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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">One thing that would be super-awesome is a lazy copy-on-write mechanism. By which I mean table2 would initially point to the same memory location as table1,
but as soon as it’s modified, the portions being modified would be copied to a new location.<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">It’s pretty rare use case to make a copy of a data structure without intending to modify it. The only instance I can really think of is passing arguments to
a function, which IIUC is already copy-on-write in R?<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">Also, I do think Natus’ example is a little different from your example on SO. In his, a new column is being added, but in yours, an existing column is being
modified.<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">Is there a doc reference showing which circumstances make `<-` do a copy-by-reference, and which do a deep copy? For example, if I do `table2 <- table1[x>1,
list(id)]` , it seems to do a deep copy:<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">> table1<-data.table(id=c(1,2,3),x=c(1,2,3))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">> table2 <- table1[x>1, list(id)]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">> table2[, id := 3:4]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">> table1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D"> id x<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">1: 1 1<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">2: 2 2<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D">3: 3 3<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">Sorry if this has already been hashed out a million times, I’m pretty new to data.table.<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">-Ken<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>
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<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@lists.r-forge.r-project.org [mailto:datatable-help-bounces@lists.r-forge.r-project.org]
<b>On Behalf Of </b>Christoph Jäckel<br>
<b>Sent:</b> Thursday, October 04, 2012 7:07 AM<br>
<b>To:</b> natus<br>
<b>Cc:</b> datatable-help@lists.r-forge.r-project.org<br>
<b>Subject:</b> Re: [datatable-help] data.table BUG : data.table assignment<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">This is actually intended behaviour and I had the problem once as well. Here is my question and the solution to it:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="http://stackoverflow.com/questions/8030452/pass-by-reference-the-operator-in-the-data-table-package" target="_blank">http://stackoverflow.com/questions/8030452/pass-by-reference-the-operator-in-the-data-table-package</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In a nutshell: Use copy() if you don't want table2 to have y as well.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I hope this helps,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Christoph<o:p></o:p></p>
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<p class="MsoNormal">On Thu, Oct 4, 2012 at 1:56 PM, natus <<a href="mailto:niparisco@gmail.com" target="_blank">niparisco@gmail.com</a>> wrote:<o:p></o:p></p>
<p class="MsoNormal">Hello,<br>
<br>
see this example :<br>
<br>
require(data.table)<br>
<br>
table1<-data.table(id=c(1,2,3),x=c(1,2,3))<br>
table2<-table1<br>
table1[,y:=sum(x)]<br>
table1<br>
table2<br>
<br>
The problem ? Both of table1 and table2 have the variable 'y' BUT only<br>
table1 should.<br>
<br>
Thx<br>
<br>
<br>
<br>
--<br>
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http://r.789695.n4.nabble.com/data-table-BUG-data-table-assignment-tp4644988.html</a><br>
Sent from the datatable-help mailing list archive at Nabble.com.<br>
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