qsort won't sort dynamically allocated array o

问题:

I have a struct

struct info {
    char firstName[100]; 
    char lastName[100]; 
    char companyName[100];
    char email[100];
    unsigned long phoneNumber; 
}; 

which is stored in the file compareElements.h

I read in a set of values into a dynamically allocated array of structs called bptr.

my comparePtr points to this function.

#include <string.h>
#include "compareElements.h"
int compareNameAscending (const void *a, const void *b) {

    struct info *part1 = (struct info *) a; 
    struct info *part2 = (struct info *) b; 




    if(part1 -> lastName[0] != '\0' && part2 -> lastName[0] != '\0') {
        return (strcmp(part1 -> lastName , part2 -> lastName));
    }   
    if (part1 -> lastName[0] == '\0' && part2 -> lastName[0] != '\0') {
        return (strcmp(part1 -> companyName , part2 -> lastName)); 
    }

    if (part1 -> lastName[0] != '\0' && part2 -> lastName[0] == '\0') {
        return (strcmp(part1 -> lastName, part2 -> companyName)); 
    }

    if (part1 -> lastName[0] == '\0' && part2 -> lastName[0] == '\0') {
        return (strcmp(part1 -> companyName, part2 -> companyName)); 
    } 

}

In the main function I have

comparePtr = &compareNameAscending 

The problem is that not all structs need to have a lastName and companyName only one of the two. So I want to sort by last name, and if they don't have a last name I'll sort by company name. The problem is that my qsort call doesn't ever actually change the order of the struct it just spits out the original. This is the qsort call.

qsort(bptr, i, sizeof(struct info), comparePtr); 

回答1:

I suppose you have problem in initialization of your code, not how to use it, here simplified version of your code that works fine:

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

struct info {
    char lastName[100]; 
    char companyName[100];
}; 

int compareNameAscending (const void *a, const void *b) {

    const struct info *part1 = a; 
    const struct info *part2 = b; 

    const char *s1 = strcmp(part1->lastName, "") == 0 ?
        part1->companyName : part1->lastName;
    const char *s2 = strcmp(part2->lastName, "") == 0 ?
        part2->companyName : part2->lastName;

    return strcmp(s1, s2);
}


int main() {
    struct info bptr[2];
    strcpy(bptr[0].lastName, "");
    strcpy(bptr[0].companyName, "Foo");
    strcpy(bptr[1].lastName, "Boo");
    strcpy(bptr[1].companyName, "");
    qsort(bptr, sizeof(bptr) / sizeof(bptr[0]), sizeof(struct info), compareNameAscending);

    size_t i;
    for (i = 0; i < sizeof(bptr) / sizeof(bptr[0]); ++i) {
        printf("company %s, name %s\n", bptr[i].companyName, bptr[i].lastName);
    }
}