C
Summary - Advanced pointers
Combine double pointer, function pointer and void* to write a sorting app.
Bringing advanced pointers together
In this group you learned four concepts: double pointer (**), array of pointers, function pointer and void *. Now we'll combine them in a sorting application.
The sort function takes an array of int * as int **. It compares through a function pointer int (*cmp)(int, int). We pass the pointer array through a void * generic pointer; this mimics the flexibility of real library APIs. So one sort function can work with different comparisons.
Exercise Sort an array of pointers
Write int greater(int a, int b) returning a > b. Define void sort_ptrs(int **items, int n, int (*cmp)(int, int)); do bubble sort, swapping neighbor pointers with an int * tmp when cmp is true. In main declare int v[3] = {5, 2, 8}; and int *ptrs[3] = {&v[0], &v[1], &v[2]};. Write void *generic = ptrs; and int **items = (int **)generic;. Call sort_ptrs(items, 3, greater), then print *items[i] in a for loop with printf.
Show example solution
#include <stdio.h>
int greater(int a, int b) { return a > b; }
void sort_ptrs(int **items, int n, int (*cmp)(int, int)) {
for (int i = 0; i < n - 1; i++) {
for (int j = 0; j < n - 1 - i; j++) {
if (cmp(*items[j], *items[j + 1])) {
int *tmp = items[j];
items[j] = items[j + 1];
items[j + 1] = tmp;
}
}
}
}
int main(void) {
int v[3] = {5, 2, 8};
int *ptrs[3] = {&v[0], &v[1], &v[2]};
void *generic = ptrs;
int **items = (int **)generic;
sort_ptrs(items, 3, greater);
for (int i = 0; i < 3; i++) printf("%d ", *items[i]);
printf("\n");
return 0;
}
Summary
In this group you learned advanced pointer concepts. A double pointer holds a pointer's address; an array of pointers is common for string lists; a function pointer selects behavior at runtime; a void * is a generic pointer with lost type that requires an explicit cast.
You combined them to write a flexible sorting application. In the next groups these concepts return in memory management and dynamic data structures. qsort and callback patterns in particular build on these foundations.
Test yourself
-
Match the concepts with their uses
- int **items
- Double pointer
- int (*cmp)(int, int)
- Function pointer
- void *generic
- Generic pointer
- int *ptrs[3]
- Pointer array
-
What does void* do in the sort function?
- Is used in the loop
- Carries the pointer array generically
- Does the comparison
- Prints values
Show answer
Carries the pointer array generically — Void* carries the pointer array generically, then is cast to int**.
Next lesson: Dynamic array
