Summing common elements of a matrix together
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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?
采纳的回答
Oleg Komarov
2012-3-22
accumarray(A.',B.')
2 个评论
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
2012-3-22
You're not wrong, it just matters with complex numbers:
' -> ctranspose: complex conjugate transpose
.' -> transpose
更多回答（1 个）
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
Vinay Pandey
2012-3-21
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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2012-3-21
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