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<div>you can just do:</div><div><br></div><div><font face="Courier New">> DT[J(letters[4:6]), list(v1=v1, 1),by=v3]</font></div><div><br></div><div>If you use `by` only the columns in it will be included in the output. But it seems like what you request is a nicer feature and I am for it. Because, when you just do, <font face="Courier New">DT[J(letters[4:6]), sum(v1)] </font>it gives you "v1". But when using `by`, it disappears. I find with your suggestion this would be more consistent.</div><div><br></div>
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<div><div><br></div><div>Arun</div><div><br></div></div>
<p style="color: #A0A0A8;">On Friday, June 21, 2013 at 10:26 PM, Frank Erickson wrote:</p>
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<span><div><div><div dir="ltr">Hi,<div><br></div><div style="">I thought that when joining with J(x) and doing by=y, all the columns involved were put into .BY. However, I see that they are not:</div><div style=""><br></div><div style=""><div><font face="courier new, monospace">DT <- data.table(v1=letters[1:10],v2=1:10,v3=c(TRUE,FALSE),key="v1")</font></div>
<div><font face="courier new, monospace">DT[J(letters[4:6]),1,by=v3]</font></div><div><br></div><div style="">I think I've just forgotten how to do this correctly (so that both v1 and v3 show up in the output). Any help would be appreciated.</div>
<div style=""><br></div><div style="">Thanks,</div><div style=""><br></div><div style="">Frank</div></div></div>
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