Diagonal/Symmetric matrix

Hi, I'm new to matlab. I wanted to make a program that takes a nxn matrix and outputs a symetric matrix such that the elements are the average of the 2 diagonals elements of the original matrix. I suceeded with a 3x3, but it won't work for a nxn. Here's my code so far.

clear

clc

PA = [81 3 15;

43 67 90;

22.5 10 68]

for i=1:length(PA)

OD(i,i) = PA(i,i);

end

for i=2:length(PA)

OD(1,i) = (PA(1,i) + PA(i,1))/2;

OD(i,1) = OD(1,i);

end

OD(2,end) = (PA(end,2)+PA(2,end)) / 2;

OD(end,2) = OD(2,end)

Answer 1

[John D'Errico]

I don't even think your code works for a 3x3 matrix. The matrix it produces is

OD
OD =
           81           23        18.75
           23           67           50
        18.75            0           68

Give the matrix PA, note that the (3,2) element does not seem to be correct, at least by my guess as to what you really want to do.

However, I think what you may be asking is how to do the following computation:

OD = (PA + PA.')/2
OD =
           81           23        18.75
           23           67           50
        18.75           50           68

I'm not sure why you would want to write a complicated routine using loops to do exactly that. Note that what I wrote will work for any square matrix. And it took one line of code.

[Kimhim Chhay]

Thank you. I didn't even realize I was adding the transpose.

[John D'Errico]

This is the standard way in MATLAB to symmetrize a matrix, having learned it on the order of 30 years ago.