How to use qsort for an array of strings?

Question

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

int sortstring(const void *str1, const void *str2) {
    const char *rec1 = str1;
    const char *rec2 = str2;
}

void sortutil(char* lines[]) {
    qsort(lines, 200, sizeof(char), sortstring);
}

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

int getarray(char *lines[]) {
    int i = 0;
    char *text = (char *)malloc(200);
    while (fgets(text, 200, stdin) != NULL) {
        lines[i] = text;
        i++;
        text = (char *)malloc(200);
    }
    return i;
}

void printarray(char *lines[], int max) {
    for (int i = 0; i < max; i++)
        printf("%s\n\n", lines[i]);
}

int main(int argc, char* argv[]) {
    char* arr[100];
    int numlines = getarray(arr);
    printf("There are %d lines\n", numlines);
    printarray(arr, numlines);

    for (int i = 1; i < argc;  i++) {
        if (strcmp(argv[i], "-s") == 0) {
            sortutil(arr);
            printarray(arr, numlines);
        }
    }
}

When I send in a file with arbitrary text, It'll read the file and print it out, but when i call -s and call the qsort function, it comes back with nulls. I'm sure I am using qsort incorrectly, what is the right way to use it for an array to char pointers?

WhozCraig · Accepted Answer · 2014-04-21 02:59:22Z

9

Your comparator is being sent each pair by-address. I.e. they're pointer-to-pointer-to-char.

Change the comparator to:

int sortstring( const void *str1, const void *str2 )
{
    char *const *pp1 = str1;
    char *const *pp2 = str2;
    return strcmp(*pp1, *pp2);
}

Likewise, your sortutil needs to know the number of items being sorted, as well as pass the correct size of each item. Change that to:

void sortutil(char* lines[], int count)
{
    qsort(lines, count, sizeof(*lines), sortstring);
}

Finally, the call from main() should look like this:

sortutil(arr, numlines);

That should do it.

edited Apr 21, 2014 at 2:59

answered Apr 21, 2014 at 1:54

WhozCraig

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2 Comments

Jakkie Chan Over a year ago

thanks that did work, only thing you missed was typecasting str1 and str2 with (const char**)

WhozCraig Over a year ago

@user3427042 actually, I missed the right type in the posted code (duh). It should have been what it is now. Sorry about that. If done correctly, no cast should be needed. Thanks for catching that. Glad it helped.

imreal · Accepted Answer · 2014-04-21 01:54:55Z

5

What the compar function gets are pointers to the elements in your array, which in this case, are pointers to char. So the parameters str1 and str2 are actually pointers to pointers to char. You must cast them like this:

int sortstring( const void *str1, const void *str2 )
{
    const char *rec1 = *(char**)str1;
    const char *rec2 = *(char**)str2;
    int val = strcmp(rec1, rec2);

    return val;
}

Then you have to use the proper element size in qsort.

qsort(lines, 200, sizeof(char*), sortstring);

edited Apr 21, 2014 at 1:54

answered Apr 21, 2014 at 1:48

imreal

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1 Comment

imreal Over a year ago

It could be because lines does not have 200 elements, maybe you can pass numlines as a parameter to sortutil and use it instead of 200.

R Sahu · Accepted Answer · 2014-04-21 01:47:03Z

3

This line is incorrect.

    qsort(lines, 200, sizeof(char), sortstring);

Change it to

    qsort(lines, 200, sizeof(char*), sortstring);

answered Apr 21, 2014 at 1:47

R Sahu

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