Randomly changing elements in an array to NaN's using a for loop

I want to be able to change a random 10% of an array to an NaN value, and save it to the matrix I have set up.

So far I have:

a = randn(100,1);

b = repmat(a,1,10);

Where, the matrix is 10 columns of the same numbers. In each column I would like to randomly change 10% of those values to NaN's and have it so these random elements do not overlap (i.e. having two or more consecutive NaN's). I want to do this for all 10 columns, so I am looking to use a for loop. I would like to have all the NaN's and random data in the matrix at the end.

Please help! Thank you in advance!

Answer 1

[Sebastian Castro]

I ran through this for one column. A while-loop worked for me

The basic algorithm is:

1. Generate a random index
2. Check if the index is valid, i.e., it is not already set to NaN and doesn't have a consecutive one already added
3. If valid, set that element to NaN
4. Else, regenerate another index
5. Keep going until the number of NaNs is 10

a = randn(100,1);

numElems = size(a,1);
numNans = numElems*0.1; % Number of NaNs is 10%
count = 1;
while count <= numNans

      % Generate random number
      idx = randi(numElems); 

      isIdxValid = false;
      if ~isnan(a(idx)) % Check if index has already been set
         % Check for NaNs in consecutive locations
         if idx==1          % beginning of vector
            if ~isnan(a(idx+1))
                isIdxValid = true;
            end
         elseif idx==numElems % end of vector
            if ~isnan(a(idx-1))
                isIdxValid = true;
            end
         else                 % middle of vector
            if ~isnan(a(idx+1)) && ~isnan(a(idx-1))
                isIdxValid = true;
            end      
         end
      end

      % If the index is valid, set that element to NaN      
      if isIdxValid
          a(idx) = NaN;
          count = count+1;
      end

end

% Display stuff at the end
a
nnz(isnan(a))

[Rebecca Johnson]

Thank you Sebastian!!! Much appreciated!