<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>