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Charms
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grouping numbers in matrix

Asked by Charms
on 23 Oct 2019
Latest activity Commented on by Akira Agata
on 23 Oct 2019
Hi, I have 2 matrices
A = [ 1, 2, 3, 2, 2, 3, 3, 1, 1]
B = [34, 20, 23, 31, 30, 33, 44, 22, 28]
and would like to group them according to their numbers in A. The answer should be a 3 x 3 matrix (A corresponds to column number).
Thank you! :)

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2 Answers

Answer by Akira Agata
on 23 Oct 2019
 Accepted Answer

More generalized solution would be:
C = splitapply(@(x){x'}, B, A);
If each group has the same number of elements, the following can generate 2D matrix.
B2 = cell2mat(C);

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Charms
on 23 Oct 2019
Hi Akira,
Could you break down/explain the first code? Am a new matlab learner :) Also for cell2mat, is there another shortcut i can use to group if they do not have the same number of elements? (I still want the 2D matrix)
Akira Agata
on 23 Oct 2019
Hi Charms-san,
Thank you for your response.
Yes, splitapply is somehow "tricky" function. But, at the same time, this is quite useful and powerful function. So I would recommend accessing the following documentation page. I believe the "Example" section will be useful to uderstand how splitapply works!
Regarding the second question, I believe padding with 'NaN' or something will be needed to adjust number of elements for each group and make the 2D matrix, when they do not have the same number of elements.

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Answer by Daniel M
on 23 Oct 2019

This is not a generalized solution, and assumes that there will always be three entries of each number (1, 2, 3) in A.
[~,sortOrder] = sort(A);
B2 = reshape(B(sortOrder),3,3)';

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