# Populate a matrix using the elements of some columns of another matrix as index and other as values

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FredMat on 6 Jul 2023
Commented: FredMat on 6 Jul 2023
I have a matrix with 4 columns. I want to use the elements of the first 3 columns as index and the element of the 4th column as value to create a new matrix. I also need the values with the same index to be summed. I can do that row by row. However, I was wondering if there is a more elegant way, with no loop.
myData = [1 2 1 1
1 2 2 2
3 1 3 1
1 2 3 1
2 3 2 3
1 2 2 1];
newMat = zeros(3,3,3);
for row = myData'
newMat(row(2),row(3),row(1)) = newMat(row(2),row(3),row(1)) + row(4);
end
newMat

Stephen23 on 6 Jul 2023
Edited: Stephen23 on 6 Jul 2023
" may have multiple values with the same indices. In my case, I need to add the values with the same index"
That is exactly what ACCUMARRAY is for:
A = [1,2,1,1; 1,2,2,2; 3,1,3,1; 1,2,3,1; 2,3,2,3; 1,2,2,1]
A = 6×4
1 2 1 1 1 2 2 2 3 1 3 1 1 2 3 1 2 3 2 3 1 2 2 1
B = accumarray(A(:,[2,3,1]),A(:,4),[],@sum)
B =
B(:,:,1) = 0 0 0 1 3 1 0 0 0 B(:,:,2) = 0 0 0 0 0 0 0 3 0 B(:,:,3) = 0 0 1 0 0 0 0 0 0
That is an odd order to store the indices in. Why not store them in order of the dimensions?
Stephen23 on 6 Jul 2023
For comparison, the loop provided by the OP:
myData = [1 2 1 1
1 2 2 2
3 1 3 1
1 2 3 1
2 3 2 3
1 2 2 1];
newMat = zeros(3,3,3);
for row = myData'
newMat(row(2),row(3),row(1)) = newMat(row(2),row(3),row(1)) + row(4);
end
newMat
newMat =
newMat(:,:,1) = 0 0 0 1 3 1 0 0 0 newMat(:,:,2) = 0 0 0 0 0 0 0 3 0 newMat(:,:,3) = 0 0 1 0 0 0 0 0 0
FredMat on 6 Jul 2023
Hi,
Thanks for the pointer. I guess I didn't search long enough.

Sahas Marwah on 6 Jul 2023
You can use the "sub2ind" function in MATLAB to convert the subscripts of the new matrix into linear indices, and then assign the values directly.
Here is my sample of the code:
myData = [1 2 1 1
1 2 2 2
3 1 3 1
1 2 3 1
2 3 2 3];
indices = myData(:, 1:3);
values = myData(:, 4);
newMat = zeros(3, 3, 3);
linearIndices = sub2ind(size(newMat), indices(:, 2), indices(:, 3), indices(:, 1));
newMat(linearIndices) = values;
newMat
Here is the documentation link to "sub2ind" for reference:
FredMat on 6 Jul 2023
Hi,
Thanks for your answer. It works fine. However, it made me realize that I may have multiple values with the same indices. In my case, I need to add the values with the same index. I modified my code to reflect this constraint.