[R] Problem Subsetting Rows that Have NA's

[Bert Gunter]

... Just to be clear:

David's end summary

"[" always returns a list.
"[[" returns only one thing, but even that thing could be a list.
Generally you want "[[" if you plan on testing for equality with a vector.


applies to indexing on a **list**, of course, and not to vectors, matrices,
etc.

Cheers,
Bert



Bert Gunter

"The trouble with having an open mind is that people keep coming along and
sticking things into it."
-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )

On Wed, Oct 25, 2017 at 11:17 AM, David Winsemius <dwinsemius at comcast.net>
wrote:

>
> > On Oct 25, 2017, at 6:57 AM, BooBoo <booboo at gforcecable.com> wrote:
> >
> > On 10/25/2017 4:38 AM, Ista Zahn wrote:
> >> On Tue, Oct 24, 2017 at 3:05 PM, BooBoo <booboo at gforcecable.com> wrote:
> >>> This has every appearance of being a bug. If it is not a bug, can
> someone
> >>> tell me what I am asking for when I ask for "x[x[,2]==0,]". Thanks.
> >> You are asking for elements of x where the second column is equal to
> zero.
> >>
> >> help("==")
> >>
> >> and
> >>
> >> help("[")
> >>
> >> explain what happens when missing values are involved. I agree that
> >> the behavior is surprising, but your first instinct when you discover
> >> something surprising should be to read the documentation, not to post
> >> to this list. After having read the documentation you may post back
> >> here if anything remains unclear.
> >>
> >> Best,
> >> Ista
> >>
> >>>> #here is the toy dataset
> >>>> x <- rbind(c(1,1),c(2,2),c(3,3),c(4,0),c(5,0),c(6,NA),
> >>> +   c(7,NA),c(8,NA),c(9,NA),c(10,NA)
> >>> + )
> >>>> x
> >>>       [,1] [,2]
> >>>  [1,]    1    1
> >>>  [2,]    2    2
> >>>  [3,]    3    3
> >>>  [4,]    4    0
> >>>  [5,]    5    0
> >>>  [6,]    6   NA
> >>>  [7,]    7   NA
> >>>  [8,]    8   NA
> >>>  [9,]    9   NA
> >>> [10,]   10   NA
> >>>> #it contains rows that have NA's
> >>>> x[is.na(x[,2]),]
> >>>      [,1] [,2]
> >>> [1,]    6   NA
> >>> [2,]    7   NA
> >>> [3,]    8   NA
> >>> [4,]    9   NA
> >>> [5,]   10   NA
> >>>> #seems like an unreasonable answer to a reasonable question
> >>>> x[x[,2]==0,]
> >>>      [,1] [,2]
> >>> [1,]    4    0
> >>> [2,]    5    0
> >>> [3,]   NA   NA
> >>> [4,]   NA   NA
> >>> [5,]   NA   NA
> >>> [6,]   NA   NA
> >>> [7,]   NA   NA
> >>>> #this is more what I was expecting
> >>>> x[which(x[,2]==0),]
> >>>      [,1] [,2]
> >>> [1,]    4    0
> >>> [2,]    5    0
> >>> ______________________________________________
> >>> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> >>> https://stat.ethz.ch/mailman/listinfo/r-help
> >>> PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> >>> and provide commented, minimal, self-contained, reproducible code.
> >
> > I wanted to know if this was a bug so that I could report it if so. You
> say it is not, so you answered my question. As far as me not reading the
> documentation, I challenge anyone to read the cited help pages and predict
> the observed behavior based on the information given in those pages.
>
> Some of us do share (or at least remember feeling) your pain. The ?Extract
> page is long and complex and there are several features that I find
> non-intuitive. But they are deemed desirable by others. I think I needed to
> read that page about ten times (with multiple different problems that
> needed explication) before it started to sink in. You are apparently on
> that same side of the split opinions on the feature of returning rows with
> logical NA's as I am. I've learned to use `which`, and I push back when the
> conoscienti says it's not needed.
>
>  After you read it a few more times you may come to a different opinion.
> Many people come to R with preconceived notions of what words like "equals"
> or "list" or "vector" mean and then complain about the documentation. You
> would be better advised to spend more time studying the language. The help
> pages are precise but terse, and you need to spend time with the examples
> and with other tutorial material to recognize the gotcha's.
>
> Here's a couple of possibly helpful rules regarding "[[" and "[" and
> logical indexing:
>
> Nothing _equals_ NA.
> Selection operations with NA logical index item return NA.  (Justified as
> a warning feature as I understand it.)
> "[" always returns a list.
> "[[" returns only one thing, but even that thing could be a list.
> Generally you want "[[" if you plan on testing for equality with a vector.
>
> The "R Inferno" by Burns is an effort to detail many more of the
> unexpected or irregular aspects of R (mostly inherited from S).
>
> --
> Best of luck in your studies.
>
>
> >
> > ______________________________________________
> > R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> > and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius
> Alameda, CA, USA
>
> 'Any technology distinguishable from magic is insufficiently advanced.'
>  -Gehm's Corollary to Clarke's Third Law
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/
> posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

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