How to select with several conditions in a matrix

I want to select the data from falling below -1 until it turns positive or crosses zero, at least two consecutive times.

An example to clarify what I mean:

In the following matrix I want to selec the bold numbers:

b = [-0.32 -0.5 -1.1 -1.9 -0.5 -0.1 0.2 NaN -1.20 NaN NaN; -0.32 -0.5 -1.1 -1.9 -0.5 -0.1 0.2 NaN -1.20 -0.9 NaN]';

Answer 1

[Image Analyst]

See if this is what you want:

b = [-0.32 -0.5 -1.1 -1.9 -0.5 -0.1 0.2 NaN -1.20 NaN NaN; -0.32 -0.5 -1.1 -1.9 -0.5 -0.1 0.2 NaN -1.20 -0.9 NaN]'
[rows, columns] = size(b)
bb = [0 0 0 0 0 0 0 0 0 0 0; 0 0 1 1 0 0 0 0 1 0 0]'
for col = 1 : columns
    % Find out where this column is less than -1.
    mask = b(:, col) < -1
    % Find out the indexes where the runs below -1 start at.
    startingIndexes = strfind(mask', [0, 1]) + 1
    % Find out where this column is less than 0.
    mask2 = b(:, col) < 0
    for k = 1 : length(startingIndexes)
        index1 = startingIndexes(k);
        for row = index1 : rows
            if mask2(row)
                index2 = row;
            else
                break; % Went 0 or above so don't increment the second index.
            end
        end
        % Set thos values of bb.
        bb(index1:index2, col) = 1;
    end
end
bb

[Luisa Andrade]

the change is this:

mask = b(1:end-1, col) <= -1 & b(2:end,col)<0

So the result is:

bb=

0 0

0 0

1 1

1 1

1 1

1 1

0 0

0 0

0 1

0 1

[Luisa Andrade]

Thank you very much!

[Image Analyst]

Thanks for accepting. Though I was wondering why the last item, where the value is -1.2, you did not want to mark that location as 1. Is it because it did not go positive before it encountered a NaN?

[Luisa Andrade]

Because of the second condition, "at least two consecutive times" ;)