reshuffle a symmetric matrix

Hi all, I have a large symmetric matrix (600*600)and need to reshuffle the elements randomly while keeping the matrix symmetric.

Could anyone help? I shall be grateful.

Regards, Shafique

Answer 1

[Andrei Bobrov]

1.

sz = size(A);
B = zeros(sz);
vtl = nonzeros(tril(reshape(1:numel(A),sz)));
B(tril(true(sz))) = A(vtl( randperm(length(vtl))));
B = B + tril(B,-1).';

2. More variant - don't modify main diagonal

sz = size(A);
B = diag(diag(A));
vtl = nonzeros(tril(reshape(1:numel(A),sz),-1));
B(tril(true(sz),-1)) = A(vtl( randperm(length(vtl))));
B = B + tril(B,-1).';

3. OR main diagonal - zeros (modify last row of the previous cod)

B = B + tril(B,-1).'

[Muhammad Shafique]

Dear Andrei,

Many thanks for taking the trouble to help. Your solution works great but it would be perfect if the values of main diagonal could be kept from getting into triangles.

Your help is immensely appreciated.

Regards,

Shafique

[Andrei Bobrov]

Hi! Muhhammad! Do you want to modify the elements of the matrix without affecting the main diagonal?

[Muhammad Shafique]

Hi Andrei. I needed to reshuffle the elements only within a triangle but excluding the diagonal. The variant you added does exactly that. Thanks a lot.

[Andrei Bobrov]

Muhhammad! Point 2.