[c-prog] Re: problems with pointer redirection

[John Matthews]

--- In c-prog@yahoogroups.com, "Rick Bowers" <[EMAIL PROTECTED]> wrote:
>
> I am trying to sort an array of pointers to a structure.

Hi Rick- below is an example program which sorts an array of
structures and an array of pointers to structures. Not sure whether
you wanted C or C++ - I only do C I'm afraid. Hope it helps.
John

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

#define MAX_ENTRIES 10

typedef struct
{
    char c;
} ENTRY;

/* Array of structures. */

static ENTRY linedata[MAX_ENTRIES] =
{
    {'q'}, {'w'}, {'e'}, {'r'}, {'t'}, {'y'}, {'u'}, {'i'}, {'o'}, {'p'}
};

/* Array of pointers to structures. */

static ENTRY *filters[MAX_ENTRIES];

/* For comparing 2 ENTRY structures. */

static int cmpEntries(const ENTRY *e1, const ENTRY *e2)
{
    printf("comparing %c %c\n", e1->c, e2->c);

    if (e1->c < e2->c) return -1;
    if (e1->c > e2->c) return  1;

    return 0;
}

/* For use with qsort() and array of structures. */

static int compareEntry(const void *a, const void *b)
{
    return cmpEntries(a, b);
}

/* For use with qsort() and array of pointers to structures. */

static int compareEntryPtr(const void *a, const void *b)
{
    ENTRY *const *pE1 = a;
    ENTRY *const *pE2 = b;

    return compareEntry(*pE1, *pE2);
}

/* Print an array of structures. */

static void printEntries(const char *s, const ENTRY *p)
{
    int i;

    printf("%s:", s);

    for (i = MAX_ENTRIES; i--; p++)
    {
        printf(" %c", p->c);
    }

    printf("\n");
}

/* Print an array of pointers to structures. */

static void printEntriesPtr(const char *s, ENTRY *const *pp)
{
    int i;

    printf("%s:", s);

    for (i = MAX_ENTRIES; i--; pp++)
    {
        printf(" %c", (*pp)->c);
    }

    printf("\n");
}

int main(void)
{
    int i;

    /* Initialise the array of pointers to structures. */

    for (i = MAX_ENTRIES; i--; )
    {
        filters[i]    = malloc(sizeof *filters[i]);
        filters[i]->c = toupper(linedata[i].c);
    }

    printEntries("linedata before", linedata);
    qsort(linedata, MAX_ENTRIES, sizeof *linedata, compareEntry);
    printEntries("linedata after", linedata);

    printEntriesPtr("filters before", filters);
    qsort(filters, MAX_ENTRIES, sizeof *filters, compareEntryPtr);
    printEntriesPtr("filters after", filters);

    return 0;
}