My solution may be a bit clearer if you define the function isFirstInRun isFirstInRun <- function(x) { if (length(x) == 0) { logical(0) } else { c(TRUE, x[-1] != x[-length(x)]) } }
Then that solution is equivalent to which(isFirstInRun(a) & a==1) If 'a' contains NA's then you have to decide how to deal with them. (The call to 'which' is not needed if you are going to be using the result as a subscript.) You may also want isLastInRun isLastInRun <- function(x) { if (length(x) == 0) { logical(0) } else { c(x[-1] != x[-length(x)], TRUE) } } Bill Dunlap TIBCO Software wdunlap tibco.com On Thu, Aug 7, 2014 at 7:36 AM, William Dunlap <wdun...@tibco.com> wrote: >> a<-c(1,1,1,0,0,1,1,1,1,1,1,0,0,0,0,1,1,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1) >> which( a==1 & c(TRUE, a[-length(a)]!=1) ) > [1] 1 6 16 23 >> which( a==0 & c(TRUE, a[-length(a)]!=0) ) > [1] 4 12 18 > > Bill Dunlap > TIBCO Software > wdunlap tibco.com > > > On Wed, Aug 6, 2014 at 7:12 PM, Johnnycz <johnn...@yeah.net> wrote: >> Hello,everybody, >> I have a sequence,like >> a<-c(1,1,1,0,0,1,1,1,1,1,1,0,0,0,0,1,1,0,0,0,0,0,1,1,1,1,1,1,1,1,1,1,1),how >> to get the position of each first 1 and 0, that's to say, how to get >> b<-c(1,6,16,23) for first 1 and d<-c(4,12,18) for first 0. >> Many thanks! >> Johnny >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> 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. ______________________________________________ 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.