qsort in C (dynamic allocation) - c

Question

I want to know this situation.
when I define this sentence
struct soccer team[100] ;
I can do qsort ;
qsort(team, MAX , sizeof(team[0]) , compare) ;
int compare(const void *a, const void *b )
{
SOC *A1 = (SOC*)a ;
SOC *B1 = (SOC*)b ;
if( A1->score > B1->score )
return -1 ;
else if ( A1->score == B1->score )
return 0 ;
else
return 1 ;
}
When I do dynamic allocation
struct soccer*team[MAX] ;
team[Index] = (SOC*)malloc(sizeof(SOC)) ;
error is existed. (qsort and compare is same )
I want to know how do use it(qsort for dynamic allocation struct)
please!
example ( when I use first way)
Man 3 1 1 16
Che 2 2 2 8
Asn 0 6 0 6
hot 6 0 0 18
City 0 0 6 0
Bar 1 5 0 8
is converted
hot 6 0 0 18
Man 3 1 1 16
Che 2 2 2 8
Bar 1 5 0 8
Asn 0 6 0 6
City 0 0 6 0

Answer 1

The first version
struct soccer team[100] ;
and the second one
struct soccer*team[MAX] ;
team[Index] = (SOC*)malloc(sizeof(SOC)) ;
are not same. The first one is an array of struct soccer and the second one is an array of struct soccer *. They are not just the same.
If you want to use the later version (including pointer) and get the same behaviour as above, you can do something like
struct soccer * team;
team = malloc(sizeof *team * SIZE) ; // SIZE is the number of elements

Answer 2

The same comparision function cannot be used for different element types. Use correct comparision function like this (pointers to elements, which are pointers, will be given, so dereference them to get the pointers to structs):
int compare2(const void *a, const void *b )
{
SOC *A1 = *(SOC**)a ;
SOC *B1 = *(SOC**)b ;
if( A1->score > B1->score )
return -1 ;
else if ( A1->score == B1->score )
return 0 ;
else
return 1 ;
}
Note: They say you shouldn't cast the result of malloc() in C.

Answer 3

Here is a demonstrative program that shows how a similar array can be sorted.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define MAX 10
typedef struct soccer
{
unsigned int score;
} SOC;
int cmp( const void *a, const void *b )
{
const SOC *lhs = *( const SOC ** )a;
const SOC *rhs = *( const SOC ** )b;
return ( lhs->score > rhs->score ) - ( rhs->score > lhs->score );
}
int main( void )
{
SOC * team[MAX];
srand( ( unsigned int )time( NULL ) );
for ( int i = 0; i < MAX; i++ )
{
team[i] = malloc( sizeof( SOC ) );
team[i]->score = rand() % MAX;
}
for ( int i = 0; i < MAX; i++ )
{
printf( "%u ", team[i]->score );
}
printf( "\n" );
qsort( team, MAX, sizeof( SOC * ), cmp );
for ( int i = 0; i < MAX; i++ )
{
printf( "%u ", team[i]->score );
}
printf( "\n" );
for ( int i = 0; i < MAX; i++ ) free( team[i] );
return 0;
}
The program output is
2 7 2 5 1 6 1 5 0 4
0 1 1 2 2 4 5 5 6 7