... or perhaps even simpler: > sz <- function(x,k)tapply(x,(seq_along(x)-1)%/%k, sum) > sz(1:10,3) 0 1 2 3 6 15 24 10
Note that this works for k>n, where the previous solution does not. > sz(1:10,15) 0 55 -- Bert On Fri, Aug 10, 2012 at 12:37 PM, David Winsemius <dwinsem...@comcast.net> wrote: > > 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 > > ______________________________________________ > R-help@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. -- Bert Gunter Genentech Nonclinical Biostatistics Internal Contact Info: Phone: 467-7374 Website: http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb-biostatistics/pdb-ncb-home.htm ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
