How to use qsort for structure members?

Question

I am still learning about C programming, and I am having a bit of an issue with my program.

So my structure is declare in q.h file

#define MAXIMUM_LENGTH 80
struct Tsunami {
    unsigned int day;
    unsigned int month;
    unsigned int year;
    unsigned fatalities;
    double height;
    char location[MAXIMUM_LENGTH];
};

and the function that uses qsort is :

double get_median_height(const struct Tsunami* begin, size_t count)
{
    double median = 0;
    double compare1,compare2;
    struct Tsunami* store = (struct Tsunami*) malloc (sizeof(struct Tsunami) * count);

    for (int i = 0; i < (int)count; i++)
    {
        store[i].month = begin[i].month;
        store[i].day = begin[i].day;
        store[i].year = begin[i].year;
        store[i].fatalities = begin[i].fatalities;
        store[i].height = begin[i].height;
        strcpy(store[i].location, begin[i].location);
    }
    qsort(store, count, sizeof(Tsunami), compare_events);
    if(count % 2  == 0)
    {
        printf("%ld",count);
        compare1 = store[(count/2)].height;
        printf("%lf",compare1);
        compare2 = store[(count/2) +1].height;
        printf("%lf",compare2);
        median = (compare1 + compare2)/2;
    }
    else
    {
        median = store[(count/2)].height;
    }
    free(store);
    return median;
}

My compare_events code is

int compare_events(const void* first, const void* second)
{
    struct  Tsunami* first = (struct Tsunami*)first;
    struct  Tsunami* second = (struct Tsunami*)second;

    return (second->height - first->height);
}

For some reason, it does not help me sort out the value of store.height from smallest to largest. Can someone explain to me why? and how should I use the qsort instead?

Side note: You can simply do `store[i] = begin[i];` instead of assigning each elements one-by-one. — MikeCAT, Nov 18 '20 at 16:22
If all you need to do is `return median` then you only need a temporary array of `height` to sort. — Weather Vane, Nov 18 '20 at 16:53
`return (second->height - first->height);` is incorrect when the 2 heights are within (0...1) as that return 0. Use `return (second->height > first->height) - (second->height < first->height);` — chux - Reinstate Monica, Nov 19 '20 at 12:12
Tip: simplify: `struct Tsunami* store = (struct Tsunami*) malloc (sizeof(struct Tsunami) * count);` --> `struct Tsunami* store = malloc(sizeof *store * count);` — chux - Reinstate Monica, Nov 19 '20 at 12:14
`qsort(store, count, sizeof(Tsunami), compare_events);` --> `Tsunami` is not defined anyplace. Are you using a C++ compiler for C code? — chux - Reinstate Monica, Nov 19 '20 at 12:16
So i have tried whatever that you all have suggested but the issue with my qsort still isn't fixed. qsort did not help to rearrange my array from smallest to biggest. Any reason why? — Milo, Nov 20 '20 at 02:45
@chux-ReinstateMonica: Your comparison code is correct for what the OP wrote, but unfortunately what the OP wrote is backwards. So reverse `first` and `second` in it. — Nate Eldredge, Nov 20 '20 at 04:37
@NateEldredge OK, yet OP's end goal is to find the median. Same regardless of sorting up or down. — chux - Reinstate Monica, Nov 20 '20 at 05:23

score 0 · Answer 1 · answered Nov 20 '20 at 04:44

Your comparison function:

Is backwards. It's supposed to return a number less than zero if first < second, but yours does the opposite. So intuitively it should be return first->height - second->height. However...
As "chux - Reinstate Monica" alluded to in comments, your heights are of type double but the comparison function must return int. So your function implicitly converts their difference to an int, but this conversion always rounds toward zero. Thus any two heights that differ by less than 1 will have a difference less than 1, which will be rounded to zero and qsort will think the two Tsunamis are of equal height, and may put them in the array in any order.

What you really want is something like sign(first->height - second->height). Unfortunately C has no standard sign function, but you can find many possible implementations at Is there a standard sign function (signum, sgn) in C/C++?. (This will also fix another bug, which is that your function will not correctly compare heights whose difference doesn't fit in an int.)

Thank you very much for the explanation. And thank everyone for your help. I managed to resolve the issue. Turn out the issue was indeed in my compare_events code and the way I declare and return the values. Thank you all very much. — Milo, Nov 21 '20 at 17:06

How to use qsort for structure members?

1 Answers1