Extract some rows of the matrix with the find command

Question

Alberto Acri 2023-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/2010052-extract-some-rows-of-the-matrix-with-the-find-command

编辑： dpb 2023-8-19

Hi. I would like to select rows from a 'matrix' array that have values between 0 and 2 in the third column of 'matrix'. I was able to select the desired rows but how can I combine them to create a unique matrix?

matrix = [6	6	5
7	6	0
8	6	10
9	6	3
10	6	7
11	6	0
12	6	2
13	6	0];
for k = 1:height(matrix)
    for selected_number = 0:2
        search = find(matrix(:,3) == selected_number);
        select_row = matrix(search,:);
    end
end

The output should be:

matrix_out = [7	6	0
6	0
6	2
6	0];

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Answer 1

Dyuman Joshi 2023-8-18

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Use ismember with logical indexing -

matrix = [6	6	5
6	0
6	10
6	3
6	7
6	0
6	2
6	0];
idx = ismember(matrix(:,3),0:2);
out = matrix(idx,:)
out = 4×3
   6     0
   6     0
   6     2
   6     0

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Answer 2

dpb 2023-8-18

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https://ww2.mathworks.cn/matlabcentral/answers/2010052-extract-some-rows-of-the-matrix-with-the-find-command#answer_1289472

编辑：dpb 2023-8-19

在 MATLAB Online 中打开

Perfect application for a little utility function I keep around -- as a sidelight, I create and add to my MATLABPATH a "Utilities" folder that contains such tidbits as this; little tools that are handy but not built into base MATLAB distribution...

Very similar to @Dyuman Joshi's solution but not quite as expensive as ismember...

>> out=matrix(iswithin(matrix(:,3),0,2),:)
out =
7     6     0
11     6     0
12     6     2
13     6     0
>>

where iswithin is the utility function to return the logical indexing array. This is an extremely handy construct to be able to throw the details of the comparison into a black box and just return the result even for simple cases like this; it's even more useful in cases with multiple conditions or the like.

>> type iswithin.m
function flg=iswithin(x,lo,hi)
% returns T for values within range of input
% SYNTAX:
%  [log] = iswithin(x,lo,hi)
%      returns T for x between lo and hi values, inclusive
flg= (x>=lo) & (x<=hi);
end
>> 