Adding two sparsematrix treemaps

Question

I'm trying to add two different sparse martices together to achieve one large matrix with all the values from the other matrix. But if they both have a value at a specific key, the values should be added together. What I can't figure out is how to reference the new matrix that is being created so I can put new the new values into it to then return.

Answer 1

I would suggest something like this- note that this is untested code.

public static SparseMatrix add(SparseMatrix a, SparseMatrix b) {
  if (a.rows != b.rows || a.cols != b.cols) {
    // They must be the same dimensions.
    return null;
  }
  return new SparseMatrix(a.rows, a.cols).add(a).add(b);
}

private SparseMatrix add(SparseMatrix a) {
  // Walk all of his.
  for (Integer i : a.matrix.keySet()) {
    // Do I have one of these?
    if (matrix.containsKey(i)) {
      // Yes! Add them together.
      TreeMap<Integer, Double> mine = matrix.get(i);
      TreeMap<Integer, Double> his = a.matrix.get(i);
      // Walk all values in there
      for (Integer j : his.keySet()) {
        // Do I have one of these?
        if (mine.containsKey(j)) {
          // We both have this one - add them.
          mine.put(j, mine.get(j) + his.get(j));
        } else {
          // I do not have this one.
          mine.put(j, his.get(j));
        }
      }
    } else {
      // I do not have any of these - copy them all in.
      matrix.put(i, new TreeMap(a.matrix.get(i)));
    }
  }
  return this;
}