Question

No matter how I try, I can’t seem to create a nice and clean algorithm for doing the following:

1. System.Array (or generic List) data of System.Object
2. System.Array (or generic List) xval of System.Object
3. System.Array (or generic List) idxs of System.Int32

xval and idxs contain the same number of elements and idxs contains no values less than zero or larger than data.Length

I need to insert all elements in xval into the data array, where the associated integer in idxs represents the insertion index in the original, unmolested data.

For example:

*data* = {A, B, C, D, E, F, G, H}
*xval* = {U, V, W, X, Y, Z}
*idxs* = {5, 0, 0, 0, 1, 1}

*output* = {V, W, X, A, Y, Z, B, C, D, E, U, F, G, H}

It’s easy enough to do, but I always end up with icky code. My best attempt so far (but I’m worried about rounding errors):

  idxs.Reverse()
  xval.Reverse()

  Dim N As Int32 = data.Count + xval.Count
  Dim map(N - 1) As Double
  Dim output(N - 1) As Object

  Dim k As Int32 = -1
  For i As Int32 = 0 To data.Count - 1
    k += 1
    map(k) = i
    output(k) = data(i)
  Next
  For i As Int32 = 0 To xval.Count - 1
    k += 1
    map(k) = idxs(i) - ((i + 1) * 1e-8)
    output(k) = xval(i)
  Next

  Array.Sort(map, output)

Answer 1

Here is my attempt, which doesn’t use IndexOf, Sort, Reverse, dictionaries, or doubles.

// input data
char[] data = { 'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H' };
char[] xval = { 'U', 'V', 'W', 'X', 'Y', 'Z' };
int[] idxs = { 5, 0, 0, 0, 1, 1 };

// algorithm starts here
var output = new List<char>(data);
for (int i = 0; i < idxs.Length; i++)
{
    // Insert the i’th item in the right place
    output.Insert(idxs[i], xval[i]);

    // Increment all the following indexes so that they
    // will insert in the right place
    for (int j = i + 1; j < idxs.Length; j++)
        if (idxs[j] >= idxs[i])
            idxs[j]++;
}

// outputs V, W, X, A, Y, Z, B, C, D, E, U, F, G, H
Console.WriteLine(string.Join(", ", output));

Of course, if you don’t want the algorithm to modify the idxs array, then simply make a copy of it first.