Adding value of a vector to another using Octave/Matlab

Question

I have a vector of values I need to add to a second vector at indices specified by another vector. How do I accomplish this using Octave/Matlab?

EDIT: v1 = [1 2 3 4]

v2 = [0 0]

indices = [1 2 1 2]

output = [4 6]

The first and third elements of v1 are added to index 1 of v2, and second and fourth element of v1 are added to second element of v2.

Answer 1

I think this is what you mean (if you provide a small example in your question it's easier to understand).

You have a vector of values

toAdd = 1:5;

You have a second, bigger vector:

bigVector = 1:10;

You want to do bigVector + toAdd , where you add the elements of toAdd at specific indices into bigVector , specified by:

indices = [1 3 5 7 9];

That is, you want the output vector:

[ bigVector(1)+toAdd(1);
  bigVector(2);
  bigVector(3)+toAdd(2);
  bigVector(4);
  bigVector(5)+toAdd(3);
  ....
 ]

In that case, you can do the following:

outputVector = bigVector;
outputVector(indices) = bigVector(indices) + toAdd;

In particular, notice the outputVector(indices) and bigVector(indices) , which selects the elements of outputVector and bigVector specified by the vector indices .

Answer 2

This should do:

for k=unique(indices),
    v2(k) = v2(k) + sum(v1(indices==k));
end

Answer 3

It's......

v2 = accumarray(indices, v1)