# [R] Find each time a value changes

William Dunlap wdunlap at tibco.com
Thu Feb 11 20:04:35 CET 2010

> -----Original Message-----
> From: r-help-bounces at r-project.org
> [mailto:r-help-bounces at r-project.org] On Behalf Of Tim Clark
> Sent: Thursday, February 11, 2010 10:53 AM
> To: r-help at r-project.org
> Subject: Re: [R] Find each time a value changes
>
> It was brought to my attention that the rle() answer to this
> question was not posted.  The following gives the correct
> answer once the last value is deleted.
>
> x<-seq(1:100)
> y1<-rep(1,10)
> y2<-rep(2,10)
> y<-c(y1,y2,y1,y1,y1,y2,y1,y2,y1,y2)
> xy<-cbind(x,y)
>
> print(xy)
> print(str(xy))
>
> # SEE WHAT RLE GIVES
> test <- rle(xy[,2])
> print(str(test)
>
> # USE JIM"S TRICK OF CUMULATIVE SUMMING
> # TO GET THE LOCATIONS
> result <- cumsum(c(1,rle(xy[,2])\$lengths))

This cumsum undoes the diff that rle does.
You could do what the first half of rle does with
isFirstInRun <- function(x)c(TRUE, x[-1]!=x[-length(x)]
or
isLastInRun <- function(x)c(x[-1]!=x[-length(x)], TRUE)
Use as
which(isFirstInRun(xy[,2]))

>
>
>
> Tim Clark
> Department of Zoology
> University of Hawaii
>
>
> --- On Wed, 2/10/10, Ben Tupper <ben.bighair at gmail.com> wrote:
>
> > From: Ben Tupper <ben.bighair at gmail.com>
> > Subject: Re: [R] Find each time a value changes
> > To: r-help at r-project.org
> > Cc: "Tim Clark" <mudiver1200 at yahoo.com>
> > Date: Wednesday, February 10, 2010, 4:16 PM
> > Hi,
> >
> > On Feb 10, 2010, at 8:58 PM, Tim Clark wrote:
> >
> > > Dear List,
> > >
> > > I am trying to find each time a value changes in a
> > dataset.  The
> > > numbers are variables for day vs. night values, so
> > what I am really
> > > getting is the daily sunrise and sunset.
> > >
> > > A simplified example is the following:
> > >
> > > x<-seq(1:100)
> > > y1<-rep(1,10)
> > > y2<-rep(2,10)
> > > y<-c(y1,y2,y1,y1,y1,y2,y1,y2,y1,y2)
> > > xy<-cbind(x,y)
> > >
> > >
> > > I would like to know each time the numbers change.
> > > Correct answer should be:
> > > x=1,11,21,51,61,71,81,91
> > >
> >
> > I think this gets close...
> >
> > which(diff(y) != 0)
> > [1] 10 20 50 60 70 80 90
> >
> > You'll need to fiddle to get exactly what you want.
> >
> > Cheers,
> > Ben
> >
> >
> >
> > > I would appreciate any help or suggestions.  It
> > seems like it should
> > > be simple but I'm stuck!
> > >
> > > Thanks,
> > >
> > > Tim
> > >
> > >
> > > Tim Clark
> > > Department of Zoology
> > > University of Hawaii
> > >
> > >
> > >
> > >
> > > ______________________________________________
> > > R-help at r-project.org
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> > > and provide commented, minimal, self-contained,
> > reproducible code.
> >
> >
>
>
>
>
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