Question

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In-place array reordering?

I have original unsorted array with structures such as:

{D, A, B, E, C}

and array of indexes of original array in sorted order:

{2, 3, 5, 1, 4} // Edited. Then I get {A, B, C, D, E}.

How can I simply rearranged the originial array by an index array?

I can’t create new array and insert elements by index-position.

Answer 1

Here’s a version that works, but note that it does invalidate the indices variable:

void sortItemsBasedOnIndices(char *items[], int indices[], size_t num)
{
    // sort them
    for (int i = 0; i < num; i++)
    {
        int newIndex = indices[i];

        // take out the item at the specified index
        char *newItem = items[newIndex];

        // take out the item in that current position
        char *oldItem = items[i];

        // swap the two items
        items[i] = newItem;
        items[newIndex] = oldItem;

        // now, tell the sorted indices the location of the old item after swap
        for (int j = i; j < num; j++)
        {
            if (indices[j] == i)
            {
                indices[j] = newIndex;
                break; // we only need the first one, so then we're done
            }
        }
    }
}

int main()
{
    char *items[] = { "D", "B", "E", "A", "C" };
    int sortedIndices[] = { 3, 1, 4, 0, 2 }; // 0-based

    int size = 5;
    sortItemsBasedOnIndices(items, sortedIndices, size);

    for (int i = 0; i < size; i++)
    {
        puts(items[i]);
    }
}