Create new smaller Matrix by summing every n-rows in old Matrix (without loops)

I've a Matrix like A =

     1     1     1
     2     2     2
     3     3     3
     4     4     4
     5     5     5
     6     6     6
     7     7     7
     8     8     8
     9     9     9

I want this 9x3 Matrix to become a (9/n)x3 Matrix (n=3) where every third row is added up. The result would then be:

B =

    12    12    12       %because 1+4+7
    15    15    15       %because 2+5+8
    18    18    18       %because 3+6+9

Is there a way e.g. to use sum() or so? I justed a loop over all rows but my actual Matrix contains a lot of data which is why that would take too long...

Thanks a lot!

[Andrei Bobrov]

it's homework?

[Tim]

nope, more something like a project...There's this older programme that I've to make more efficient..It handels huge amount of data and only exists of while-and for-loops so far...But I don't have that much experience in this programming style of using vectors and matlab-functions..yet ;-) So thanks a lot (again)!

Answer 1

[Andrei Bobrov]

s1 = size(A);
s2 = [3 3]; % size of the matrix- result
ii = rem((1:s1(1))'-1,s2(1))+1;
[i1,i2] = ndgrid(ii,1:s1(2));
B = accumarray([i1(:),i2(:)],A(:));

or just:

B = sum(reshape(A',size(A,2),3,[]),3)';

[Tim]

I used the second solution! works perfectly!