<div dir="ltr">I wanted to do<div><br><div><font face="monospace, monospace">X[ Y, newx := yvar[1], by=yvar2 ]</font></div></div><div><br></div><div>Does what I'm trying to do here somehow not make sense? This seems to work fine:</div><div><br></div><div><div><font face="monospace, monospace">X[ Y, newx := yvar[1], by=.EACHI ]</font></div></div><div><br></div><div>My full example is below.</div><div><br></div><div>Thanks,</div><div><br></div><div>Frank</div><div><br></div><font face="monospace, monospace">DT <- data.table(Address=c('A','B','C','A','D','C','E'), Transaction=c(1,1,2,3,3,4,4))<br>setkey(DT,Address)<br>t0DT <- DT[,.(t0=Transaction[1]),by=Address]<br>DT[t0DT,Entity:=t0[1],by=Transaction]</font><div><font face="monospace, monospace"><br></font></div><div><font face="monospace, monospace">based on <a href="http://stackoverflow.com/questions/31011741/creating-an-entity-id-using-data-table">http://stackoverflow.com/questions/31011741/creating-an-entity-id-using-data-table</a></font></div></div>