MATLAB: How to use firs value from array column when threshold is reached without for loop

2d arrayarrayfor loopif statement

Hello Everyone,
I have the following problem I am trying to solve. The situation is like this, if we look at the column there can’t be two numbers in a row that are above or below predefined threshold value. For example if we have array D:
D=-2 2 3 -1 -5
      1 4 -2 3 4
      5 3 -4 6 -5
     -7 -2 1 4 -1
And we use threshold 0, then it should fulfill this logic
If D>0
B=D
And other values of B are zeros till D<0, then
B=D;
And other values of B are zeros till D>0, and so on.
As in our example final result should look like this:
   B=0 2 3 0 0
     1 0 -2 3 4
     0 0 0 0 -5
    -7 -2 1 0 0
The idea is to track when the threshold value is crossed and keep the values of D when threshold was crossed, the result matrix would be used for further calculations. Maybe someone would have an idea how to do this. In reality I would be using big arrays like 100000×20000, thus would be great to avoid for loops.

Best Answer

This actually returns the requested matrix, and follows the description in the question:
>> thr = 0;
>> D = [-2,2,3,-1,-5;1,4,-2,3,4;5,3,-4,6,-5;-7,-2,1,4,-1]
D =
  -2   2   3  -1  -5
   1   4  -2   3   4
   5   3  -4   6  -5
  -7  -2   1   4  -1
>> X = D>thr;
>> Y = D<thr & cumsum(X)>0;
>> Z = [X(1,:); diff(X)>0 | diff(Y)>0];
>> B = zeros(size(D));
>> B(Z) = D(Z)
B =
   0   2   3   0   0
   1   0  -2   3   4
   0   0   0   0  -5
  -7  -2   1   0   0
Compared to the requested output matrix:
>> [0 2 3 0 0; 1 0 -2 3 4; 0 0 0 0 -5; -7 -2 1 0 0]
ans =
   0   2   3   0   0
   1   0  -2   3   4
   0   0   0   0  -5
  -7  -2   1   0   0
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