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<p class="MsoNormal">Dear Data Table Help List,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I am using data.table version 1.6 (with R version 2.12.2, 64-bit on Windows 7). Suppose I have a table whose key does not give me a unique ordering. Then the output of the “roll” option will be arbitrary (i.e., it will depend on what
one does between the two executions). Is this something noteworthy? <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Please see output of the following:<o:p></o:p></p>
<p class="MsoNormal">> DT = data.table(A=c(1,2,2),B=c("b1","b3","b2"),key="A")<o:p></o:p></p>
<p class="MsoNormal">> DT[J(1:3),roll=TRUE] # output 1<o:p></o:p></p>
<p class="MsoNormal"> A B<o:p></o:p></p>
<p class="MsoNormal">[1,] 1 b1<o:p></o:p></p>
<p class="MsoNormal">[2,] 2 b3<o:p></o:p></p>
<p class="MsoNormal">[3,] 2 b2<o:p></o:p></p>
<p class="MsoNormal">[4,] 3 b2<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> key(DT)="B" # change keys to do other stuff...<o:p></o:p></p>
<p class="MsoNormal">> key(DT)="A" # get back to key A<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">> DT[J(1:3),roll=TRUE] # output 2 does not match output 1<o:p></o:p></p>
<p class="MsoNormal"> A B<o:p></o:p></p>
<p class="MsoNormal">[1,] 1 b1<o:p></o:p></p>
<p class="MsoNormal">[2,] 2 b2<o:p></o:p></p>
<p class="MsoNormal">[3,] 2 b3<o:p></o:p></p>
<p class="MsoNormal">[4,] 3 b3<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">(Also, as an aside, I get identical output in the two executions of DT[J(1:3),roll=TRUE] when I start with the table DT = data.table(A=c(1,2,2),B=c("b1","b2","b3"),key="A") instead.)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I’m sure there must also be other reverberations—beyond the effect on the roll option.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any insight would be of interest. Thank you.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">-Alex<o:p></o:p></p>
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