randomizing location of N zeros in matrix of ones

I need to make a matrix of ones (say 20X20).

Inside that matrix I need zero to appear randomly (in random locations) N times.

Answer 1

[Matt Fig]

EDIT: Thanks to Jan Simon for nitpicking ;-).

Say you want a 20-by-20 matrix of zeros with 7 ones put in:

N = 7;
A = ones(20);
B = randperm(numel(A));
A(B(1:N)) = 0

[Tali]

thank you so much! (made my day!)

[Jos (10584)]

See http://www.mathworks.com/matlabcentral/fileexchange/10924-ntrue

Answer 2

[Jan]

EDITED: First version completely deleted after Walter found out, that Matt's algorithm works ~30% faster.

For larger matrices RANDPERM is slow. You can use the C-mex Shuffle from the FEX instead: http://www.mathworks.com/matlabcentral/fileexchange/27076

N = 7;
M = 20;
A = ones(M);
A(Shuffle(numel(A), 'index', N)) = 0;

For M=20, N=7 this is 4 times faster, for M=200, N=70 this is even 30 times faster than the method using RANDPERM.

[Tali]

thanks a lot!!! i may need a larger matrix!