Question

Normally sorting Array of structure objects is easy. Consider an Array of a structure (AOS)

#define ITEMS 10

typedef struct MyStruct
{
 char a;
 int  b;
}tMyStruct;

tMyStruct arr_mystruct[ITEMS];

I first fill this array of structure with values of < a , b> pairs.

If I now want to sort this array of structures according to the integer field, I can do it using libc qsort function by using a comparison function which takes two integer arguments.

Now consider that I replace the above structure in AOS format with SOA format

#define ITEMS 10

typedef struct MyStruct
{
 char a[ITEMS];
 int  b[ITEMS];
}tMyStruct;

tMyStruct mystruct;

Now I can still use qsort to sort the array of integers b field, however this time I need to additionally sort a (array of characters) w.r.t sorting order of b.

So my question is what is a possible efficient way to do sorting for data laid out in SOA format instead of usual AOS format?

Can anyone help me with this ? Thanks!

Answer 1

Probably the best way would be to use a helper array:

int helper[ITEMS];

int helper_cmp(const void *a, const void *b);

for (i = 0; i < ITEMS; ++i)
    helper[i] = i;
qsort(helper, ITEMS, sizeof(*helper), helper_cmp);

The helper_cmp function will take helper values as indices to mystruct.b and compares those values:

int helper_cmp(const void *a, const void *b)
{
    int first = *(const int *)a;
    int second = *(const int *)b;
    int b_first = mystruct.b[first];
    int b_second = mystruct.b[second];
    // compare b_first and b_second
}

Then, after the qsort, the helper array will contain rearranged indices of b, telling how it should be ordered.

You can use this array to both rearrange mystruct.a and mystruct.b.

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Note that this is not as efficient as sorting “array of struct”. I don’t know your application, but my guess is that “struct of array”, in which the fields of the arrays are correlated is not a good idea in the first place. That is, if sorting of b affects sorting of a, they probably belong to the same conceptual element (call it class if you want), and it is better if they are grouped together in one struct.