[R] summing subsets of rows matrices

[BXC (Bendix Carstensen)]

What you need is matrix multiplication:

rbind( c(1,1,0,0,0,0),
       c(0,0,1,1,0,0),
       c(0,0,0,0,1,1) ) %*% M

Bendix
----------------------
Bendix Carstensen
Senior Statistician
Steno Diabetes Center
Niels Steensens Vej 2
DK-2820 Gentofte
Denmark
tel: +45 44 43 87 38
mob: +45 30 75 87 38
fax: +45 44 43 07 06
bxc at steno.dk
www.biostat.ku.dk/~bxc
----------------------



> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of 
> Stephen Nyangoma
> Sent: Friday, January 14, 2005 10:18 AM
> To: r-help at stat.math.ethz.ch
> Subject: [R] summing subsets of rows matrices
> 
> 
> 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.
> 	[[alternative HTML version deleted]]
> 
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