[R] blockwise sums

[Dimitris Rizopoulos]

Hi Lutz,

you could try the following:

blockwisesum <- function(x, n){
  nx <- length(x)
  if(nx%%n) x. <- c(x, rep(0., n*ceiling(nx/n)-nx)) else x. <- x
  x. <- matrix(x., ncol=n, byrow=TRUE)
  rowSums(x.)
}

blockwisesum(1:10, 3)

I hope this helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Doctoral Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/16/396887
Fax: +32/16/337015
Web: http://www.med.kuleuven.ac.be/biostat/
     http://www.student.kuleuven.ac.be/~m0390867/dimitris.htm


----- Original Message ----- 
From: "Lutz Prechelt" <prechelt at pcpool.mi.fu-berlin.de>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, August 31, 2004 2:19 PM
Subject: [R] blockwise sums


> I am looking for a function like
>   my.blockwisesum(vector, n)
> that computes sums of disjoint subsequences of length n from vector
> and can work with vector lengths that are not a multiple of n.
>
> It should give me for instance
>   my.blockwisesum(1:10, 3) == c(6, 15, 24, 10)
>
> Is there a builtin function that can do this?
> One could do it by coercing the vector into a matrix of width n,
> and then use apply,
> but that is cumbersome if the length is not divisible by n,
> is it not?
> Any other ideas?
>
>   Lutz
>
> Prof. Dr. Lutz Prechelt;  prechelt at inf.fu-berlin.de
> Institut fuer Informatik; Freie Universitaet Berlin
> Takustr. 9; 14195 Berlin; Germany
> +49 30 838 75115; http://www.inf.fu-berlin.de/inst/ag-se/
>
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