Question

is there a sensible way to replace x% of each value in matrix/vector with a new value, and have the the element(s) to be changed be selected randomly? That is, in A, if I wanted to change 20% of the values (1 element per existing value) to the value 5, how do I make sure that each of the 5 elements per existing value in A has an equal probability of changing to the new value (e.g. 5)? I would appreciate some guidance on a method to complete the task described above.

Thank you kindly.

% Example Matrix
% M = 5;
% N = 5;
% A = zeros(M, N);
A = [0 0 0 0 0;
     1 1 1 1 1;
     2 2 2 2 2;
     3 3 3 3 3;
     4 4 4 4 4];   

% Example Matrix with 20% of elements per value replaced with the value '5'
A = [0 0 5 0 0;
     1 5 1 1 1;
     2 5 2 2 2;
     3 3 3 3 5;
     4 4 5 4 4];  

Answer 1

Using information from here and here I was able to achieve my objective. The code below will replace x% of each value in matrix with a new value and then randomize its location within that value in the matrix.

M = 5;
N = 5;
A = zeros(M, N);
PC = 20; % percent to change
nCells = round(100/PC); % # of cells to replace with new value
A = [0 0 0 0 0;
     1 1 1 1 1;
     2 2 2 2 2;
     3 3 3 3 3;
     4 4 4 4 4]; 
A2 = A+1; % Pad the cell values for calculations (bc of zero)
newvalue = 6;
a=hist(A2(:),5);% determine qty of each value
for i=1:5
    % find 1st instance of each value and convert to newvalue
    A2(find(A2==i,round(a(i)/nCells)))=newvalue;
end;
out = A2-1; % remove padding
[~,idx] = sort(rand(M,N),2); % convert column indices into linear indices
idx = (idx-1)*M + ndgrid(1:M,1:N); %rearrange each newvalue to be random
A = out;
A(:) = A(idx);