[datatable-help] Force evaluation of first argument to [

[Matt Dowle]

Hi John,

In examples like this I'd use set() and [[,  since it's a bit easier to 
write but memory efficient too.

for (x in c("foo", "bar", "qux"))   set(fbq, is.na(fbq[[x]]), x, 
FALSE)           [untested]

A downside here is one repetition of the "fbq" symbol,  but can live 
with that.  If you have a large number of columns  (and I've been 
surprised just how many columns some poeple have!) then calling set() 
many times has lower overhead than DT[, :=],  see ?set.   Note also that 
[[ is base R, doesn't copy the column and often useful to use with 
data.table.

Or, use get() in either i or j rather than eval().

HTH, Matt


On 12/02/14 17:24, John Laing wrote:
> Let's say I merge together several data.tables such that I wind up
> with lots of NAs:
>
> require(data.table)
> foo <- data.table(k=1:4, foo=TRUE, key="k")
> bar <- data.table(k=3:6, bar=TRUE, key="k")
> qux <- data.table(k=5:8, qux=TRUE, key="k")
> fbq <- merge(merge(foo, bar, all=TRUE), qux, all=TRUE)
> print(fbq)
> #    k  foo  bar  qux
> # 1: 1 TRUE   NA   NA
> # 2: 2 TRUE   NA   NA
> # 3: 3 TRUE TRUE   NA
> # 4: 4 TRUE TRUE   NA
> # 5: 5   NA TRUE TRUE
> # 6: 6   NA TRUE TRUE
> # 7: 7   NA   NA TRUE
> # 8: 8   NA   NA TRUE
>
> I want to go through those columns and turn each NA into FALSE. I can
> do this by writing code for each column:
>
> fbq.cp <- copy(fbq)
> fbq.cp[is.na <http://is.na>(foo), foo:=FALSE]
> fbq.cp[is.na <http://is.na>(bar), bar:=FALSE]
> fbq.cp[is.na <http://is.na>(qux), qux:=FALSE]
> print(fbq.cp)
> #    k   foo   bar   qux
> # 1: 1  TRUE FALSE FALSE
> # 2: 2  TRUE FALSE FALSE
> # 3: 3  TRUE  TRUE FALSE
> # 4: 4  TRUE  TRUE FALSE
> # 5: 5 FALSE  TRUE  TRUE
> # 6: 6 FALSE  TRUE  TRUE
> # 7: 7 FALSE FALSE  TRUE
> # 8: 8 FALSE FALSE  TRUE
>
> But I can't figure out how to do it in a loop. More precisely, I can't
> figure out how to make the [ operator evaluate its first argument in
> the context of the data.table. All of these have no effect:
> for (x in c("foo", "bar", "qux")) fbq[is.na <http://is.na>(x), 
> eval(x):=FALSE]
> for (x in c("foo", "bar", "qux")) fbq[is.na <http://is.na>(eval(x)), 
> eval(x):=FALSE]
> for (x in c("foo", "bar", "qux")) fbq[eval(is.na <http://is.na>(x)), 
> eval(x):=FALSE]
>
> I'm running R 3.0.2 on Linux, data.table 1.8.10.
>
> Thanks in advance,
> John
>
>
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