Reorder vector using a vector of indices

Question

I\'d like to reorder the items in a vector, using another vector to specify the order:

char   A[]     = { \'a\', \'b\', \'c\' };
size_t ORDER[] = { 1, 0, 2 }         


        

Answer 1

You could do it recursively, I guess - something like this (unchecked, but it gives the idea):

// Recursive function
template<typename T>
void REORDER(int oldPosition, vector<T>& vA, 
             const vector<int>& vecNewOrder, vector<bool>& vecVisited)
{
    // Keep a record of the value currently in that position,
    // as well as the position we're moving it to.
    // But don't move it yet, or we'll overwrite whatever's at the next
    // position. Instead, we first move what's at the next position.
    // To guard against loops, we look at vecVisited, and set it to true
    // once we've visited a position.
    T oldVal = vA[oldPosition];
    int newPos = vecNewOrder[oldPosition];
    if (vecVisited[oldPosition])
    {
        // We've hit a loop. Set it and return.
        vA[newPosition] = oldVal;
        return;
    }
    // Guard against loops:
    vecVisited[oldPosition] = true;

    // Recursively re-order the next item in the sequence.
    REORDER(newPos, vA, vecNewOrder, vecVisited);

    // And, after we've set this new value, 
    vA[newPosition] = oldVal;
}

// The "main" function
template<typename T>
void REORDER(vector<T>& vA, const vector<int>& newOrder)
{
    // Initialise vecVisited with false values
    vector<bool> vecVisited(vA.size(), false);

    for (int x = 0; x < vA.size(); x++)
    {
        REORDER(x, vA, newOrder, vecVisited);
    }
}

Of course, you do have the overhead of vecVisited. Thoughts on this approach, anyone?

Answer 2

To iterate through the vector is O(n) operation. Its sorta hard to beat that.