How to select with several conditions in a matrix

Question

Luisa Andrade 2021-11-23

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https://ww2.mathworks.cn/matlabcentral/answers/1594154-how-to-select-with-several-conditions-in-a-matrix

编辑： Luisa Andrade 2021-11-25

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]';

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Answer 1

Image Analyst 2021-11-23

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https://ww2.mathworks.cn/matlabcentral/answers/1594154-how-to-select-with-several-conditions-in-a-matrix#answer_838749

在 MATLAB Online 中打开

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

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Image Analyst 2021-11-24

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 2021-11-24