How to sort two-dimensional array using qsort() in C

Question

I want to sort the two-dimensional array of integers "d" according to the first column in decreasing order, using qsort(). However, I want each element of the first column to match the element that was in the second column in the same line on the beginning.

For example:

Array in the beginning:

column1: { 4, 5, 1, 3, 0}
column2: { 1, 2, 3, 4, 5}

and the result should be:

column1: { 5, 4, 3, 1, 0}
column2: { 2, 1, 4, 3, 5}

I have written the code for sorting an one-dimensional array, but I can't figure out how to do that for the two-dimensional one.

int main(){

    FILE *file_in, *file_out;

    file_in=fopen("file.in", "r");
        fscanf(file_in, "%d", &N);

    for( i = 1; i <= N; i = i + 1 ){
            fscanf(file_in, "%d", &a);

            fscanf(file_in, "%d", &b);

            fscanf(file_in, "%d", &c);

            d[i][1] = a+b+c;
            d[i][2] = a*b*c
    }
    fclose(file_in);

    int cmpfunc (const void * a, const void * b) {
        return ( *(int*)b - *(int*)a );
    }
    qsort(d, N, sizeof(int), cmpfunc);

    file_out=fopen("file.out","w");
    fprintf(file_out, "%d", M);
    for ( i=1; i<=N; i = i + 1){
        fprintf(file_out, "%d", d);
    }
    fclose(file_out);
    return 0;
}

Thank you in advance for your help.

If the two columns are always treated a pair, one option could be to make it a one-dimensional array of struct containing two fields. — Arkku
– Arkku, Commented Jan 1, 2019 at 0:44
Also, why do your arrays contain float when you treat the contents as int everywhere? Or, alternatively, why is everything else (including format strings) treating them as int? In particular, a - a * (b/3000) - c * a /40 is very unlikely to give the results you expect when a, b, and c are int. — Arkku
– Arkku, Commented Jan 1, 2019 at 0:45
Also, your range comparisons are impossible: while (a < 1 && a > 1000000000) continues as long as a is less than 1 and at the same time greater than 1000000000… I'm sorry, but I'm tempted to say that you need to fix a lot of other stuff before qsort even becomes relevant. — Arkku
– Arkku, Commented Jan 1, 2019 at 0:48
Make the size of your element be the size of a row. Not super efficient, but works. Alternatively, allocate an array of indices, sort them according to the contents of the first column. Then allocate another buffer the size of the original array and copy each row into the proper location. — Mad Physicist
– Mad Physicist, Commented Jan 1, 2019 at 0:59
@Arkku arrays were declared as float by mistake because I had tried something else earlier. You are also right about the while, I confused the data check conditions. Thank you for your advice. — Giannis D
– Giannis D, Commented Jan 1, 2019 at 1:33

rcgldr · Accepted Answer · 2019-01-01 21:10:27Z

2

C doesn't allow the assignment of arrays. You could create an array of pointers, sort the pointers according to one of the arrays, the reorder the arrays according to the pointers using memcpy or a loop to move rows (the link below shows an efficient way to do this). (Using an array of indexes would require the compare function to be able to reference an array's elements only given the indexes.)

The second part of the answer in this thread shows the pointer method. For qsort(), the compare function input parameters would be pointer to pointer to (the first integer of a) row.

Sorting two arrays based on one with standard library (copy steps avoided)

An alternative would be to use structures, where each structure contains an array, since C does allow structures to be assigned.

edited Jan 1, 2019 at 21:10

answered Jan 1, 2019 at 3:06

rcgldr

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