How to properly use qsort()? - c

Question

I am trying to write a simple program to find the median of an array. To do that first I tried to use qsort to put my values in order but I can't quite get it to work. I believe it has something to do with my function parameter and pointers.
#include <stdio.h>
#include <stdlib.h>
int cmpfunc (const void * a, const void * b)
{
return ( *(int*)a - *(int*)b );
}
int median(const int* array)
{
double median;
int length =0;
while(array[length])
length++;
qsort(*array, length, sizeof(int), cmpfunc);
if(length %2 ==0)
{
median=(array[length/2] + array[(length/2)+1])/2;
}
else
{
median = array[(length +1/2)];
}
return median;
}
int main ()
{
int array [] = {5, 3, 2, 7, 9};
printf( "%f", median(array));
}

Answer 1

Beside that you are getting wrong value for length with snippet
while(array[length]) // Array is not ended with zero
length++; // Length would get some unexpected value
you need to return double from function median and it also need one more parameter for length.
double median(int* array, const int length){...}
// ^Removed const qualifier and added an extra parameter for array size
Call median as
double result = median(array, sizeof(array)/sizeof(array[0]))

Answer 2

From the qsort documentation:
void qsort(void *base, size_t nmemb, size_t size,
int (*compar)(const void *, const void *));
The qsort() function sorts an array with nmemb elements of size size.
The base argument points to the start of the array.
First you need to get the length right; second, in your original call to qsort you used qsort(*array...) which is not what you want. Just use qsort(array...).
It is also useful when writing these small tests to view the actual data; you can either use a debugger, or just print out the contents of your array.
#include <stdio.h>
#include <stdlib.h>
int cmpfunc(const void *a, const void *b)
{
return (*(int *) a - *(int *) b);
}
void printArray(int *array, int length)
{
int i;
for (i = 0; i < length; i++) {
printf("%d ", array[i]);
}
printf("\n");
}
int median(int *array, int length)
{
double median;
printArray(array, length);
qsort(array, length, sizeof(int), cmpfunc);
printArray(array, length);
if (length % 2 == 0) {
median = (array[length / 2] + array[(length / 2) + 1]) / 2;
} else {
median = array[(length + 1 / 2)];
}
return median;
}
int main()
{
int array[] = { 5, 3, 2, 7, 9 };
printf("%d\n", median(array, sizeof(array) / sizeof(array[0])));
return 0;
}