Change an element in diagonal of matrix

As example

A=magic(5);
result=diag(A,-1);

How can I change an element of this diagonal array when I don't know their indices?

[Kirby Fears]

The size of diag(A,n) will always be (length(A)-abs(n)) by 1.

The indices of your diagonal array are 1 through 4. They correspond to elements (2,1), (3,2), (4,3), and (5,4) of A. This correspondence can be formulated just in terms of n and the size of A.

Does this help? I'm not sure what information you are seeking.

Answer 1

[Tushar Athawale]

I understand that you want to update the elements stored at a specified diagonal without knowing the element indices.

One way to do this could be to create a new matrix of ones along the specified diagonal with rest of the elements set to 0. This matrix then can be used as a reference to update the elements in original matrix. Try the following code snippet to increment the elements at the specified diagonal by 2:

% out: updated matrix, n: size of the input matrix, shift: diagonal selection

function out = update_diagonal(n,shift)

A=magic(n)
result=diag(A,shift);

% Create matrix of ones along specified diagonal
iden = diag(ones(n-abs(shift),1),shift);

% Modify the elements at location of ones in 'iden'. Here, increment all elements
% along the specified diagonal by 2
 A(iden(:,:)~=0) = A(iden(:,:)~=0)+2;
 out = A;

I hope this answers your question.

Answer 2

[Andrei Bobrov]

e.g.:

A=magic(5);

ii = diag(true(4,1),-1);
A(ii) = 100;

Answer 3

[Sean de Wolski]

The -1st diagonal is 2:n+1:end. You can index into that directly without ever needing diag/tril

n = 5;
x = magic(n);
didx = 2:n+1:numel(x);

% Change third element on -1st diagonal
x(didx(3)) = -100

x =
  17    24     1     8    15
  23     5     7    14    16
   4     6    13    20    22
  10    12  -100    21     3
  11    18    25     2     9