How to make a 31x31 matrix of zeros with 300 1's in random position

x1 = randi([0, 1], [31,31]); %random grid

I have this so far, but I want only 300 1's and not a random amount.

Answer 1

[DGM]

I'm sure this can be cleaned up, but here's one way.

sz = [31 31];
X = zeros(sz); % an array of zeros
X(1:300) = 1; % put 300 ones in there
X = reshape(X(randperm(prod(sz))),sz) % shuffle it

X = 31×31

0 0 1 1 0 0 0 0 0 1 0 0 0 0 0 0 1 1 0 0 0 1 0 0 0 1 0 1 0 0 1 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 0 1 0 0 0 0 1 0 1 0 0 0 0 0 0 0 1 0 0 0 0 1 0 1 1 1 0 0 0 0 0 0 1 0 0 0 1 1 0 0 1 0 1 0 1 1 0 0 0 0 0 1 1 1 1 0 0 0 0 1 0 1 0 1 0 0 0 0 1 0 1 0 0 1 1 0 0 0 0 1 0 1 0 1 0 0 0 0 1 0 0 1 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 1 0 1 0 0 1 1 0 0 1 0 1 0 1 1 0 1 0 0 0 0 1 1 1 1 0 0 0 1 1 1 1 0 1 1 0 0 0 0 0 0 0 0 0 1 0 1 1 1 0 0 0 0 1 0 0 0 0 0 0 0 0 1 0 0 1 0 1 1 0 1 0 0 0 1 1 1 0 0 0 0 1 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0 1 1 0 0 0 0 0 0 0 0 1 0 1 1 0 1 1 0 0 1 0 0 0 0 0 0 0 1 0 0 1 0

nnz(X) % check number of nonzero elements

ans = 300

[Bilese]

Thank you!