MATLAB: Can this be done without a for loop

matrix manipulation

I have a huge matrix M such that each row is ascending ordered. I find if/where each row exceeds a certain threshold by looking at the last column of cumsum(M>threshold,2).
Rows that do not exceed threshold are left unchanged. For each row that does exceed the threshold, I find the first element on that row that's larger than the threshold and set it to -1. Note that these elements will be on different columns. Then, all elements between -1 and the end of the row are set to zero. For example,
M=[1 2 3 4; 1 4 6 8; 2 6 7 9]; threshold=5;
I need
result=[1 2 3 4; 1 4 -1 0; 2 -1 0 0];
Thanks!

Best Answer

The following will put all elements above your threshold value to 0.
[row, col, index] = find( M > threshold);
M(index) = 0;
This next part will put the -1 at the first element like you want.
for i = 1:length(row)
    if(i > 1)
       if(row(i) < row(i-1))
          %the previous row was larger than the current row, break the loop
          break;
       end
    end
    %found the first element of the row that exceeds the threshold
    M(row(i),col(i)) = -1;
 end
Another option, is to do a loop and look at it row by row.
   [row, col] = size(M);
   for i = 1:row
       index = find(M(i,:) > threshold, 1);
   if(exists(index))
M(i,index) = -1;
   M(i,index+1:end) = zeros(1,col-index);
   end
   end
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