Allow logical specification in .SDcols

[stefanfritsch]

Hi,

these are two connected feature requests for .SDcols:

.1. First, allow logical specifications of columns which would allow stuff like this:

DT[,`:=`(colnames(.SD), lapply(.SD,as.character)), .SDcols=sapply(.SD, is.factor)]

Note that this fails currently because

• colnames(.SD) in the := specification doesn't work as I mentioned in issue names(.SD) := ... should work #795
• .SD is not accessible in .SDcols. This would mean that there are two definitions of .SD around (one pre- and one post-.SDcols-selection), but imho the difference is rather intuitive.
• The resulting logical vector doesn't work.

.2. As a second step it would be really cool if .SDcols would accept selection functions, kind of like Filter:

DT[,(.SDcols):=lapply(.SD,as.character), .SDcols=is.factor]

Other possible use cases apart from type-checks would be basic thresholds and the like.

.3. I smuggled a third wish into the last example. .SDcols already looks like one of the comfort-variables, it would be really nice if we could use it to access the available columns. :)

Thanks for having a look at it even if it doesn't get implemented

Ciao,
Stefan

[arunsrinivasan]

Please file 1 issue per FR/bug, even if they are related. It helps close the issues as and when they are implemented / discussed.

I'll consider point (1) alone for this issue, and will implement .SDcols taking logical argument. Could you please re-file the other issues when you find time? Thanks.

The use of colnames(.SD) in the LHS would be a completely different FR (and I think one already exists) as well.

[stefanfritsch]

Hi Arun,

thanks a lot for the patch. 👍

I'd consider the colnames(.SD) issue part of #795 as mentioned in the report.

I'll have to think about the functions-as-.SDcols-value thing a bit. I put it in here as an aside because it could be implemented as a quick hack the way you did the logicals (if(is.function) ...) but before I make it its own issue I'd like to find a more flexible and less hacky way; even if it's just a feature request / conversation starter. :)

Ciao,
Stefan