Cut out all of a square array except for a specified NxN section in the middle.

How can I take the following square array (or any other square array ebtered by the user) and take the center NxN square?

For example:

1 2 3 4

5 6 7 8

9 10 11 12

13 14 15 16

is entered along with specifications for the middle 2x2, so it becomes

6 7

10 11

Here is my code thus far:

function [FinalMatrix] = arraycenter(x, m)
    % arraycenter takes a large square NxN matrix and trims away everything
    % other than the center 'm'x'm' submatrix.
    % x must be a square 2 dimension matrix that is at least 3x3
    % m must be an integer less than or equal to N, and if N is even, m
    % must be even, and vice versa
    
    % Find dimensions of x and check to make sure it is a square, 2D matrix
    columndim = size(x, 1);
    rowdim = size(x, 2);
    pagedim = size(x, 3);
    if columndim < 2 || rowdim < 2 || pagedim ~= 1
        error('Your first matrix entered was not two dimensional. Try again.')
    end
    if columndim ~= rowdim
        error('Your first matrix was not a square. Try again.')
    end
    
    % Make sure m is the correct size
    if m >= columndim || m >= rowdim
        error('m is too large. Try again.')
    end
    
    % Make sure N and m match (N is odd, m is odd; N is even, m is even)
    if rem(rowdim, 2) == 0 && rem(m, 2) == 1
        error('N is even and m is odd. Try again.')
    end
    if rem(rowdim, 2) == 1 && rem(m, 2) == 0
        error('N is odd and m is even. Try again.')
    end
    
    % Perform the operation to find the center matrix

end

As you can see, I have done all the data validation. I am stuck on the actual performance of the task.

Thank you in advance!

Answer 1

[Timo Dietz]

Try this:

rowOffset = (rowdim-m)/2;

columnOffset = (columndim-m)/2;

centerMatrix = x(rowOffset+1:end-rowOffset, columnOffset+1:end-columnOffset);

[Timo Dietz]

Since rowOffset always equals cloumnOffset, you can certainly use one variable, only.

Answer 2

[Bruno Luong]

This works on nd-array, any size (>=m) in all dimension:

i = num2cell(floor((size(x)-m)/2)+(1:m)',2)
FinalMatrix = x(i{:})