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9.3 Array Sort Function

To sort an array using an arbitrary comparison function, use the qsort function. The prototype for this function is in stdlib.h.

— Function: void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)

The qsort function sorts the array array. The array contains count elements, each of which is of size size.

The compare function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.

Warning: If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects.

If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary.

Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see Comparison Functions):

            double *array;
            int size;
            qsort (array, size, sizeof (double), compare_doubles);

The qsort function derives its name from the fact that it was originally implemented using the “quick sort” algorithm.

The implementation of qsort in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array.