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<p class="MsoNormal">Is there a way to subset a data table by the result of a grouped aggregation without adding an interim column to the table? For example, if I want to select all rows for which the group mean value of x is less than 10, I can do the following:<o:p></o:p></p>
<p class="MsoNormal"><span style="font-family:"Courier New""><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">data <- data.table(x = 1:20, g = rep(c("a", "b"), each = 10))<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">data[, mean.x := mean(x), by = .(g)]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Courier New"">data[mean.x < 10,]<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But I’m not really interested in “mean.x”. Can I do the same thing without adding it to the table?<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">- Elliot<span style="font-size:10.0pt;color:black;background:white"><o:p></o:p></span></p>
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