[R] Find each time a value changes

[Bill.Venables at csiro.au]

something like this?

> c(1, which(c(0, diff(y)) != 0))
[1]  1 11 21 51 61 71 81 91

Bill Venables
CSIRO/CMIS Cleveland Laboratories


-----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, 11 February 2010 11:59 AM
To: r-help at r-project.org
Subject: [R] Find each time a value changes

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 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




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