How to sort a struct using qsort?

Question

I have a struct declared like this

struct data
{
char * Date;
char * String;
};
struct data **RegArray = NULL;

int ArrayCount = 0;

I add new items to the array this way:

struct data **tmp = ( struct data ** )realloc( RegArray, ( ArrayCount + 1 ) * sizeof( struct data * ) );
if ( tmp == NULL )
{
    printf( "\nRealloc failed!" );
    return;
}
RegArray = tmp;

RegArray[ ArrayCount ] = ( struct data * )malloc( sizeof **RegArray );
if ( RegArray[ ArrayCount ] == NULL )
{
    printf( "\nMalloc failed!" );
    return;
}

RegArray[ ArrayCount ]->Date = _strdup( cDate );
RegArray[ ArrayCount ]->String = _strdup( cString );

ArrayCount++;

The function which compares the values:

int CompareByDate( const void *elem1, const void *elem2 )
{
//return ( ( data* )elem1 )->Date > ( ( data* )elem2 )->Date ? 1 : -1;
return strcmp( ( ( data* )elem1 )->Date, ( ( data* )elem2 )->Date );

}//CompareByDate

And finally I call qsort like this:

qsort( RegArray, ArrayCount-1, sizeof( data ), CompareByDate );

The problem is, that the data won't be sorted. So what am I doing wrong?

Thanks!

Answer 1

In your qsort call and comparison function, you forget that you're dealing with an "array" of pointers. The easiest change is to not use an array of pointers:

struct data *RegArray = NULL;

/* ... */

struct data *tmp = realloc( RegArray, ( ArrayCount + 1 ) * sizeof( struct data ) );
if ( tmp == NULL )
{
    printf( "\nRealloc failed!" );
    return;
}
RegArray = tmp;

RegArray[ ArrayCount ].Date = _strdup( cDate );
RegArray[ ArrayCount ].String = _strdup( cString );

ArrayCount++;

This will make your qsort call (and comparison function) work as they are shown in the question.

---

If you don't want to change the code as outlined above, you have to change the qsort call and comparison function:

qsort( RegArray, ArrayCount-1, sizeof( data * ), CompareByDate );

/* ... */

int CompareByDate( const void *elem1, const void *elem2 )
{
    struct data **d1 = (struct data **) elem1;
    struct data **d2 = (struct data **) elem2;

    return strcmp((*d1)->Date, (*d2)->Date);
}