Sorting an array using qsort()

Question

I am trying to check if two arrays contain the same elements. Since I don't know if the input file is sorted correctly I want to use qsort() to sort the array first and them compare it against the array cmpvalues.

However I can't get qsort to sort the array like the array cmpvalues. The array where the values are stored should be sorted like the array cmpvalues so that it's a lot easier to compare.

#include <stdio.h>

#define MAXCOLR   14
#define MAXLINE  100
#define MAXCHR  1024   
#define _GNU_SOURCE

typedef struct {
    char color[MAXCOLR];
    int value;
} colorval_t;

int cmpfunc(const void *a, const void *b) {
    int aa, bb;
    aa = *(int *)a;
    bb = *(int *)b; 
    return (aa - bb);   
}

int main(int argc, char **argv) {
    size_t n;
    int cmpvalues[] = {
        65,
         2,
         3,
         4,
         5,
         6,
         7,
         8,
         9,
        10,
        74,
        81,
        75,
        65,
         2,
         3,
         4,
         5,
         6,
         7,
         8,
         9,
        10,
        74,
        81,
        75
    };
    size_t ndx = 0;
    char buf[MAXCHR];
    colorval_t arr[MAXLINE] = {{ .color = "" }};

    FILE *fp = argc > 1 ? fopen(argv[1], "r") : stdin;

    if (!fp) { 
        perror("file open failed");
        return 1;
    }

    while (ndx < MAXLINE && fgets(buf, MAXCHR, fp)) {
        char c;
        if (sscanf(buf, "%13s %d", arr[ndx].color, &arr[ndx].value) == 2)
            ndx++;
        else if (sscanf(buf, "%13s %c", arr[ndx].color, &c) == 2) {
            arr[ndx].value = c;
            ndx++;
        }
    }
    if (fp != stdin)
        fclose(fp);   

    for (size_t i = 0; i < ndx; i++)
        printf("arr[%2zu] : %s %d\n", i, arr[i].color, arr[i].value);

    //sorts the array
    qsort(arr, 26, sizeof(arr[26]), cmpfunc);
    for (n = 0 ; n < 26; n++) {   
        printf("%d ", arr[n].value);
    }

    //checks if arrays have the same element
    int j;
    for (j = 0; j < 26; j++) {
        if (arr[j].value != cmpvalues[j]) {
            printf("wrong");
            break;
        }
    }
    return 0;
}

input:

RED A
RED 2
RED 3
RED 4
RED 5
RED 6
RED 7
RED 8
RED 9
RED 10
RED J
RED Q
RED K
BLACK 10
BLACK J
BLACK Q
BLACK K
BLACK A
BLACK 2
BLACK 3
BLACK 4
BLACK 5
BLACK 6
BLACK 7
BLACK 8
BLACK 9

You pass colorval_t * to qsort, yet your cmpfunc expects int* argument. The cmpfunc does not sort according to your cmpvalues array just only subtracts the ints. You need to change it to compare coloarval_t values and also take the color into account. You need to define _GNU_SOURCE before any #include statement for it to work. You are just lucky sizeof(arr[26]) works and is valid, however I can guess it doesn't mean what you think it means. You have posted the input to your program - please post the output of your program and the expected output of you program. — KamilCuk
– KamilCuk, Commented Dec 23, 2018 at 18:57

Some programmer dude · Accepted Answer · 2018-12-23 18:55:45Z

1

Your comparison function is wrong.

The qsort function passes pointers to the elements in the array you sort, so if you have an array of colorval_t then the arguments are of type colorval_t *.

Since you treat the pointers are int * you have a mismatch, and that will lead to undefined behavior.

That means your comparison function should look something like

int cmpfunc (const void * a, const void * b) {
    colorval_t *first = (colorval_t *) a;
    colorval_t *second = (colorval_t *) b;

    return first->value - second->value;
}

answered Dec 23, 2018 at 18:55

Some programmer dude

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2 Comments

chqrlie Over a year ago

return first->value - second->value; is incorrect: subtracting int values coud overflow, causing undefined behavior. You could instead write: return (first->value > second->value) - (first->value < second->value); which is defined for all arithmetic types.

chqrlie Over a year ago

It is also advisable to avoid removing constness to prevent warnings: use const colorval_t *first = (const colorval_t *)a;...

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