    float sum = 0 , i = first;

    va_list marker;

    va_start(marker , first);
    while(i != -1)
    {
        sum += i;
        count++;
        i = va_arg(marker , float);
    }

    va_end(marker);
    return(sum ? (sum / count) : 0);
}


int main(int argc , char* argv[])
{
    float avg = average(12.0f , 34.0f);

    printf("The average is : %f\n" , avg);
    scanf("a\n");
}

Answer 1

You forgot to end the call with a -1, so your while loop causes undefined behaviour because it tries to keep getting more arguments than you passed. It should be:

float avg = average(12.0f, 34.0f, -1f);

Also, float arguments are promoted to double when being passed to variadic functions, so you can’t use float with va_arg. You should be using double for all this:

double average(double first, ...)
{
    int count = 0;
    double sum = 0, i = first;

    va_list marker;

    va_start(marker, first);
    while(i != -1)
    {
        sum += i;
        count++;
        i = va_arg(marker, double);
    }

    va_end(marker);

    return sum ? sum / count : 0;
}

double avg = average(12.0, 34.0, -1.0);