Trouble with handling values in anti-diagonal of a square matrix

I am trying to create a square matrix whose diagonal and antidiagonal have the same value (let say, 8). However the antidiagonal values in the matrix I create do not act in the way I expect. Here is my code:

function outXmatrix = sqrtmatx(s)   %a function whose input is the dimension of the square matrix
nrows = s;
A = zeros(nrows, nrows);
for c = 1:nrows
    for r = 1:nrows
        for i = 0:(nrows - 1)
        if r == c
            A(r,c) = 8; %Assign value 8 to the diagonal
        elseif r == i+1 && c == nrows-i %Assign value 8 to the antidiagonal
            A(r,c) = 8;
        else    %Assign value 1 to the other elements
            A(r,c) = 1;
        end
        end

    end
    outXmatrix = A;
end

For example, s = 5

My expected matrix is

A = [8 1 1 1 8,
     1 8 1 8 1,
     1 1 8 1 1,
     1 8 1 8 1,
     8 1 1 1 8]

The matrix my function produced is

sqrtmatx(5)

I hope someone could point out the problem in my code. Thanks a lot!

Answer 1

[Dyuman Joshi]

1 - Preallocate using ones() instead of zeros() and remove the else part.

2 - Update the condition for checking the anti-diagonal element, and remove the 3rd for loop.

y = sqrtmatx(5)

y = 5×5

8 1 1 1 8 1 8 1 8 1 1 1 8 1 1 1 8 1 8 1 8 1 1 1 8

y = sqrtmatx(8)

y = 8×8

8 1 1 1 1 1 1 8 1 8 1 1 1 1 8 1 1 1 8 1 1 8 1 1 1 1 1 8 8 1 1 1 1 1 1 8 8 1 1 1 1 1 8 1 1 8 1 1 1 8 1 1 1 1 8 1 8 1 1 1 1 1 1 8

function outXmatrix = sqrtmatx(s)   %a function whose input is the dimension of the square matrix
nrows = s;
A = ones(nrows, nrows);
for c = 1:nrows
    for r = 1:nrows
        if r == c
            A(r,c) = 8; %Assign value 8 to the diagonal
        %% updated check for antidiagonal term
        elseif r + c == s+1 %Assign value 8 to the antidiagonal
            A(r,c) = 8;
        end
    end
    outXmatrix = A;
end
end

[Minh Hoang]

It worked! Thank you so much!

[Dyuman Joshi]

You're welcome!

You could vectorize your code as well. See - eye, flip

Answer 2

[Chuguang Pan]

diagVal=8;
otherVal=1;
nrows=8;

if mod(nrows,2) % odd
    A=(diagVal-otherVal)*eye(nrows) + (diagVal-otherVal)*fliplr(eye(nrows))+...
    otherVal*ones(nrows);
    A(ceil(nrows/2),ceil(nrows/2))=A(ceil(nrows/2),ceil(nrows/2))-(diagVal-otherVal);
else
    % even
    A=(diagVal-otherVal)*eye(nrows) + (diagVal-otherVal)*fliplr(eye(nrows))+...
    otherVal*ones(nrows);
end


disp(A)

     8     1     1     1     1     1     1     8
     1     8     1     1     1     1     8     1
     1     1     8     1     1     8     1     1
     1     1     1     8     8     1     1     1
     1     1     1     8     8     1     1     1
     1     1     8     1     1     8     1     1
     1     8     1     1     1     1     8     1
     8     1     1     1     1     1     1     8