On Dec 10, 2012, at 3:29 PM, Marc Schwartz <marc_schwa...@me.com> wrote:
> > On Dec 10, 2012, at 2:52 PM, David Winsemius <dwinsem...@comcast.net> wrote: > >> >> On Dec 10, 2012, at 11:29 AM, Sam Steingold wrote: >> >>> How do I sum portions of a vector into another vector? >>> E.g., for >>> --8<---------------cut here---------------start------------->8--- >>>> vec <- 1:10 >>>> breaks <- c(3,8,10) >>> --8<---------------cut here---------------end--------------->8--- >>> I want to get a vector of length 3 with content >>> --8<---------------cut here---------------start------------->8--- >>> 6 = 1+2+3 >>> 30 = 4+5+6+7+8 >>> 19 = 9+10 >>> --8<---------------cut here---------------end--------------->8--- >>> Obviously, I could write a loop, but I would rather have a vectorized >>> version. >> >>> tapply(vec, cut(vec, breaks=c(-Inf, breaks), include.lowest=TRUE), sum) >> [-Inf,3] (3,8] (8,10] >> 6 30 19 > > > One gotcha there David, as I think you were on the right track earlier with > findInterval(). The result with this approach, using cut(), takes advantage > of the idiosyncrasy of Sam's example, which uses a sorted vector (1:10) that > is equivalent to the indices of the same vector (1:10). > > If Sam really is using 'breaks' as the indices into 'vec' and not as ranges > for the values to be summed, as cut() does, then findInterval() works: > > set.seed(1) > vec2 <- sample(vec) > >> vec2 > [1] 3 4 5 7 2 8 9 6 10 1 > > > [-Inf,3] = 3+2+1 = 6 > (3,8] = 4+5+7+8+6 = 30 > (8,10] = 9+10 = 19 > >> tapply(vec2, cut(vec2, breaks=c(-Inf, breaks), include.lowest=TRUE), sum) > [-Inf,3] (3,8] (8,10] > 6 30 19 > > > as compared to: > > > 3+4+5 = 12 > 7+2+8+9+6 = 32 > 10+1 = 11 > >> as.vector(sapply(split(vec2, findInterval(seq(along = vec), breaks + 1)), >> sum)) > [1] 12 32 11 Ack, copied and pasted the above with a typo. Same result, but should be: > as.vector(sapply(split(vec2, findInterval(seq(along = vec2), breaks + 1)), > sum)) [1] 12 32 11 Regards, Marc ______________________________________________ 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.
