randomly change signs of certain elements in a matrix

Dear All,

I have a matrix of elements like this: a=[1 -1 1 ; 1 1 1 ; -1 -1 1] and i want to change the signs of say 3 or 4 elements of this matrix by random means so that the result could be a=[-1 -1 1 ; -1 -1 1 ; -1 1 1] or anything that changes the signs of 3 or 4 elements in the matrix.

Anyone with idea, please assist, am a beginner in matlab

Thanks

Answer 1

[Stephen23]

Do NOT use a loop for this! You can do this very simply with logical indexing:

>> a = [1,-1,1;1,1,1;-1,-1,1]
a =
   1  -1   1
   1   1   1
  -1  -1   1
>> idx = randperm(numel(a),4);
>> a(idx) = -a(idx)
a =
  -1  -1   1
   1   1   1
   1   1  -1

You can even randomly pick 3 or 4 elements:

   idx = randperm(numel(a),randi([3,4]));

For versions prior to R2011b you will need to return the entire vector and select from that:

vec = randperm(numel(a));
idx = vec(1:4);
a(idx) = -a(idx)

[mac-nic ifegbo]

Thanks so much for assisting