how to extract another matrix out of a big matrix

Question

farfar 2017-9-11

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/356126-how-to-extract-another-matrix-out-of-a-big-matrix

评论： farfar 2017-9-11

Hello I have a big matrix including many rows and 2 columns. first column is the wavelengths. I need to extract another matrix out of it based on some specific wavelength value and I dont need the rest of data. for example this is my main matrix:

A=[420 0.23;421 0.26;422 0.25;423 0.24;424 0.28;425 0.27]

but I know that I just need values corresponding to (420 422 425) in the first column , so what i need is:

B=[420 0.23;422 0.25;425 0.27]

how can I ask to extract data for these three specific values in the first column? I cant refer to them by their position in matrix since it is not fix. thanks !

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Answer 1

James Tursa 2017-9-11

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https://ww2.mathworks.cn/matlabcentral/answers/356126-how-to-extract-another-matrix-out-of-a-big-matrix#answer_281120

在 MATLAB Online 中打开

v = [420,422,425];
B = A(ismember(A(:,1),v),:);

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Answer 2

Stephen23 2017-9-11

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/356126-how-to-extract-another-matrix-out-of-a-big-matrix#answer_281121

在 MATLAB Online 中打开

>> A=[420 0.23;421 0.26;422 0.25;423 0.24;424 0.28;425 0.27]
A =
          420         0.23
          421         0.26
          422         0.25
          423         0.24
          424         0.28
          425         0.27
>> idx = ismember(A(:,1),[420,422,425]);
>> A(idx,:)
ans =
          420         0.23
          422         0.25
          425         0.27