qsort did not sort the array of string

Question

I tried to sort an array of string using qsort. here is the content of my array:

{"a","orange","apple","mobile","car"}

this is how I use qsort:

int myCompare (const void * a, const void * b ) {
  const char *pa = (const char*)a;
  const char *pb = (const char*)b;
  return strcmp(pa,pb);
}

int stringLen = sizeof(input)/sizeof(char *);
qsort(input, stringLen, sizeof (char*), myCompare);

However, when I print the array, nothing is changed. is there something wrong w/ this?

an array of pointers to char, is not the same as a contiguous memory block of char of the same size. — Mitch Wheat, Feb 21 '13 at 02:05
@MitchWheat what do you mean? I can't use stringLen? so what should I do? even if I put an array of string {"aaa","vvv","abc","mmm"} it still output the same thing — jack stov, Feb 21 '13 at 02:13

score 10 · Answer 1 · answered Feb 21 '13 at 02:17

10

I've changed your myCompare function to what Mitch Wheat had posted earlier, and that works properly. Here's the example:

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

int myCompare (const void * a, const void * b ) {
    const char *pa = *(const char**)a;
    const char *pb = *(const char**)b;

    return strcmp(pa,pb);
}

int main() {
    int i;
    const char *input[] = {"a","orange","apple","mobile","car"};

    int stringLen = sizeof(input) / sizeof(char *);
    qsort(input, stringLen, sizeof(char *), myCompare);

    for (i=0; i<stringLen; ++i)
        printf("%d: %s\n", i, input[i]);
}

This will return:

0: a
1: apple
2: car
3: mobile
4: orange

answered Feb 21 '13 at 02:17

Bill Lynch

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I just copy paste ur code, but it doesn't work T.T is it possible that something is wrong w/ the compiler? – jack stov Feb 21 '13 at 02:30
1

No. As I said some time ago: it's how you declare your 'array'. THE COMPILER ISN"T BROKEN! Point of diminishing return reached.... – Mitch Wheat Feb 21 '13 at 02:32
@MitchWheat but according to sharth it works properly – jack stov Feb 21 '13 at 02:40
1

It also works correctly here: http://ideone.com/UqvehY – Bill Lynch Feb 21 '13 at 02:58
To keep const qualifier between castings, these lines must change a little: const char * pa = *(const char * const * ) a; const char * pb = *(const char * const * ) b; – AshkanVZ May 21 '19 at 05:14

score 4 · Answer 2 · answered May 24 '15 at 09:57

qsort(input, stringLen, sizeof (char*), myCompare) calls myCompare to compare strings which are sorted.

myCompare gets pointers to compared values. In our case we get pointers to strings (const char**). So we should compare *(const char**)a and *(const char**)b, which are strings pointed by a and b.

score 2 · Answer 3 · answered Feb 21 '13 at 02:05

2

Start debugging with:

int myCompare (const void * a, const void * b ) {
  const char *pa = (const char*)a;
  const char *pb = (const char*)b;

  printf("Comparing %s vs. %s for result %d\n", pa, pb, strcmp(pa,pb));

  return strcmp(pa,pb);
}

I think shortly after that, you'll figure out the problem. :)

answered Feb 21 '13 at 02:05

abelenky

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nothing is wrong. it return a correct value for every a and b – jack stov Feb 21 '13 at 02:10

qsort did not sort the array of string

3 Answers3

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