MATLAB: Sum columns of matrix

columnsloopsumsummationvectorization

If I have a 3×6 matrix like this
1 5 2 4 3 2
2 3 1 4 4 1
4 5 2 2 3 1
how can i sum(without a loop) columns 1:2, 3:4, 5:6, to form a new 3×3 matrix?
And if have a 3×12 like this
2 1 2 1 1 5 5 4 4 2 4 5
3 5 1 3 1 2 5 3 3 4 5 4
2 4 2 1 5 5 4 3 2 2 1 5
how can i sum(without a loop) columns 1:3,4:6,7:9,10:12, to form a new 3×4 matrix?
Is there a general formula that will work for both situations?

Best Answer

>> M = [1,5,2,4,3,2;2,3,1,4,4,1;4,5,2,2,3,1]
M =
   1   5   2   4   3   2
   2   3   1   4   4   1
   4   5   2   2   3   1
>> M(:,1:2:end) + M(:,2:2:end)
ans =
   6   6   5
   5   5   5
   9   4   4
>> reshape(sum(reshape(M.',2,[]),1),[],3).'
ans =
   6   6   5
   5   5   5
   9   4   4
And
>> M = [2,1,2,1,1,5,5,4,4,2,4,5;3,5,1,3,1,2,5,3,3,4,5,4;2,4,2,1,5,5,4,3,2,2,1,5]
M =
   2   1   2   1   1   5   5   4   4   2   4   5
   3   5   1   3   1   2   5   3   3   4   5   4
   2   4   2   1   5   5   4   3   2   2   1   5
>> reshape(sum(reshape(M.',3,[]),1),[],3).'
ans =
    5    7   13   11
    9    6   11   13
    8   11    9    8
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