Pointers to structures and functions - c

Question

I have a simple structure called entry defined which contains name and age. Given an array of these structures, I want to sort the array based on age.
Below is my attempt at applying this, at the moment I can't even get this to compile. I think my pointer logic is incorrect in both the if statement comparison and the subsequent swapping of the pointers. I've tried various ways to do the same thing, but I'm not getting anywhere. I'm pretty new to C, and I'm still trying to get my head around pointers, so it's probably something basic I'm misunderstanding. Can anybody please explain what I'm doing wrong below?
Any help would be greatly appreciated.
#include <stdio.h>
struct entry {
char name[15];
int age;
};
void entrySort( struct entry *dict);
void entrySort( struct entry *dict){
int i,j; // counters
int ct = 4;
struct entry *tmp; // temporary holder
for( i = 0; i < ct; i++){
for( j = 0; j < ct; j++ ){
if ((*dict[i].age) > (*dict[j].age)){
tmp = (dict + i);
(dict+i) = (dict+j);
(dict+j) = tmp;
}
}
}
int main (void){
int ct = 4, i;
struct entry reg[4] =
{{ "John", 24 },
{ "Alan", 18 },
{ "Jim", 40 },
{ "Sarah",32 }};
entrySort(reg);
for( i = 0; i < ct; i++)
printf("name: %s. Age: %d\n", reg[i].name, reg[i].age);
return 0;
}

Answer 1

You pass an array of struct entry objects as a pointer: struct entry *dict, but you are treating it as it would be an array of pointers to struct entry objects: (*dict[i]).age.
(dict+i) is still just a pointer pointing to the memory where i+1. element is stored, i.e. &dict[i]. To actually access this element at index i, you need to use dereference operator: *(dict + i), which is equal to dict[i].
And also note that your swapping of elements at i and j is wrong. "Temporary holder" tmp should be an object that will temporarily hold data, not just a pointer to memory, that you are going to rewrite, thus declare it as struct entry tmp;:
struct entry tmp;
for( i = 0; i < ct; i++) {
for( j = 0; j < ct; j++ ) {
if ((dict[i].age) > (dict[j].age)) {
tmp = dict[i];
dict[i] = dict[j];
dict[j] = tmp;
}
}
}
By the way in the code you have posted, the ending curly brace (}) of your if is missing.

Answer 2

Try:
#include <stdio.h>
struct entry {
char name[15];
int age;
};
void entrySort( struct entry *dict, int);
void entrySort( struct entry *dict, int ct){
int i,j; // counters
/* int ct = 4; */
struct entry tmp; // temporary holder
for( i = 0; i < ct; i++){
for( j = 0; j < ct; j++ ){
if ((dict[i].age) > (dict[j].age)){ /* no * */
tmp = *(dict + i);
*(dict+i) = *(dict+j);
*(dict+j) = tmp;
}
}
}
int main (void){
int ct = 4, i;
struct entry reg[4] =
{{ "John", 24 },
{ "Alan", 18 },
{ "Jim", 40 },
{ "Sarah",32 }};
entrySort(reg, ct);
for( i = 0; i < ct; i++)
printf("name: %s. Age: %d\n", reg[i].name, reg[i].age);
return 0;
}

Answer 3

For completeness, here's how you'd do it with qsort:
#include <stdlib.h>
int sort_entry(const void *va, const void *vb) {
const struct entry *a = va;
const struct entry *b = vb;
if(a->age < b->age) return -1;
else if(a->age == b->age) return 0;
return 1;
}
...
qsort(reg, ct, sizeof(struct entry), sort_entry);