sum in multidimensional arrays matlab

Question

I have an I x K x V array, where V = S x R, ie, the third dimension has "S" observation for "R" different categories. I would like to end up with an I x K x S that has, in the third dimension, the sum across R within each S.

e,g: I = 3, K = 3, S=2 and R=2, I want to end up with a matrix C that is 3x3x2 that sums the third dimension in the following way.

A = [5 7 8; 0 1 9;4 3 6]; 
A(:,:,2)=[1 0 4; 3 5 6;9 8 7]
A(:,:,3)=[3 2 1 ; 4 5 6; 3 4 5]
A(:,:,4)=[1 2 3 ; 3 4 5; 5 6 7]
C=A(:,:,1)+A(:,:,2)
C2=A(:,:,3)+A(:,:,4)
C(:,:,2)=C2

I cannot do this manually b/c R and S are very large in my "real" case.

Thanks!

You say you want to "sum across R" (fourth dimension), but then you say you want a matrix "that sums the third dimension". I consider both cases in my solution — Luis Mendo
– Luis Mendo, Commented Nov 24, 2013 at 3:05

Luis Mendo · Accepted Answer · 2013-11-24 03:08:08Z

1

Separate the S and R dimensions with reshape, and then sum across the third dimension, which is S:

I = 3; K = 3; S = 2; R = 2;
C = squeeze(sum(reshape(A,[I K S R]),3));

If you want to sum across R, that's the fourth dimension:

C = sum(reshape(A,[I K S R]),4);

edited Nov 24, 2013 at 3:08

answered Nov 24, 2013 at 2:50

Luis Mendo

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1 Comment

user3026357 Over a year ago

I am sorry if I lead to confusion, A was meant to be constructed with only three dimensions, so your first answer (reshaping first) is what I needed. Works beautifully, thank you!

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