# How to select the row of an element in a vector and put it in the same row for a different matrix?

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lauuser1 on 16 Feb 2016
Commented: lauuser1 on 17 Feb 2016
For example, vector A is a 3x1 and has the value 1 in its third row. I want to then take the sum of the entire vector A and put this sum in a 3x5 matrix B (currently filled with NaN's) in the first column third row. I want to keep doing this for each subsequent column-- so for a second round, if vector A (3x1) has the value 1 in the first row, I want to put the sum of all numbers into the second column first row of matrix B.

Kirby Fears on 16 Feb 2016
Edited: Kirby Fears on 16 Feb 2016
Here's an example of creating an index and using it appropriately:
A = [1;2;3];
B = NaN(3,5);
% find where A is equal to 1
idx1 = (A==1);
% Set B's column 1 to be the sum(A) in the same row where A is 1
B(idx1,1) = sum(A);
Hope this helps.
lauuser1 on 17 Feb 2016
I've figured it out.

dpb on 16 Feb 2016
Well, much is undefined but the indexing is pretty simple...
B(A==1,icol)=sum(A);
Now, you'll have to iterate or otherwise set the column index and regenerate A and it's presumed there will only be a single value of '1' in A (else't you'll get more than one value set in B or none if didn't happen to have any). Also A must be a vector of length<=size(B,1) or you'll also be changing the size of the B array.