Trying to change columns in a Matrix

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!!

Answer 1

[Dave B]

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

[Justin Noland]

Oh ok. The linear indexing is a cool feature. Im coming from c++, and java, and so im a little new to matlab. Thanks for the help!!!

[Dave B]

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++)