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

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

Answer 1

[Stephen23]

" 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:

https://www.mathworks.com/help/matlab/ref/accumarray.html

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]

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]

Hi,

Thanks for the pointer. I guess I didn't search long enough.

Answer 2

[Sahas Marwah]

Hi @FredMat,

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:

https://in.mathworks.com/help/matlab/ref/sub2ind.html

[FredMat]

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.