sort array of pointers to structures qsort

Question

I'm trying to sort an array of pointers to structures where the key to compare is one of the structure's property.

I think that probably is the compare method.

Here's an example code.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

struct BINARY_ARRAY_RECORD {
    char *name;
};

int compare(const void *node1, const void *node2) {
    return strcmp(
        ((struct BINARY_ARRAY_RECORD *) node1)->name,
        ((struct BINARY_ARRAY_RECORD *) node2)->name
    );
}

int main()
{
    struct BINARY_ARRAY_RECORD **records;

    records = malloc(sizeof(struct BINARY_ARRAY_RECORD *) * 2);

    records[0] = malloc(sizeof(struct BINARY_ARRAY_RECORD));
    records[1] = malloc(sizeof(struct BINARY_ARRAY_RECORD));

    records[0]->name = malloc(sizeof(char) * (strlen("string2") + 1));
    records[1]->name = malloc(sizeof(char) * (strlen("string1") + 1));

    strcpy(records[0]->name, "string2");
    strcpy(records[1]->name, "string1");

    qsort(records, 2, sizeof(records[0]), compare);

    printf("%s\n", records[0]->name);
    printf("%s\n", records[1]->name);

    return 0;
}

Ajai · Accepted Answer · 2011-12-09 20:09:49Z

9

I guess this should be simpler..

  int compare(const void *node1, const void *node2) {
      BINARY_ARRAY_RECORD *ptr1 = *(BINARY_ARRAY_RECORD * const *)node1;
      BINARY_ARRAY_RECORD *ptr2 = *(BINARY_ARRAY_RECORD * const *)node2;
      return strcmp(ptr1->name, ptr2->name);
    }

And also I think the qsort function call could be definitely right if it were something like this,

qsort(records, 2, sizeof(BINARY_ARRAY_RECORD*), compare);

I think the third argument must be the size of the structure and you can be definitely be sure if it were something like the above..

answered Dec 9, 2011 at 20:09

Ajai

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3 Comments

tomas Over a year ago

struct BINARY_ARRAY_RECORD **records so records[0] its the same as struct BINARY_ARRAY_RECORD *

tomas Over a year ago

I thought it was ok the first answer but when i try with more than 2 pointers on the array fails. the correct answer is that gave me Ajai. I edit a little to get rid of the variables. int compare(const void *node1, const void *node2) { return strcmp( (*(struct BINARY_ARRAY_RECORD * const *) node1)->name, (*(struct BINARY_ARRAY_RECORD * const *) node2)->name ); }

tomas Over a year ago

why? the subscript operator dereferer from struct BINARY_ARRAY_RECORD ** to struct BINARY_ARRAY_RECORD *

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