How to change the elements by different values in a matrix simultaneously in Octave?

Question

I want to change the individual elements in a matrix by different values simultaneously. How do I do that?

For example: I want to change the first element in matrix A by certain amount and the second element by a different amount simultaneously.

{ A = [1; 2]

% instead of doing A(1) = ..... A(2) = .....

}

Answer 1

You can access the elements of a vector or matrix and replace them.

For a vector this is intuitive.

octave:16> A = 1:9
A =

   1   2   3   4   5   6   7   8   9

octave:17> A([1 3 5 7 9]) = 0
A =

   0   2   0   4   0   6   0   8   0

This can be done for a matrix as well. The elements of a matrix are arranged in a column-first manner. You can use a single index to access the elements of a matrix.

octave:18> A = [1 2 3; 4 5 6; 7 8 9]
A =

   1   2   3
   4   5   6
   7   8   9

The 2nd element of A is the same as A(2, 1). The 4th element of A is the same as A(1, 2).

octave:21> A(2)
ans =  4
octave:22> A(4)
ans =  2

So, you can set all the odd elements of A to 0 in one go like this:

octave:19> A([1 3 5 7 9]) = 0
A =

   0   2   0
   4   0   6
   0   8   0