How can I use meshgrid to isolate the center values of a matrix and change them? So if it is 10x10, I want to change the 2x2 at the center. At 20x20, I want to change the 4x4

clc, clear, close all; % clear the command window
size = 10;  % set the size for matrix, can be any number
x = zeros(size); % create matrix

for i = 1:size^2    % open loop for all values in matrix
    if i == 1 % defines first value
        x(i) = 1;
% define fixed points before the points in the middle
    elseif i == (size^2 - size+1)
        x(i) = 3;  % defines the top right corner of any matrix size
    elseif i == (size)
        x(i) = 7;   % defines the bottom left corner of any matrix size
    elseif i == (size^2)
        x(i) = 5; % defines the bottom right corner of any matrix size
% We can now define the rows after the corners are defined
    elseif i >= 1 && i <= size
        x(i) = 8; % Defines first column
    elseif mod(i,size) == 1
        x(i) = 2; % Defines the top row
    elseif mod(i,size) == 0
        x(i) = 6; % Defines bottom row
    elseif i >= (size^2 - size+1) && i <= size^2
        x(i) = 4; % Defines the last column
% The following fills in all center values for the matrix
% This is because the remaining values are not predefined
    else 
        x(i) = 9;
    end
end


disp(x)

This is what I so far. At n = 10 it should have a 2x2 square in the center of value 10

At n = 20 it should have a 4x4 square in the center of value 10

Answer 1

[Shubham]

Hey @codeconstructo,

For modifying the centre values of the matrix, you can index them using array positions. For example, In the case of a 6x6 matrix, check out the following:

A = zeros(6);
A(3:4,3:4)=10

A = 6×6

0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 10 0 0 0 0 10 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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You can take hints from the above example and generalise it for your use case.

I hope this would help!

[codeconstructo]

How could I code it so that it will work for any size matrix?

[Stephen23]

"How could I code it so that it will work for any size matrix?"

For even values of N:

N = 6;
M = N/2;
A = zeros(N);
A(M:M+1,M:M+1) = 10

A = 6×6

0 0 0 0 0 0 0 0 0 0 0 0 0 0 10 10 0 0 0 0 10 10 0 0 0 0 0 0 0 0 0 0 0 0 0 0

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Odd values of N are left as an exercise for the reader.

[Shubham]

Adding on to Stephen's comment,

For your code to work on a matrix of any size, first understand the pattern you wish to have.

Notice you mentioned that you want:

• for n=10 a matrix of 2x2
• for n=20 a matrix of 4x4

Based on your requirement, do you see if you could generalise this? I hope this would give you some hint.