Certainly ... but this is of course limited to the few C coded functions available. Back to apply-type stuff for, say, median as a summary statistic.
-- Bert On Fri, Aug 10, 2012 at 3:58 PM, David Winsemius <dwinsem...@comcast.net> wrote: > > On Aug 10, 2012, at 3:42 PM, Michael Weylandt wrote: > >> I wouldn't be surprised if one couldn't get an *apply-free solution by >> using diff(), cumsum() and selective indexing as well. > > > What about colSums on a matrix extended with the right number of zeros. > >> colSums(matrix (c(v, rep(0, 3- length(v)%%3) ) , nrow=3) ) > [1] 6 15 24 10 > > (My experience is that tapply is generally fairly fast anyway, much faster > than apply.data.frame. So I do not lump all *apply solutions in the same > efficiency category.) > > -- > David. >> >> >> Cheers, >> Michael >> >> On Aug 10, 2012, at 5:07 PM, David Winsemius <dwinsem...@comcast.net> >> wrote: >> >>> >>> On Aug 10, 2012, at 12:57 PM, Bert Gunter wrote: >>> >>>> ... 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 >>> >>> >>> I agree that it is more elegant, but I do not get an error or an >>> unexpected result with my method. >>> >>>> N=10 >>>> k=15 >>>> w <- tapply( v ,rep(1:(N/k +1), each=k, len=N ) , sum) >>>> w >>> >>> 1 >>> 55 >>> >>> A different label but the same result. I'm protected from the typical 1:0 >>> problem that seq_along solves by including +1 in the second argument to >>> ":"/seq(). Unless, of course, you set N to a negative number, but that >>> wouldn't make much sense would it, and you get an error from rep() anyway. >>> >>> Best; >>> David. >>> >>>> >>>> -- 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 >>> >>> >>> 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. > > > David Winsemius, MD > Alameda, CA, USA > -- 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.
