[R] summarize a vector

[David Winsemius]

On Aug 10, 2012, at 12:20 PM, Sam Steingold wrote:

> I have a long numeric vector v (length N) and I want create a shorter
> vector of length N/k consisting of sums of k-subsequences of v:
>
> v <- c(1,2,3,4,5,6,7,8,9,10)
>
> N=10, k=3
> ===> [6,15,24,10]
>
> I can, of course, iterate:
>
>> w <- vector(mode="numeric",length=ceiling(N/k))
>> for (i in 1:length(w)) w[i] <- sum(v(i*k:(i+1)*k))
>
> (modulo boundary conditions)
> but I wonder if there is a better way.

Well, using v with parentheses instead of square-brackets might not be  
the right way, since v is not a function.

Consider this alternate (no need to pre-allocate 'w'):

 > w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum)
 > w
  1  2  3  4
  6 15 24 10

-- 

David Winsemius, MD
Alameda, CA, USA