<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:x="urn:schemas-microsoft-com:office:excel" xmlns:p="urn:schemas-microsoft-com:office:powerpoint" xmlns:a="urn:schemas-microsoft-com:office:access" xmlns:dt="uuid:C2F41010-65B3-11d1-A29F-00AA00C14882" xmlns:s="uuid:BDC6E3F0-6DA3-11d1-A2A3-00AA00C14882" xmlns:rs="urn:schemas-microsoft-com:rowset" xmlns:z="#RowsetSchema" xmlns:b="urn:schemas-microsoft-com:office:publisher" xmlns:ss="urn:schemas-microsoft-com:office:spreadsheet" xmlns:c="urn:schemas-microsoft-com:office:component:spreadsheet" xmlns:odc="urn:schemas-microsoft-com:office:odc" xmlns:oa="urn:schemas-microsoft-com:office:activation" xmlns:html="http://www.w3.org/TR/REC-html40" xmlns:q="http://schemas.xmlsoap.org/soap/envelope/" xmlns:rtc="http://microsoft.com/officenet/conferencing" xmlns:D="DAV:" xmlns:Repl="http://schemas.microsoft.com/repl/" xmlns:mt="http://schemas.microsoft.com/sharepoint/soap/meetings/" xmlns:x2="http://schemas.microsoft.com/office/excel/2003/xml" xmlns:ppda="http://www.passport.com/NameSpace.xsd" xmlns:ois="http://schemas.microsoft.com/sharepoint/soap/ois/" xmlns:dir="http://schemas.microsoft.com/sharepoint/soap/directory/" xmlns:ds="http://www.w3.org/2000/09/xmldsig#" xmlns:dsp="http://schemas.microsoft.com/sharepoint/dsp" xmlns:udc="http://schemas.microsoft.com/data/udc" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:sub="http://schemas.microsoft.com/sharepoint/soap/2002/1/alerts/" xmlns:ec="http://www.w3.org/2001/04/xmlenc#" xmlns:sp="http://schemas.microsoft.com/sharepoint/" xmlns:sps="http://schemas.microsoft.com/sharepoint/soap/" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:udcs="http://schemas.microsoft.com/data/udc/soap" xmlns:udcxf="http://schemas.microsoft.com/data/udc/xmlfile" xmlns:udcp2p="http://schemas.microsoft.com/data/udc/parttopart" xmlns:wf="http://schemas.microsoft.com/sharepoint/soap/workflow/" xmlns:dsss="http://schemas.microsoft.com/office/2006/digsig-setup" xmlns:dssi="http://schemas.microsoft.com/office/2006/digsig" xmlns:mdssi="http://schemas.openxmlformats.org/package/2006/digital-signature" xmlns:mver="http://schemas.openxmlformats.org/markup-compatibility/2006" xmlns:m="http://schemas.microsoft.com/office/2004/12/omml" xmlns:mrels="http://schemas.openxmlformats.org/package/2006/relationships" xmlns:spwp="http://microsoft.com/sharepoint/webpartpages" xmlns:ex12t="http://schemas.microsoft.com/exchange/services/2006/types" xmlns:ex12m="http://schemas.microsoft.com/exchange/services/2006/messages" xmlns:pptsl="http://schemas.microsoft.com/sharepoint/soap/SlideLibrary/" xmlns:spsl="http://microsoft.com/webservices/SharePointPortalServer/PublishedLinksService" xmlns:Z="urn:schemas-microsoft-com:" xmlns:st="" 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><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Well, I don’t know what’s faster, but I’ll have to agree with Farrel’s philosophy. In addition, I KNOW that if I were to look at the code for $i ‘trick’ a few weeks later, I would have uneasy feelings about it. I would prefer that I can easily read my code—the less thought, the better.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Here are the two ways I would think about doing it. No idea of speed issues.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>dt <- data.table(a=c('a','a','a','a','b','b','b','b'),<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'> b=c('a','b','a','b','a','b','b','a'),c=1:8,key=c('a','b'))<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'># method 1. Just include all possible values of the first key in J. To me, this is conceptually the simplest<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>dt[J(unique(a),"b")]<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'># method 2. Swap the keys, twice<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>setkey(dt,b,a)<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>dt<-dt[J("b")]<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>setkey(dt,a,b)<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> datatable-help-bounces@r-forge.wu-wien.ac.at [mailto:datatable-help-bounces@r-forge.wu-wien.ac.at] <b>On Behalf Of </b>Farrel Buchinsky<br><b>Sent:</b> Thursday, January 19, 2012 8:59 PM<br><b>To:</b> Steve Lianoglou<br><b>Cc:</b> datatable-help@r-forge.wu-wien.ac.at<br><b>Subject:</b> Re: [datatable-help] using J() to select for a value that is in something other than the first key<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>I do not know if that is how all indexes work. I am not really a card-carrying database manager or programmer. I just play one in my spare time. The price I pay is not remembering how to write syntax when I need to do something. That to me, is a higher price than slow subsetting. If the syntax is not easy I would rather just use the traditional vector scan methods that one sees in conventional data.frame subset commands. <o:p></o:p></p><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Notwithstanding my idiosyncratic needs, I thank you very much for your explanation.<br clear=all>Farrel <o:p></o:p></p><div><p class=MsoNormal style='margin-bottom:12.0pt'><br><br><br><o:p></o:p></p><div><p class=MsoNormal>On Thu, Jan 19, 2012 at 18:26, Steve Lianoglou <<a href="mailto:mailinglist.honeypot@gmail.com">mailinglist.honeypot@gmail.com</a>> wrote:<o:p></o:p></p><div><p class=MsoNormal style='margin-bottom:12.0pt'>On Thu, Jan 19, 2012 at 6:09 PM, Farrel Buchinsky <<a href="mailto:fjbuch@gmail.com">fjbuch@gmail.com</a>> wrote:<br>> Oy gevalt!.Am I correct to believe that the technique is rearranging the<br>> data.table so that J can accept the input as pertaining to a secondary key?<br>> That seems as if it is too much work for me and my computer. I will rather<br>> stick to the vector scan methods for now.<o:p></o:p></p></div><p class=MsoNormal>Not the entire data.table, just the key columns.<br><br>Depending on how many queries you're going to make against the 2nd key<br>only, the pay off for your troubles could be anywhere from zero to<br>mucho. Of course if you simply don't have the RAM to make the idx<br>data.table in the first place, then that's that.<br><br>That's how all indexes work though, no? In a database for instance, if<br>you have a compound key/index over two or more columns, the index will<br>only help queries that work any prefix (or whole) part of the key, and<br>not just any subset elements of it (as you want to do here), right?<br><br>HTH,<br>-steve<o:p></o:p></p><div><div><p class=MsoNormal><br>--<br>Steve Lianoglou<br>Graduate Student: Computational Systems Biology<br> | Memorial Sloan-Kettering Cancer Center<br> | Weill Medical College of Cornell University<br>Contact Info: <a href="http://cbio.mskcc.org/~lianos/contact" target="_blank">http://cbio.mskcc.org/~lianos/contact</a><o:p></o:p></p></div></div></div><p class=MsoNormal><o:p> </o:p></p></div></div></div></body></html>