[R-Forge #4996] provide a message when x[y,newcol:=my_ycol] does not create a new column

[arunsrinivasan]

Submitted by: Frank Erickson; Assigned to: Arun ; R-Forge link

If I attempt to make a new column in x with

x[y,newcol:=my_ycol]

and fail to do so (because x[y] has no matches), I would like a message.

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One idea: there is already a nice message (with verbose turned on) for

x[logi,newcol:=my_val]

where logi is FALSE:

No rows pass i clause so quitting := early with no changes made.

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Another idea: there is already a nice error when assigning to NA after bad subsetting:

x <- data.table(a=1:2)
x[c(FALSE,FALSE,TRUE),newcol:=1]

which gives:

Error in [.data.table(x, c(FALSE, FALSE, TRUE), :=(newcol, 1)) :
i[1] is NA. Can't assign by reference to row 'NA'.

And a similar error message is given for x[3,newcol:=1].

[mattdowle]

@arunsrinivasan Came across this one via #1150. Not sure why closed so reopening to come back to.

[jangorecki]

duplicate of #759, IMO it should not be a message but a NA valued column added, as in SQL when using outer join and there is no matching values.

[arunsrinivasan]

@mattdowle as Jan points out, this is a duplicate. I must have forgotten to add the issue at the time.