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

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

[Kevin Maakomane]

extract=data(row_nu,:)

or

tranpose data using ' operator and extract column wise

[baby]

great, thank you, works for me as well.

Answer 1

[Mukul Rao]

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,:);

[Nathan Paul]

How would i do this on a bunch of conditions e.g. I would have to find the indices for 28 different conditions for the column?

[Stephen23]

@Nathan Paul: I answered this in your next question:

http://www.mathworks.com/matlabcentral/answers/283139-extracting-rows-from-a-matrix-based-on-values-in-columns

[Suchet Nanda]

Hi, What would be the simplest way to automate it, e.g, for ind(i), with i=n?

[Herbert Nyakoojo]

Thank you people

[Abdelrahman Mohamed]

Thanks you Mukul Rao its working

Answer 2

[Steven Lord]

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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