使用C qsort（）的排序结构

Question

Having trouble getting my head around implementing the qsort() built into C to sort an array of structs by a stored int value (hitCount).

My struct:

typedef struct words {
    const char *word;
    int hitCount;
} words;

I'm trying to use the example given by Microsoft ( http://support.microsoft.com/kb/73853 ).

So I've got at the top:

typedef int (*compfn)(const void*, const void*);

and the comparision method:

int compare (words *a, words *b) {
    if (a->hitCount > b->hitCount) {
        return -1;
    } else if (a->hitCount < b->hitCount) {
        return 1;
    } else {
        return 0;
    }
}

then within another method I call qsort with my array name and other details replacing the Microsoft example:

qsort((void *) &output, outputLength, sizeof(words), (compfn)compare);

This gives a segmentation fault.

I don't fully understand how to use qsort so I assume where I've adapted it from Microsoft's example I've done it incorrectly.

I hope I've included the mistake and can get some enlightenment as to what I should be doing in order for this to work correctly.

Many Thanks!

Answer 1

You have to pass the array not the address of the array to qsort.

qsort( output, ... );

Also your compare function must return an int and accept two const void* arguments. Casting your function int compare (words *a, words *b) to a different( yet correct ) type which is then called by qsort() will cause undefined behaviour.

The compare function must be:

int compare (const void *a, const void *b)...

Then you cast a and b to correct types:

((words*)a)->hitCount < ((words*)b)->hitCount

Answer 2

I suspect that outputLength is computed incorrectly. A complete working example:

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

typedef struct words {
    const char *word;
    int hitCount;
} words;

int compare(const void * left, const void * right) {
    const words * a = (const words *) left;
    const words * b = (const words *) right;
    if (a->hitCount > b->hitCount) {
        return -1;
    } else if (a->hitCount < b->hitCount) {
        return 1;
    } else {
        return 0;
    }
}

int main() {
    struct words output[] = {
      { "hello", 314 },
      { "world", 42 },
      { "answer", 42 }
    };
    int outputLength = sizeof(output) / sizeof(output[0]);
    int i;

    output[0].word = "hello";
    output[0].hitCount = 314;
    output[1].word = "world";
    output[1].hitCount = 42;
    qsort(output, outputLength, sizeof(words), compare);

    for (i = 0; i < outputLength; ++i) {
        printf("%d %s\n", output[i].hitCount, output[i].word);
    }

    return 0;
}

Answer 3

The prototype of the standard library function qsort is

void qsort(void *base, size_t nmemb, size_t size, 
           int (*compar)(const void *, const void *));

Note the signature of the compare function. You cannot typecast a pointer to a function of different signature and make it work correctly. Therefore, typecasting your compare function will not work. It must have the same signature as declared in the prototype of qsort . Change your compare function to -

int compare(const void *a, const void *b) {
    int c = ((words *) a)->hitCount;
    int d = ((words *) b)->hitCount;

    if(c > d) return -1;
    if(c < d) return 1;
    return 0;
}

The first argument base of qsort is the base address of the buffer which contains the elements to be sorted. Also, any pointer type is assignment compatible to a void * variable and as such you don't need to cast the base address. Therefore, you should call the qsort function as -

qsort(output, outputLength, sizeof output[0], compare);

Answer 4

Got it working with:

    int compare (const void *a, const void *b) {
        if (((words *)a)->hitCount > ((words *)b)->hitCount) {
            return -1;
        } else if (((words *)a)->hitCount < ((words *)b)->hitCount) {
            return 1;
        } else {
            return 0;
        }
    }

and call to sort:

    qsort(output, outputLength, sizeof(words), compare);

Thanks to everyone's help but majority credit to "self".