How to extract rows based on column values in a matrix?

Question

qualiaMachine 2015-6-22

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https://ww2.mathworks.cn/matlabcentral/answers/224877-how-to-extract-rows-based-on-column-values-in-a-matrix

评论： Abdelrahman Mohamed 2022-11-24

I am working with data that is in a 152867x2 matrix. The first column contains one of three values ranging from 1-3. The second column, however, has a unique value for each row (see example data below). I would like to know how I can write a program that can extract 3 matrices according to the value of the first column (see example output). I'm new to MATLAB, so explicit instructions would be fantastic. Thanks.

Example Data

3, 0.1234

1, 0.1345

1, 0.1456

2, 0.1567

1, 0.1678

1, 0.1789

Desired Output - Three Matrices

1) 3, 0.1234

2) 2, 0.1567

3) 1, 0.1345

   1, 0.1456
   1, 0.1678
   1, 0.1789

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Kevin Maakomane 2022-8-19

extract=data(row_nu,:)

or

tranpose data using ' operator and extract column wise

baby 2022-9-20

great, thank you, works for me as well.

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Answer 1

Mukul Rao 2015-6-23

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编辑：Mukul Rao 2015-6-24

在 MATLAB Online 中打开

Here are links to a few concepts that would be useful for performing such operations:

1. Matrix Indexing : Using Logicals in Array Indexing

http://www.mathworks.com/company/newsletters/articles/matrix-indexing-in-matlab.html

2. Find array elements based on a specified condition

http://www.mathworks.com/help/matlab/matlab_prog/find-array-elements-that-meet-a-condition.html

Finally, here is an example on how you can implement these concepts. There are many ways to do this, here is one such method.

      %Example matrix
      A = [3, 0.1234
      1, 0.1345
      1, 0.1456
      2, 0.1567
      1, 0.1678
      1, 0.1789];
      %Find indices to elements in first column of A that satisfy the equality
      ind1 = A(:,1) == 1;
      ind2 = A(:,1) == 2;
      ind3 = A(:,1) == 3;
      %Use the logical indices to index into A to return required sub-matrices
      A1 = A(ind1,:);
      A2 = A(ind2,:);
      A3 = A(ind3,:);

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Herbert Nyakoojo 2020-8-15

Thank you people

Abdelrahman Mohamed 2022-11-24

Thanks you Mukul Rao its working

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Answer 2

Steven Lord 2020-11-26

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在 MATLAB Online 中打开

This wasn't an option when the question was asked originally but depending what you're trying to do with those "3 matrices", using some of the functions in the 'Grouping and Binning Data' section on this documentation page may be useful. For example, to compute the mean of values in the second column that have the same value in the first column:

A = [3, 0.1234;
1, 0.1345;
1, 0.1456;
2, 0.1567;
1, 0.1678;
1, 0.1789];
G = groupsummary(A, A(:, 1), @mean)
G = 3×2
    1.0000    0.1567
    2.0000    0.1567
    3.0000    0.1234
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