How Can I randomly select 20% of my array elements AND save their indices (row,column) ?

I have a matrix A with n points.

A = [230,30,40,40,80;40 40 30 80 230;40 40 40 230 80]; n=numel(A);

How can i now randomly select a percentage of the points (example p=20% of n) ?

The output should be a new matrix with these random elements. AND another array that tells me "which" elements(row, column) were selected.

Any help appreciated :)

Answer 1

[Jan]

A = [230,30,40,40,80;40 40 30 80 230;40 40 40 230 80];
n = numel(A);
Index    = randperm(numel(A), ceil(numel(A) * 0.20));
[Xp, Yp] = ind2sub(size(A), Index);
Selected = A(Index);

This chooses 20% of the values randomly and replies the values in Selected and the indices in Xp, Yp.

[Omar Shehata]

Thank you very much :)

what if I now want to have the "Selected" MATRIX in the same dimension as my data matrix "A". and all data which are not selected could be NaN or 0 then ?

[Jan]

@Omar: What did you try so far?

B        = nan(size(A));
B(Index) = Selected;

Answer 2

[ES]

A = [230,30,40,40,80;40 40 30 80 230;40 40 40 230 80]; 
n=numel(A);
B=reshape(A, 1, n); %1-D array
p = randperm(n);%random permutation of numbers till n

k = p(1:ceil(numel(p)/8));%Very poor algo for 1st to 20% of elements. You can improve here
  C=B(p);%Target Matrix