Sorting array of strings with qsort

Question

This program takes an integer value which then determines the amount of strings that can be inputted, once the user has entered the amount of strings that they specified they may input another integer and then input those amount of strings. Once done the program sorts the strings based on their lengths in descending order. However the qsort doesn't work, it ends up outputting how the order of when the strings were originally entered.

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


int sort(const void * a, const void * b){
    size_t fa = strlen((const char *)a);
    size_t fb = strlen((const char *)b);
    return (fa < fb) - (fa > fb);

}

int main(void){

char pointer[100];
int n;
scanf("%d", &n);
char** strings = malloc(n * sizeof(char*));

int i;
for (i = 0; i < n; i++){
    scanf("%s", pointer);
    strings[i] = malloc(sizeof(char) * (strlen(pointer) + 1));
    strcpy(strings[i], pointer);

}

int m;
scanf("%d", &m);
strings = realloc(strings, (n + m) * sizeof(char*));
for (i = n; i < m + n; i++){

    scanf("%s", pointer);
    strings[i] = malloc(sizeof(char) * (strlen(pointer) + 1));
    strcpy(strings[i], pointer);
}

int a;
int g;
int k = m + n;
qsort(strings , a, 100, sort);
for (g = 0; g < k; g++){
    printf("%s", strings[g]);

    printf("\n");


}
}

have you tried debugging this? putting a break point when you hit sort should show you what values are being passed in and if you're handling them correctly. — Chris Turner
– Chris Turner, Commented Mar 2, 2017 at 13:50
Well I'm not entirely sure if I used the qsort correctly. I know the data is getting read in, otherwise it wouldn't print anything. — user7644794
– user7644794, Commented Mar 2, 2017 at 14:07

Chris Turner · Accepted Answer · 2017-03-02 15:52:39Z

1

You're not calling qsort correctly at all.

The 2nd parameter is the number of elements in the "array" that you are sorting. You're currently passing it a which as others have pointed out isn't set to anything. Compiling your code with the "-Wall" option will show you those kinds of errors.

The 3rd parameter is the size of one of the elements in the "array", but you've confused this with the size of an unrelated variable pointer? You could write it like sizeof(strings[0]).

The finished call should look like qsort(strings,k,sizeof(strings[0]),sort);.

But that still won't work because your sort() function is being passed two pointers to elements in your "array" (char **) and you're treating them as two elements of the "array" (char *). So you'd want something like

size_t fa = strlen(*(char **)a);

answered Mar 2, 2017 at 15:52

Chris Turner

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Comments

unwind · Accepted Answer · 2017-03-02 13:51:39Z

0

Well, this:

int a;
int g;
int k = m + n;
qsort(strings , a, 100, sort);

makes no sense at all, a has no value so this is undefined behavior.

Also sort() is broken, it should just be strcmp().

answered Mar 2, 2017 at 13:51

unwind

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5 Comments

Chris Turner Over a year ago

yes the sort() is broken, but you can't replace it with strcmp() because it takes two char * when qsort is calling the sort routine with what amounts to two char **

StoryTeller - Unslander Monica Over a year ago

Note the the OP wants to sort by length, and not lexicogrpahically

user7644794 Over a year ago

So if I put strcmp() instead of my sort() function it will fix my issue? Sorry I'm fairly new to C and I find sorting quite difficult.

Roy Avidan Over a year ago

also, why using char** instead of char*[]?

user7644794 Over a year ago

Well I got it to run without compiler or run time errors the first time so I kept it that way.

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