How can add the rows in n-by-m matrix that have the same indicator in the last column?

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Abdulatif Alabdulatif
Abdulatif Alabdulatif on 6 Jun 2016
Edited: Stephen23 on 6 Jun 2016
Hi,
How can add the rows in n-by-m matrix that have the same indicator in the last column?
For example: 10-by-3 matrix, the third column is the indecator:
253638 3309 2
223389 3309 2
255968 3309 2
243943 33309 2
245568 63309 1
242490 93309 3
268464 23309 2
272665 53309 1
241206 83309 1
259310 13309 2
I want to add all rows (all values in first column with the same indicator) with ( all values in second column with the same indicator).

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Accepted Answer

Jos (10584)
Jos (10584) on 6 Jun 2016
X = [253638 3309 2
223389 3309 2
255968 3309 2
243943 33309 2
245568 63309 1
242490 93309 3
268464 23309 2
272665 53309 1
241206 83309 1
259310 13309 2]
OUT = cell2mat(arrayfun(@(k) [sum(X(X(:,3)==k,[1 2]),1) k], unique(X(:,3)),'un',0))
  2 Comments
Abdulatif Alabdulatif
Abdulatif Alabdulatif on 6 Jun 2016
Thanks for a single line answer!!
Could you add another column which contains how many data points belong to each class?
Jos (10584)
Jos (10584) on 6 Jun 2016
Juts modify the function used by arrayfun
OUT = cell2mat(arrayfun(@(k) [sum(X(X(:,3)==k,[1 2]),1) k sum(X(:,3)==k)], unique(X(:,3)),'un',0))

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More Answers (2)

Azzi Abdelmalek
Azzi Abdelmalek on 6 Jun 2016
Edited: Azzi Abdelmalek on 6 Jun 2016
You can use accumarray function
A=[253638 3309 3
223389 3309 2
255968 3309 2
243943 33309 2
245568 63309 1
242490 93309 3
268464 23309 2
272665 53309 1
241206 83309 1
259310 13309 2]
[ii,~,jj]=unique(A(:,3))
out=accumarray(jj,(1:numel(jj))',[],@(x) {sum(A(x,1:2),1)})
out=[ cell2mat(out) ii]
Walter Roberson
Walter Roberson on 6 Jun 2016
unique() to find the unique indicators and the number of indicators. Allocate an zero array of appropriate size to hold the results. Loop over the unique indicators finding the rows that match and totaling them and storing that in the results array.
accumarray() is not suitable for your purposes because accumarray cannot operator on row vectors of values.
Alternately if you convert the data into a table, you could perhaps use varfun()
  4 Comments
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Abdulatif Alabdulatif
Abdulatif Alabdulatif on 6 Jun 2016
Edited: Abdulatif Alabdulatif on 6 Jun 2016
I got the following error for the second line of the loop. The marix name is y
Index exceeds matrix dimensions.
Error in kmc (line 46)
results(K, :) = sum( y( y(:,end) == ind, 1:end-1) );
what is the issue in this case?

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