r - When add a column to a data.table, what's then best way to add if else conditions? -

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for example, like:

 dt[isflag, newcol:= a]  dt[!isflag, newcol:=b]

or should this?

 dt[, newcol:= b]  dt[isflag, newcol:=a]

or this?

 dt[, newcol:= if (isflag) else b] # works if isflag scalar

or

dt[, newcol:= ifelse(isflag, a, b)] # works if isflag vector

what best way?


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