having really hard time with qsort()

Question

i've been trying to sort 2D array with qsort like:

I want to sort it with qsort increasing or decreasing by x value, and if x values are same, i want it to compare the y values. it should be printing like this:

1, 2 - 2, 3 - 2, 4

I tried so hard, please help me.

#include <stdio.h> #include <stdlib.h> struct testt{ int start; int len; }; int comparePlease(const void* p, const void* q) { struct testt* x = (struct testt*)p; struct testt* y = (struct testt*)q; return *x - *y; } int main() { struct testt test[5] = { {40, 20}, {50, 30}, {30, 100}, {50, 35}, {25, 15} }; qsort(test, sizeof(test) / sizeof(test[0]), sizeof(int), comparePlease); }

enter image description here

Where is the code that cals qsort? Please provide a minimal reproducible example. — Jabberwocky
– Jabberwocky, Commented Nov 22, 2020 at 8:04
@MikeCAT i added the error image on post, can u please check it? — userandnuser
– userandnuser, Commented Nov 22, 2020 at 8:15

Jabberwocky · Accepted Answer · 2020-11-22 08:49:49Z

There are many issues:

use struct test everywhere and not struct testt.
qsort(test, sizeof(test) / sizeof(test[0]), sizeof(int), comparePlease) should be qsort(test, sizeof(test) / sizeof(test[0]), sizeof(test[0]), comparePlease)
the struct members are x and y instead of start and len.
your comparePlease function was totally bogous, you cannot naively substract structs, it doesn't make any sense. What would {40,20} - {30,40} be?

You want this:

struct test { int x; int y; }; int comparePlease(const void* p, const void* q) { struct test* lhs = (struct test*)p; // left hand side struct test* rhs = (struct test*)q; // right hand side /* if x values are same */ if (lhs->x == rhs->x) { /* compare the y values */ return lhs->y < rhs->y ? -1 : lhs->y > rhs->y; } else { /* compare the x values */ return lhs->x < rhs->x ? -1 : 1; } } int main() { struct test test[5] = { {40, 20}, {50, 30}, {30, 100}, {50, 35}, {25, 15} }; qsort(test, sizeof(test) / sizeof(test[0]), sizeof(test[0]), comparePlease); }

comparePlease is a quick and dirty fix, there is possibly room form improvement, but it should give you an idea.

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