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names(.SD) := ... should work #795
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That would be very nice.
Another implementation would be that, when.SDcolsis not specified, and .SD is present inj, the LHS of:=is understood as.SDcols.Reacted by Uwe Block and sweepydodoAlternate is to create a
.SDcolsobject that is usable in LHS of:=.Also, using
.SDin LHS of:=should probably throw an error instead of kind of work. More discussion on SO.the straight "workaround" for this would be to use
with=FALSE
example in: http://jangorecki.github.io/blog/2014-11-07/Data-Anonymization-in-R.html#minimal-scriptstill the
.SDcolsobject usable inj(bothLHSandRHS) seems to be the best idea.Jan, not sure
with=Fis necessary, since you can do:dt <- data.table(a=1:10, b=1:10, c=rep(c(T,F), 5)) cols <- 1:2 dt[, cols:=lapply(.SD, `*`, 2), .SDcols=cols, with=F]or
dt[, (cols):=lapply(.SD, `*`, 2), .SDcols=cols] # add parens dtequivalently, though this is a good reminder that
withcan help in situations when wanting to usedata.tableprogramatically, which is another issue I've been discussing with Arun.
Also, note both fail with:cols <- -3Another inconsistency with this currently is that
this works:
A<-data.table(x=1:10,y=10:1,z=rnorm(10)) A[,`:=`(colnames(A),.SD)]
and this doesn't:
A[,`:=`(colnames(.SD),.SD)]
The second fails with:
Error in `[.data.table`(A, , `:=`(colnames(.SD), .SD)) : LHS of := isn't column names ('character') or positions ('integer' or 'numeric')That precludes some elegant .SDcols syntax.
Unfortunate that this workaround is blocked by
:=:set.seed(23940) DT = setDT(lapply(integer(10), function(...) sample(1e7, 100))) DT[ , do.call(`:=`, lapply(.SD, .POSIXct, tz = 'UTC'))]Error in (function (...) :
Check that is.data.table(DT) == TRUE. Otherwise, := and:=(...) are defined for use in j, once only and in particular ways. See help(":=").It's unfortunate since the result of
lapplyis already named, so this is a shorthand for the\`:=\`(V1 = .POSIXct(V1, tz = 'UTC'), ...)approach of explicitly naming columnsDT[ , str(lapply(.SD, .POSIXct, tz = 'UTC'))] List of 10 $ V1 : POSIXct[1:100], format: "1970-02-28 08:44:45" "1970-01-26 19:24:38" ... $ V2 : POSIXct[1:100], format: "1970-03-14 20:40:49" "1970-01-04 23:53:16" ... $ V3 : POSIXct[1:100], format: "1970-01-09 03:32:08" "1970-02-12 06:18:31" ... $ V4 : POSIXct[1:100], format: "1970-04-13 04:15:52" "1970-03-17 19:10:23" ... $ V5 : POSIXct[1:100], format: "1970-03-22 07:57:03" "1970-02-10 19:42:45" ... $ V6 : POSIXct[1:100], format: "1970-01-28 05:56:39" "1970-04-20 11:43:32" ... $ V7 : POSIXct[1:100], format: "1970-01-02 03:41:31" "1970-04-01 23:58:52" ... $ V8 : POSIXct[1:100], format: "1970-03-05 05:58:29" "1970-03-05 23:27:10" ... $ V9 : POSIXct[1:100], format: "1970-04-13 20:29:31" "1970-01-24 12:18:58" ... $ V10: POSIXct[1:100], format: "1970-04-22 15:01:36" "1970-03-08 00:33:20" ... NULLI actually lean towards allowing
.SDon the LHS of:=. More concise and I think the intent is clear. We're doing this with NSE so we can just capture.SD-->names(.SD)anyway.DT[ , .SD := lapply(.SD, rev), .SDcols = -c(1,3,8)]That & whatever comes out of #3795 would make adding/editing many columns much less clunky
Reacted by Yoann PAGEAUD, Grant McDermott, iagogv3 and r2evansor eventually which is more like
names(.SD)DT[ , .SDcols := lapply(.SD, rev), .SDcols = -c(1,3,8)]
Reacted by Frank, Yoann PAGEAUD, JenspederM, Vincent Arel-Bundock, Yun Peng, Uwe Block, Grant McDermott, Henrik-P, Peter Kress, iagogv3 and 1 moreI've been thinking about this FR again after having to do quite a bit of "manual" LHS creation in a current project. (FWIW my own preferred option is @MichaelChirico's
DT[, .SD := ....], but would support any of the proposed solutions.)Another possible syntax variant — which would involve even less typing if it is feasible to code up — would be to enable
:=directly in .SDcols. I'm not sure how others would feel about this, though.DT[ , lapply(.SD, rev), .SDcols := c(1,3,8)]
Reacted by iagogv3 and Peter KressIt does ~basically read well here, but I would be against that...
:=semantics are (based on legion user reports/SO Q&A) confusing enough without opening up another API surface for it. Being able to consistently look only forjto know if a table is being updated by reference will keep code more readable than if:=could show up in other[arguments, possibly on other lines, possible separated fromjby dozens of lines.Reacted by Grant McDermott and Jan Gorecki
This would allow the following type of code:
DT[, names(.SD) := lapply(.SD, rev), .SDcols = -c(1,3,8)]To reverse every column except 1, 3, and 8 in
DTby reference. See discussion in #787. Maybe potentially even:DT[, := lapply(.SD, rev), .SDcols = -c(1,3,8)]and have the names of the columns to be updated inferred from the names of the return value to
lapply.