Why does R do this?

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
Team Biometrie & Kwaliteitszorg / Team Biometrics & Quality Assurance
thierry.onkelinx at inbo.be
Havenlaan 88 bus 73, 1000 Brussel
www.inbo.be

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what the experiment died of. ~ Sir Ronald Aylmer Fisher
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Op di 8 jan. 2019 om 10:29 schreef Nick Wray via R-help <
r-help at r-project.org>:

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
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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 <r-help-bounces at r-project.org> On Behalf Of Nick Wray via R-
help
Sent: Tuesday, January 8, 2019 10:29 AM
To: r-help <r-help at r-project.org>
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]]

______________________________________________
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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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