Summing common elements of a matrix together

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Vinay Pandey
Vinay Pandey 2012-3-21
Suppose I have a matrix
A = [1 1 2 2 3 3 3]
and
B = [1 2 3 4 5 6 7]
I want to create a matrix which sums the elements of B, by grouping them according to A, which means:
sum = [3 7 18]
which we got by: from B, we took 1, 2 because A matrix tells us that first two elements belong to first group, similarly 3, 4 for second and 5, 6, 7 for the third. Is there a non-loop way to do this?

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采纳的回答

Oleg Komarov
Oleg Komarov 2012-3-22
accumarray(A.',B.')
  2 个评论
Geoff
Geoff 2012-3-22
I've always used the syntax A' to transpose, but just checked the help for transpose() and see I'm apparently wrong! Is A' just an alias for A.', or is there some obscure difference?
Oleg Komarov
Oleg Komarov 2012-3-22
You're not wrong, it just matters with complex numbers:
' -> ctranspose: complex conjugate transpose
.' -> transpose

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更多回答（1 个）

Onomitra Ghosh
Onomitra Ghosh 2012-3-21
May be something like:
A = [1 1 2 2 3 3 3];
B = [1 2 3 4 5 6 7];
C = [];
for idx = unique(A)
C(end+1) = sum(B(A==idx));
end
C
  2 个评论
Onomitra Ghosh
Onomitra Ghosh 2012-3-22
I have not tried it; it might be difficult to get rid of the loop because it is not straightforward vectorization.

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