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

[John Laing]

Thanks, Matt! With a slight amendment that works great:
for (x in c("foo", "bar", "qux")) set(fbq, which(is.na(fbq[[x]])), x, FALSE)

Which highlights an opportunity to say that I really appreciate the
unusually helpful error messages in this package.

-John


On Wed, Feb 12, 2014 at 12:44 PM, Matt Dowle <mdowle at mdowle.plus.com> wrote:

>
> 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(foo), foo:=FALSE]
> fbq.cp[is.na(bar), bar:=FALSE]
> fbq.cp[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(x), eval(x):=FALSE]
> for (x in c("foo", "bar", "qux")) fbq[is.na(eval(x)), eval(x):=FALSE]
> for (x in c("foo", "bar", "qux")) fbq[eval(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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>
>
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