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Extracting rows from a matrix based on values in columns

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Nathan Paul
Nathan Paul on 8 May 2016
Commented: Stephen Cobeldick on 16 Oct 2017
Following on from this question
I have a large number of conditions lets say i needed 28 different values in the columns to find, how would i do that without finding the indices in 28 different lines, not sure how to do the for loop? :/

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Answers (1)

Stephen Cobeldick
Stephen Cobeldick on 8 May 2016
Edited: Stephen Cobeldick on 8 May 2016
This is easy using accumarray:
A = [3, 0.1234;
1, 0.1345;
1, 0.1456;
2, 0.1567;
1, 0.1678;
1, 0.1789];
C = accumarray(A(:,1),A(:,2),[],@(n){n});
Which places each collection into one cell of cell array C:
>> C
C =
[4x1 double]
[ 0.1567]
[ 0.1234]
>> C{1}
ans =
0.1345
0.1456
0.1678
0.1789
This will automatically create as many cells as needed in one output variable C. You need to use cell array indexing to access the contents of the cells of C. Note that you should not attempt to create 28 separate variables and access them dynamically in a loop: this is very poor programming practice (sadly much loved by beginners, who think it is great) that is slow, buggy, and obfuscated:

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Maria445
Maria445 on 8 Oct 2017
What if I have more than 1 column? That is, I want A to be a 6-by-4 matrix for example, so to have C{1} = 4-by-3 matrix?
Stephen Cobeldick
Stephen Cobeldick on 16 Oct 2017
@Maria445: make the second input to accumarray the row indices, and change the anonymous function to use those indices to extract the required rows:
>> R = 1:size(A,1);
>> C = accumarray(A(:,1),R.',[],@(r){A(r,:)})

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