changing specific values in diagonal

Hello,

I'm trying to change the values in the diagonal based on its column.

let says this is my matrix ( 1/ ,2/ ,3/ ,4/ . . . . are the rows and columns number)

	1/	2/	3/	4/	5/	6/	7/	8/	9/	10/	11/	12/
1/	1	1	0	0	1	0	0	0	0	0	0	0
2/	1	1	1	0	0	1	0	0	0	0	0	0
3/	0	1	1	1	0	0	1	0	0	0	0	0
4/	0	0	1	1	0	0	0	1	0	0	0	0
5/	1	0	0	0	1	1	0	0	1	0	0	0
6/	0	1	0	0	1	1	1	0	0	1	0	0
7/	0	0	1	0	0	1	1	1	0	0	1	0
8/	0	0	0	1	0	0	1	1	0	0	0	1
9/	0	0	0	0	1	0	0	0	1	1	0	0
10/	0	0	0	0	0	1	0	0	1	1	1	0
11/	0	0	0	0	0	0	1	0	0	1	1	1
12/	0	0	0	0	0	0	0	1	0	0	1	1

in Diag (XX,-1), I want to change those values to 999:

	1/	2/	3/	4/	5/	6/	7/	8/	9/	10/	11/	12/
1/	1	1	0	0	1	0	0	0	0	0	0	0
2/	999	1	1	0	0	1	0	0	0	0	0	0
3/	0	1	1	1	0	0	1	0	0	0	0	0
4/	0	0	1	1	0	0	0	1	0	0	0	0
5/	1	0	0	0	1	1	0	0	1	0	0	0
6/	0	1	0	0	999	1	1	0	0	1	0	0
7/	0	0	1	0	0	1	1	1	0	0	1	0
8/	0	0	0	1	0	0	1	1	0	0	0	1
9/	0	0	0	0	1	0	0	0	1	1	0	0
10/	0	0	0	0	0	1	0	0	999	1	1	0
11/	0	0	0	0	0	0	1	0	0	1	1	1
12/	0	0	0	0	0	0	0	1	0	0	1	1

How to know which column to change ?

I defined those varibles:

x = 4

y = 3

I should change those:

column 1

column x+1

column 2x+1

The required:

How to do it? are there any different way to make it automatic for any matrix size?

[A Aln]

This is my attempt to change it, but it only works for this matrix

for i = 1:y*x
    for j = 5
        if (j-i) == -1
         if (XX(i,j) == 1)
             XX(i,j)=99;
         end
        end 
    end
end
   

Are there any way to aotumate it ?

[Walter Roberson]

I do not understand why row 3 column 2 is not being changed as well ? Or row 4 column 3 ?

[KALYAN ACHARJYA]

mat=[1 1 0 0 1 0 0	0 0	0 0	0;1 1 1	0 0 1 0 0 0 0 0 0;
    0 1	1 1	0 0	1 0	0 0	0 0;0 0	1 1	0 0	0 1	0 0	0 0;
    1 0	0 0	1 1	0 0	1 0	0 0;0 1	0 0	1 1	1 0	0 1	0 0;
    0 0	1 0	0 1	1 1	0 0	1 0;0 0	0 1	0 0	1 1	0 0	0 1;
    0 0	0 0	1 0	0 0	1 1	0 0;0 0	0 0	0 1	0 0	1 1	1 0;
    0 0	0 0	0 0	1 0	0 1	1 1;0 0	0 0	0 0	0 1	0 0	1 1];
[rows colm]=size(mat);
r1=2:4:rows;
c1=1:4:colm;
mat(r1,c1)=999 % Modification needed for individual elementwise pair of r1 and c2

[Walter Roberson]

ind = sub2ind(size(m), r1, c1);
mat(ind) = 999;

Answer 1

[Image Analyst]

Sounds like homework. Some hints: check out toeplitz(), circshift(), and eye()