# Sum alternaing columns of a matrix

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Cecilia Martinelli on 12 May 2021
Edited: DGM on 12 May 2021
Hi,
I have a matrix 191x4034 and I would like to sum alternating columns and put the results in a vector, but I can't understand how to do that.
It would be something like that
A= 1 3 4 5
3 6 7 8
5 6 7 9
B = (9, 18, ...) % 1st column = 9, 3rd column = 18, 5th column = ...
C = (15, 22, ...) % 2nd column = 9, 4th column = 18, 6th column = ...
Thanks for the help

DGM on 12 May 2021
Edited: DGM on 12 May 2021
Something like
A = randi(9,3,10)
a0 = sum(A,1) % show all column sums as an example
a1 = sum(A(:,1:2:end),1) % odd col sums
a2 = sum(A(:,2:2:end),1) % even col sums
gives
A =
8 2 6 9 3 8 9 3 5 2
9 5 9 1 2 5 4 5 4 4
9 8 9 4 2 9 4 3 3 5
a0 =
26 15 24 14 7 22 17 11 12 11
a1 =
26 24 7 17 12
a2 =
15 14 22 11 11
If you need one or the other, just pick the line you need. If you need to calculate both even and odd column sums, it's generally going to be faster to do it like this, especially for large arrays.
a0 = sum(A,1); % get all column sums at once
a1 = a0(1:2:end); % sort out odd cols
a2 = a0(2:2:end); % sort out even cols