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!

You may want to tag this as C rather than C++. There is little need to deal with this kind of thing in C++. — juanchopanza
– juanchopanza, Commented Dec 4, 2013 at 14:38
Why use struct data ** when you only allocate a single data structure? Why not simple struct data * = realloc(...)? Changing this would incidentally make your qsort and comparison function work as expected. — Some programmer dude
– Some programmer dude, Commented Dec 4, 2013 at 14:43
@BitFiddlingCodeMonkey If the pointer passed to realloc is NULL then it works just like malloc. — Some programmer dude
– Some programmer dude, Commented Dec 4, 2013 at 14:49
@kampi Then you're not compiling as C, but as C++. They're different languages, it's rarely a good idea to do this, even deliberately. — unwind
– unwind, Commented Dec 4, 2013 at 14:51

Some programmer dude · Accepted Answer · 2013-12-04 14:48:58Z

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);
}

answered Dec 4, 2013 at 14:48

Some programmer dude

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1 Comment

kampi Over a year ago

Thank you! That solved the problem, however I made one more mistake. When calling qsort I used ArrayCount-1, and it should be only ArrayCount.

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