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Trying to change columns in a Matrix

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Justin Noland
Justin Noland on 12 Sep 2021
Commented: Dave B on 12 Sep 2021
So lets say I have an array = [1 2 3 4; 3 3 4 5; 4 4 4 7], a 3 by 4 matrix. I want to change a certain column for a specified row. so lets say I want to change the columns 1,3,2 of the corresponding rows 1,2,3. All of the columns im changing for the corresponding rows will be changed to a value of 10. So my ideal answer when printing out the new matrix should be [10 2 3 4; 3 3 10 5; 4 10 4 7]. I'm trying to not use any loops to save on run time. And I've tried doing something like this: array(1:3, [1,3,2]) = 10. But I got the result [10 10 10 4; 10 10 10 5; 10 10 10 7]. which is not the answer I want. Is it possible to do this without using any loops. Any help would be appreciated thanks!!

Accepted Answer

Dave B
Dave B on 12 Sep 2021
Specifying rows and columns makes MATLAB point to rectangles: a(1:2,1:4) refers to rows 1 and 2, columns 1 through 4, i.e. MATLAB doesn't take pairs of row/column but a range.
To change non-neighboring values you can specify their index.
There are two ways to index values in a matrix, you can do it by row and column or by a linear index. (You don't need to know this to solve your problem, but...) The linear index looks like this:
a = [1 2 3 4; 3 3 4 5; 4 4 4 7];
reshape(1:numel(a),size(a))
ans = 3×4
1 4 7 10 2 5 8 11 3 6 9 12
MATLAB has a nice function called sub2ind to convert rows and columns to linear index, which will let you address the non-neighboring values that you want to change:
ind = sub2ind(size(a),[1 2 3],[1 3 2])
ind = 1×3
1 8 6
With that index in hand, it's easy to adjust the values:
a(ind) = 10
a = 3×4
10 2 3 4 3 3 10 5 4 10 4 7
  2 Comments
Dave B
Dave B on 12 Sep 2021
sub2ind makes it easy but you could also have done something like: (cols-1)*height(a) + rows
(I only mention this because I often do a sub2ind-like or ind2sub-like calculation when I'm in c++)

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