I have a large data matrix (4460X3500) and I want to sum row subsets in groups of 10 (bin). Is there an efficient way to do this in R without using loops. Looping takes forever to perform this task!

For example suppose we have the matrix
> matrix(1:12,6,2)
     [,1] [,2]
[1,]    1    7
[2,]    2    8
[3,]    3    9
[4,]    4   10
[5,]    5   11
[6,]    6   12

my problem is to sdum for example:
1. the first and second row 
2. the third and fourth
and
3. the fifth and the sixth.

To obtain

    [,1] [,2]
[1,]    3   15
[2,]    7   19
[3,]   11   23



Thank you.
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