Question

How to sort an array of structs by multiple columns in C

Answer 1

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

typedef struct {
    int a;
    int b;
    const char *label;
} Item;

Item data[] = {
    {7, 2, "python"},
    {8, 3, "c"},
    {3, 5, "c++"},
    {4, 1, "c#"},
    {3, 2, "java"},
    {7, 1, "go"},
    {1, 2, "rust"},
};

int cmp_items(const void *p1, const void *p2) {
    const Item *x = p1;
    const Item *y = p2;

    if (x->a != y->a)
        return x->a - y->a;   // sort by first column

    return x->b - y->b;       // then by second column
}

int main(void) {
    int count = sizeof(data) / sizeof(data[0]);

    qsort(data, count, sizeof(Item), cmp_items);

    for (int i = 0; i < count; i++) {
        printf("(%d, %d, %s)\n", data[i].a, data[i].b, data[i].label);
    }

    return 0;
}



/*
run:

(1, 2, rust)
(3, 2, java)
(3, 5, c++)
(4, 1, c#)
(7, 1, go)
(7, 2, python)
(8, 3, c)

*/

 

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