Why does R do this?

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Why does R do this?

R help mailing list-2
y<-c(1,2,3)
z<-which(y>3)
z
y<-y[-z]
y

In the work I'm doing I often have this situation and have to make sure that I condition on z being non-zero as y is now numeric(0) rather than the set c(1,2,3).  Why does R do this?  Wouldn't it be more sensible for R to simply leave the host set unchanged if there are no elements to take out?

Any thoughts?

Thanks, Nick Wray
        [[alternative HTML version deleted]]

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Re: Why does R do this?

R help mailing list-2
Dear Nick,

The best solution is not to use which() but directy use the logical test.
This will work in case the condition is always FALSE and which() returns a
integer(0). And it is much faster too.
z <- y > 3
y[!z]

library(microbenchmark)
microbenchmark(
  y[!y > 3],
  y[-which(y > 3)]
)

Best regards,




ir. Thierry Onkelinx
Statisticus / Statistician

Vlaamse Overheid / Government of Flanders
INSTITUUT VOOR NATUUR- EN BOSONDERZOEK / RESEARCH INSTITUTE FOR NATURE AND
FOREST
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Op di 8 jan. 2019 om 10:29 schreef Nick Wray via R-help <
[hidden email]>:

> y<-c(1,2,3)
> z<-which(y>3)
> z
> y<-y[-z]
> y
>
> In the work I'm doing I often have this situation and have to make sure
> that I condition on z being non-zero as y is now numeric(0) rather than the
> set c(1,2,3).  Why does R do this?  Wouldn't it be more sensible for R to
> simply leave the host set unchanged if there are no elements to take out?
>
> Any thoughts?
>
> Thanks, Nick Wray
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> [hidden email] 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.
>

        [[alternative HTML version deleted]]

______________________________________________
[hidden email] mailing list -- To UNSUBSCRIBE and more, see
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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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Re: Why does R do this?

PIKAL Petr
In reply to this post by R help mailing list-2
Hi

It is documented behaviour.

"An empty index selects all values: this is most often used to replace all the entries but keep the attributes."

so I presume that changing it could break huge amount of code. The only workaround could be to check "z" before using it for indexing.

e.g.
> if(length(z)==0) z <- length(y) + 1
> y[-z]
[1] 1 2 3
>
Cheers
Petr

> -----Original Message-----
> From: R-help <[hidden email]> On Behalf Of Nick Wray via R-
> help
> Sent: Tuesday, January 8, 2019 10:29 AM
> To: r-help <[hidden email]>
> Subject: [R] Why does R do this?
>
> y<-c(1,2,3)
> z<-which(y>3)
> z
> y<-y[-z]
> y
>
> In the work I'm doing I often have this situation and have to make sure that I
> condition on z being non-zero as y is now numeric(0) rather than the set
> c(1,2,3).  Why does R do this?  Wouldn't it be more sensible for R to simply
> leave the host set unchanged if there are no elements to take out?
>
> Any thoughts?
>
> Thanks, Nick Wray
> [[alternative HTML version deleted]]
>
> ______________________________________________
> [hidden email] 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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Re: Why does R do this?

Duncan Murdoch-2
In reply to this post by R help mailing list-2
On 08/01/2019 4:28 a.m., Nick Wray via R-help wrote:
> y<-c(1,2,3)
> z<-which(y>3)

At this point z is a vector with no entries in it.

> z
> y<-y[-z]

-z is the same vector.  So y[z] and y[-z] are the same.

> y
>
> In the work I'm doing I often have this situation and have to make sure that I condition on z being non-zero as y is now numeric(0) rather than the set c(1,2,3).  Why does R do this?  Wouldn't it be more sensible for R to simply leave the host set unchanged if there are no elements to take out?

No, it wouldn't.  You asked for no entries, so you get no entries.

Follow Thierry's advice, and don't use which() unless you really need a
vector of indices, and are prepared for an empty one.

Duncan Murdoch

______________________________________________
[hidden email] mailing list -- To UNSUBSCRIBE and more, see
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and provide commented, minimal, self-contained, reproducible code.