How can I select rows with specific column values from a matrix?

Question

I have a matrix train3.

1    2    3    4    5    6    7
2   12   13   14   15   16   17
3   62   53   44   35   26   17
4   52   13   24   15   26   37

I want to select only those rows of whose 1st columns contain specific values (in my case 1 and 2).

I have tried the following,

>> train3
train3 =
    1    2    3    4    5    6    7
    2   12   13   14   15   16   17
    3   62   53   44   35   26   17
    4   52   13   24   15   26   37

>> ind1 = train3(:,1) == 1
ind1 =
   1
   0
   0
   0

>> ind2 = train3(:,1) == 2
ind2 =
   0
   1
   0
   0

>> mat1 = train3(ind1, :)
mat1 =
   1   2   3   4   5   6   7

>> mat2 = train3(ind2, :)
mat2 =
    2   12   13   14   15   16   17

>> mat3 = [mat1 ; mat2]
mat3 =                                                                                                                                         
    1    2    3    4    5    6    7
    2   12   13   14   15   16   17

>>

Is there any better way to do this?

grantnz · Accepted Answer · 2016-11-30 20:22:41Z

2

Presumably you are trying to get mat3 in a single step which you can do with:

mat3 = train3(train3(:,1)==1 | train3(:,1)==2,:)

answered Nov 30, 2016 at 20:22

grantnz

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craigim · Accepted Answer · 2016-11-30 20:32:46Z

2

A more general way to do this would be to use ismember to get all of the rows that match the values in a list:

train3 =[
    1    2    3    4    5    6    7
    2   12   13   14   15   16   17
    3   62   53   44   35   26   17
    4   52   13   24   15   26   37];

chooseList = [1 2];

colIndex = ismember(train3(:, 1), chooseList);

subset = train3(colIndex, :);

subset =

     1     2     3     4     5     6     7
     2    12    13    14    15    16    17

answered Nov 30, 2016 at 20:32

craigim

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