Evan: You misunderstand the concept of a lagged variable.
Ulrik: Well, yes, that is certainly a general solution that works. However, given the *specific* structure described by the OP, an even more direct (maybe more efficient?) way to do it just uses (logical) subscripting: odds <- (seq_len(nrow(mydata)) %% 2) == 1 newdat <-data.frame(mydata[odds,1 ],mydata[!odds,2] - mydata[odds,2]) names(newdat) <- names(mydata) Cheers, Bert Bert Gunter "The trouble with having an open mind is that people keep coming along and sticking things into it." -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip ) On Fri, Mar 17, 2017 at 9:58 AM, Ulrik Stervbo <ulrik.ster...@gmail.com> wrote: > Hi Evan > > you can easily do this by applying diff() to each exp group. > > Either using dplyr: > library(dplyr) > mydata %>% > group_by(exp) %>% > summarise(difference = diff(rslt)) > > Or with base R > aggregate(mydata, by = list(group = mydata$exp), FUN = diff) > > HTH > Ulrik > > > On Fri, 17 Mar 2017 at 17:34 Evan Cooch <evan.co...@gmail.com> wrote: > >> Suppose I have a dataframe that looks like the following: >> >> n=2 >> mydata <- data.frame(exp = rep(1:5,each=n), rslt = >> c(12,15,7,8,24,28,33,15,22,11)) >> mydata >> exp rslt >> 1 1 12 >> 2 1 15 >> 3 2 7 >> 4 2 8 >> 5 3 24 >> 6 3 28 >> 7 4 33 >> 8 4 15 >> 9 5 22 >> 10 5 11 >> >> The variable 'exp' (for experiment') occurs in pairs over consecutive >> rows -- 1,1, then 2,2, then 3,3, and so on. The first row in a pair is >> the 'control', and the second is a 'treatment'. The rslt column is the >> result. >> >> What I'm trying to do is create a subset of this dataframe that consists >> of the exp number, and the lagged difference between the 'control' and >> 'treatment' result. So, for exp=1, the difference is (15-12)=3. For >> exp=2, the difference is (8-7)=1, and so on. What I'm hoping to do is >> take mydata (above), and turn it into >> >> exp diff >> 1 1 3 >> 2 2 1 >> 3 3 4 >> 4 4 -18 >> 5 5 -11 >> >> The basic 'trick' I can't figure out is how to create a lagged variable >> between the second row (record) for a given level of exp, and the first >> row for that exp. This is easy to do in SAS (which I'm more familiar >> with), but I'm struggling with the equivalent in R. The brute force >> approach I thought of is to simply split the dataframe into to (one >> even rows, one odd rows), merge by exp, and then calculate a difference. >> But this seems to require renaming the rslt column in the two new >> dataframes so they are different in the merge (say, rslt_cont n the odd >> dataframe, and rslt_trt in the even dataframe), allowing me to calculate >> a difference between the two. >> >> While I suppose this would work, I'm wondering if I'm missing a more >> elegant 'in place' approach that doesn't require me to split the data >> frame and do every via a merge. >> >> Suggestions/pointers to the obvious welcome. I've tried playing with >> lag, and some approaches using lag in the zoo package, but haven't >> found the magic trick. The problem (meaning, what I can't figure out) >> seems to be conditioning the lag on the level of exp. >> >> Many thanks... >> >> >> mydata <-*data.frame*(x = c(20,35,45,55,70), n = rep(50,5), y = >> c(6,17,26,37,44)) >> >> >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> 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. >> > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > 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. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see 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.