vectorization

hi,

is there a quick way to change the values of the diagonal of a matrix, instead of

for i = 1:length(A)
A(i,i) = 0;
end

thx

[Andrei Bobrov]

A-diag(diag(A))

Answer 1

[Dr. Seis]

If A is an NxN matrix:

A(1:N+1:N*N) = 0;

[Dr. Seis]

N = 10000;

A = rand(N);

tic

A(1:N+1:N*N)=0;

toc % Took 0.000532 seconds

A = rand(N);

tic

A(logical(speye(length(A)))) = 0;

toc % Took 0.001379 seconds

A = rand(N);

tic

A = A - diag(diag(A));

toc % Took 0.215796 seconds

Answer 2

[Sean de Wolski]

One way:

A = magic(100); %sample matrix
A(logical(speye(length(A)))) = 0;

Also diag if you're building a matrix