qsort
- C course
- Intermediate
- Lesson 26/116
- 5 min
Learn to sort with qsort using function pointers.
The qsort function
qsort is the standard library's generic sort function. It works regardless of array type; it takes the comparison via a function pointer.
Signature: qsort(arr, n, sizeof(int), cmp);. Parameters are: array start, element count, element size and the comparator function. The comparator takes two const void * parameters and returns int. It returns negative, zero or positive; this decides the sort direction. qsort needs <stdlib.h>.
The comparator function
The qsort comparator takes two const void * parameters and returns int. Negative means first is smaller; positive means first is larger; zero means equal.
Example: int cmp(const void *a, const void *b) { return *(int *)a - *(int *)b; }. Here the void * parameters are first cast to int *, then dereferenced. The subtraction gives negative/zero/positive; that's what qsort expects. The comparator decides the sort direction; reversing the subtraction gives descending order.
Practice Sort an array with qsort
Declare int arr[5] = {5, 2, 8, 1, 9};. Write int cmp(const void *a, const void *b) returning *(int *)a - *(int *)b. Sort with qsort(arr, 5, sizeof(int), cmp);. Print with printf("%d ", arr[i]) in a for loop, then printf("\n"). Include <stdlib.h>.
Starter code
#include <stdio.h>
#include <stdlib.h>
// Write the cmp comparator
// Sort the array with qsort
// Print with a for loop
int main(void) {
return 0;
}Sample solution
#include <stdio.h>
#include <stdlib.h>
int cmp(const void *a, const void *b) {
return *(int *)a - *(int *)b;
}
int main(void) {
int arr[5] = {5, 2, 8, 1, 9};
qsort(arr, 5, sizeof(int), cmp);
for (int i = 0; i < 5; i++) printf("%d ", arr[i]);
printf("\n");
return 0;
}Expected output
1 2 5 8 9
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Complete the qsort call
int arr[5] = {5, 2, 8, 1, 9};
____(arr, 5, sizeof(int), cmp);int arr[5] = {5, 2, 8, 1, 9};
qsort(arr, 5, sizeof(int), cmp);Explanation: qsort is the generic sort function.
2 Match the qsort parameters
| Term | Meaning |
|---|---|
arr |
Array start |
5 |
Element count |
sizeof(int) |
Element size |
cmp |
Comparator |
3 What should the qsort comparator return?
- Only 0 or 1
- A pointer
- Boolean
- Negative, 0 or positive Correct answer
Explanation: The comparator returns negative, 0 or positive.