Select specific number of random ones placed in a zeros matrix

Hi guys, I am trying to make a 1024 by 1024 zeros matrix with specifically 20 ones randomly placed anywhere within the zeros matrix. I feel like it's easy but can anyone help?

Answer 1

[Guillaume]

m = zeros(1024, 1024);
m(randperm(numel(m), 20)) = 1;  %place 20 ones randomly in m.