在C中使用qsort（）和Structs

Question

For our exam practice sheet, we have to create a struct using river data. The struct is:

struct river
{
    char name[40]; //name of river max 40 char long
    int length; 
    int drainage_area;
}

And the data is supposed to be organised from largest to smallest river length using a file called data.txt. This file has the following information:

Nile 6650 334900

Amazon 6400 6915000

Yangtze 6300 1800000

Mississippi-Missouri 6275 2980000

Yenisei-Angara-Selenga 5539 2580000

Yellow 5464 745000

Ob-Irtysh 5410 2990000

Congo-Chambeshi 4700 3680000

So I know for using the function qsort(), you need a comparison function. However I'm not so sure how to do this when there's a file involved. I have the following for the comparison function (general comparison function):

int compare_rivers(void *r1, void *r2)
{
    int *_r1 = (int *)r1;
    int *_r2 = (int *)r2;

    if( *_r1 > *_r2)
    {
        return -1;
    }
    if (*_r1 == *_r2)
    {
        return 0;
    }

    return 1;
}

My question is: how to implement the compare function and then the main? I know how to open and read files, but is this different than using arrays?

An explanation would be really helpful! I'm not asking for a full-fledged code. I just want an explanation/example how I would do this! Thanks in advance!

Answer 1

You have to read the file into memory first. Store it as an (unsorted) array of structs.

You probably don't want to sort this array(?) because it contains "large data". Ok in this case it really doesn't, but for all I know it might be the purpose of the assignment to recognize this. qsort will do a lot of data shuffling if you sort whole structs.

So in parallel to this array, you can create an array of struct pointers, each pointing to a struct item of the unsorted array.

Apply qsort on this array of pointers. The comparison function should have the form

int (*compar)(const void* obj1, const void* obj2)

But note that since you are sorting an array of pointers, qsort will actually pass you the address to a pointer . So you'll have to do something like:

  const struct river* r1 = *(const struct river**)obj1;

Then compare the contents in some meaningful way.

Answer 2

When you hold the file in memory as an array of struct river the compare function gets two struct river pointers as parameter. So cast those first and then do your compare logic.

struct river rivers[10]; // filled with the file

qsort(rivers, 10, sizeof *rivers, compare_rivers);

    int compare_rivers(const void *r1, const void *r2)
    {
        const struct river*_r1 = (const struct river *)r1;
        const struct river*_r2 = (const struct river*)r2;

        // do compare logic
    }