qsort with structs in C - c

Question

I have to write a program that uses the qsort function to sort a vector
of points in the Cartesian plane. Each point is formed by a pair of
coordinates (x, y).
Points must be sorted by ascending x-axis. With the same x-axis,
y-axis is ordered by descending.
This is a sample input:
5 (Struct numbers)
2 5
12 2
2 7
3 4
2 2
With the output:
2 7
2 5
2 2
3 4
12 2
Now, this is my code:
#include <stdio.h>
#include <stdlib.h>
typedef struct x_y
{
int x;
int y;
}coordinates;
typedef coordinates *coordinatesList;
int compare(const void *a, const void *b)
{
coordinates *a1 = (coordinates *)a;
coordinates *b1 = (coordinates *)b;
if (a1->x > b1->x)
return 1;
else if (a1->x < b1->x)
return (-1);
else if (a1->x == b1->x)
{
if (a1->y < b1->y)
return 1;
else if (a1->y > b1->y)
return (-1);
else
return 0;
}
}
int main()
{
int n, i;
scanf("%d", &n);
coordinatesList *A = (coordinatesList*)malloc(n * sizeof(coordinates));
for (i = 0; i < n; i++)
{
A[i] = (coordinatesList)malloc(sizeof(coordinates));
scanf("%d%d", &A[i]->x, &A[i]->y);
}
qsort(A, n, sizeof(coordinates*), compare);
for (i = 0; i < n; i++)
printf("%d %d\n", A[i]->x, A[i]->y);
return 0;
}
whith his wrong output:
2 7
3 4
2 2
2 5
12 2
If I try to separate the structs by element:
int compare(const void *a, const void *b)
{
coordinates *a1 = (coordinates *)a;
coordinates *b1 = (coordinates *)b;
int a_x = a1->x;
int a_y = a1->y;
int b_x = b1->x;
int b_y = b1->y;
if (a_x > b_x)
return 1;
else if (a_x < b_x)
return (-1);
else if (a_x == b_x)
{
if (a_y < b_y)
return 1;
else if (a_y > b_y)
return (-1);
else
return 0;
}
}
...gives me a different wrong output:
2 2
12 2
2 7
3 4
2 5

Answer 1

The compare function gets pointers to the elements to be sorted, so here it gets pointers to coordinates pointers. The correct beginning is:
int compare(const void *a, const void *b)
{
const coordinates *a1 = *(const coordinates **)a;
const coordinates *b1 = *(const coordinates **)b;
I added const because you shouldn't cast away const-ness, even if it doesn't matter here. You would notice if you used warnings with the compilation.
You should also use sizeof(coordinates) in the call to qsort, not sizeof(coordinates*), because otherwise the sort function can't know how big your elements are, but these two probably have the same value here.

Answer 2

For starters according to the C Standard the function main without parameters shall be declared like
int main( void )
In this declaration
coordinatesList *A = (coordinatesList*)malloc(n * sizeof(coordinates));
you have to use expression sizeof( coordinatesList ) instead of the expressions sizeof( coordinates ).
In the function compare you have to write
int compare(const void *a, const void *b)
{
const coordinatesList a1 = *( const coordinatesList *)a;
const coordinatesList b1 = *( const coordinatesList *)b;
//...
Here is a demonstrative program
#include <stdio.h>
#include <stdlib.h>
typedef struct x_y
{
int x;
int y;
} coordinates;
typedef coordinates *coordinatesList;
int compare( const void *a, const void *b )
{
const coordinatesList a1 = *( const coordinatesList * )a;
const coordinatesList b1 = *( const coordinatesList * )b;
if ( a1->x < b1->x )
{
return -1;
}
else if ( b1->x < a1->x )
{
return 1;
}
else
{
if ( a1->y < b1-> y )
{
return 1;
}
else if ( b1->y < a1->y )
{
return -1;
}
else
{
return 0;
}
}
}
int main(void)
{
const size_t N = 5;
coordinatesList *A = malloc( N * sizeof( coordinatesList ) );
for ( size_t i = 0; i < N; i++ )
{
A[i] = malloc( sizeof( coordinates ) );
}
A[0]->x = 2; A[0]->y = 5;
A[1]->x = 12; A[1]->y = 2;
A[2]->x = 2; A[2]->y = 7;
A[3]->x = 3; A[3]->y = 4;
A[4]->x = 2; A[4]->y = 2;
for ( size_t i = 0; i < N; i++ )
{
printf( "%d\t%d\n", A[i]->x, A[i]->y );
}
putchar( '\n' );
qsort( A, N, sizeof( coordinatesList ), compare );
for ( size_t i = 0; i < N; i++ )
{
printf( "%d\t%d\n", A[i]->x, A[i]->y );
}
putchar( '\n' );
return 0;
}
Its output is
2 5
12 2
2 7
3 4
2 2
2 7
2 5
2 2
3 4
12 2