MATLAB: How to extract rows based on column values in a matrix

accumarrayindexingmatrix manipulations

I am working with data that is in a 152867×2 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

Best Answer

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,:);
Related Question