C sort array of strings

Question

I'm trying to sort a array of strings in C. But it can't run. It seems I misuse qsort. Program crash in calling qsort. How can I fix my code. Should i use const char* [] instead of char[][]? Why?

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

#define N 40
#define MIN 4
#define MAX 10

int generateRandomNumber(int low, int high)/*generate random number between low and high inclusive*/
{
    return rand() % (high + 1 - low) + low;
}

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

    return strcmp(pa, pb);
}

int main()
{
    char words[N][MAX + 1];
    int i, j;
    int length;
    srand(time(NULL));

    for (i = 0; i < N; ++i)
    {
        length = generateRandomNumber(MIN, MAX);
        for (j = 0; j < length; ++j)
        {
            words[i][j] = generateRandomNumber('a', 'z');
        }
        words[i][length] = '\0';
    }

    qsort(words, N, sizeof(char*), comp);

    for (i = 0; i < N; ++i)
    {
        printf("%s\n", words[i]);
    }

    return 0;
}

Start by enabling all your compiler warnings, and by reading the manuals. — Kerrek SB
– Kerrek SB, Commented Nov 24, 2016 at 10:02
1) sizeof(char*) --> sizeof(*words) 2) const char* pa = *(const char**)a; const char* pb = *(const char**)b; --> const char* pa = (const char*)a; const char* pb = (const char*)b; — BLUEPIXY
– BLUEPIXY, Commented Nov 24, 2016 at 10:08
A pointer-to-pointer has no relation what-so-ever to a 2D array. That's why the code doesn't work. — Lundin
– Lundin, Commented Nov 24, 2016 at 10:28

utnapistim · Accepted Answer · 2016-11-24 10:08:05Z

1

You are not casting properly:

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

    return strcmp(pa, pb);
}

should be:

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

    return strcmp(pa, pb);
}

I do not know if there are other problems in your code, but when you cast a void* to a const char** you are sending the processor to access memory addresses that are bogus.

answered Nov 24, 2016 at 10:08

utnapistim

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Comments

freestyle · Accepted Answer · 2018-03-28 09:39:50Z

1

You write your program as words has type char *words[N] (array of pointers to C-strings). But your type is char words[N][MAX + 1]. Its flat array, compiler can translate it to char words[N * (MAX + 1)].

So, if you really want to save type of words then:

qsort(words, N, sizeof(*words), comp);

int comp(const void* a, const void* b)
{
    return strcmp(a, b);
}

edited Mar 28, 2018 at 9:39

answered Nov 24, 2016 at 10:09

freestyle

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5 Comments

Sir Jo Black Over a year ago

I think it's better: return strcmp((char *)a,(char *)b);

Simon Kraemer Over a year ago

@SergioFormiggini You forgot the const in both casts

freestyle Over a year ago

Why? It's redundantly in C. More over, I would write: qsort(words, N, (sizeof *words), (int (*)(void const *, void const *))strcmp);

Sir Jo Black Over a year ago

Doing some tests I've understand that the function comp shall contain: return strcmp(*(char **)a, *(char **)b); and qsort may be: qsort(words, sizeof(words)/sizeof(char *), sizeof(char *), comp);

freestyle Over a year ago

This would be right if words has type char *words[N].

Zebrafish · Accepted Answer · 2016-11-24 10:10:07Z

0

In your string compare function change

const char* pa = *(const char**)a;
const char* pb = *(const char**)b;

to

 const char* pa = (const char*)a;
 const char* pb = (const char*)b;

answered Nov 24, 2016 at 10:10

Zebrafish

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Comments

BLUEPIXY · Accepted Answer · 2016-11-25 01:04:54Z

0

const char* pa = *(const char**)a;
const char* pb = *(const char**)b;

should be:

const char* pa = (const char*)a;
const char* pb = (const char*)b;

and

qsort(words, N, sizeof(char*), comp);

should be:

qsort(words, N, sizeof(words[0]), comp);

the third parameter means the length of the array

edited Nov 25, 2016 at 1:04

BLUEPIXY

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answered Nov 24, 2016 at 10:46

朋00439

1

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