Picking indices randomly

Suppose I have a binary vector B of size n. I'd like to randomly pick the indices with value 1. How would one go about doing this?

Answer 1

[Sean de Wolski]

idx1 = find(A==1);
idxkeep = randperm(numel(idx1),how_many_you_want);
idx12keep = idx1(idxkeep);

[Walter Roberson]

Note: this requires R2011b or later (I think it is). Before that,

idx1 find(A==1);

idxkeep = randperm(numel(idx1));

idx12keep = idx1(idxkeep(1:how_many_you_want));