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
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