<html xmlns:v="urn:schemas-microsoft-com:vml" xmlns:o="urn:schemas-microsoft-com:office:office" xmlns:w="urn:schemas-microsoft-com:office:word" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns="http://www.w3.org/TR/REC-html40"><head><meta http-equiv=Content-Type content="text/html; charset=us-ascii"><meta name=Generator content="Microsoft Word 12 (filtered medium)"><style><!--
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</o:shapelayout></xml><![endif]--></head><body lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal>Here is a simple example of a simple question:<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>dt<-data.table(a=rep(1:5,1:5),b=1,c=rep(1:3,5))<o:p></o:p></p><p class=MsoNormal>dt<o:p></o:p></p><p class=MsoNormal>dt[,seq_along(b),by=a] # expected behavior (note: dt is already in order of a)<o:p></o:p></p><p class=MsoNormal>setkey(dt,c) # to sort by c<o:p></o:p></p><p class=MsoNormal>dt<o:p></o:p></p><p class=MsoNormal>dt[,seq_along(b),by=a] # expected behavior? Appears to be in order of unique(dt$a)<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Thanks,<o:p></o:p></p><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>Joe Voelkel<o:p></o:p></p></div></body></html>