R devel - Feb 2010 - Confusing error message for [[.factor (PR#14209)

Full_Name: George Russell
Version: 2.10.0 and 2.11.0 Under development (unstable) (2010-02-08 r51108)
OS: Windows
Submission from: (NULL) (217.111.3.131)

> c("a","b")[[c(TRUE,FALSE)]]
Error in `[[.default`(factor(c("a", "b")), c(TRUE, FALSE)) :
  recursive indexing failed at level 1

I find this error message confusing, though after reading the HELP carefully I
think I know what is going on. Would not something like "[[ does not work
with
logical index vectors" be more appropriate?

sessionInfo is (for 2.11) :
R version 2.11.0 Under development (unstable) (2010-02-08 r51108) 
i386-pc-mingw32 

locale:
[1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252   
LC_MONETARY=German_Germany.1252
[4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252    

attached base packages:
[1] stats     graphics  grDevices datasets  utils     methods   base

On 09.02.2010 10:50, g.russell at eos-solutions.com
wrote:
> Full_Name: George Russell
> Version: 2.10.0 and 2.11.0 Under development (unstable) (2010-02-08 r51108)
> OS: Windows
> Submission from: (NULL) (217.111.3.131)
>
>
>> c("a","b")[[c(TRUE,FALSE)]]
> Error in `[[.default`(factor(c("a", "b")), c(TRUE,
FALSE)) :
>    recursive indexing failed at level 1
>
> I find this error message confusing, though after reading the HELP
carefully I
> think I know what is going on. Would not something like "[[ does not
work with
> logical index vectors" be more appropriate?

Well, it is nonsense to use it, but works since TRUE is coerced to 1 as in:

list(list(11, 12), 13)[[c(TRUE, TRUE)]]


In your example will also fail with integers without a chance to do the 
recursion.

Uwe Ligges



> sessionInfo is (for 2.11) :
> R version 2.11.0 Under development (unstable) (2010-02-08 r51108)
> i386-pc-mingw32
>
> locale:
> [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252
> LC_MONETARY=German_Germany.1252
> [4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252
>
> attached base packages:
> [1] stats     graphics  grDevices datasets  utils     methods   base
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

g.russell at eos-solutions.com wrote:
> Full_Name: George Russell
> Version: 2.10.0 and 2.11.0 Under development (unstable) (2010-02-08 r51108)
> OS: Windows
> Submission from: (NULL) (217.111.3.131)
> 
> 
>> c("a","b")[[c(TRUE,FALSE)]]
> Error in `[[.default`(factor(c("a", "b")), c(TRUE,
FALSE)) :
>   recursive indexing failed at level 1
> 
> I find this error message confusing, though after reading the HELP
carefully I
> think I know what is going on. Would not something like "[[ does not
work with
> logical index vectors" be more appropriate?
It didn't take particularly careful reading to find this:

  "The most important distinction between [, [[ and $ is that
   the [ can select more than one element whereas the other two
   select a single element."

Try this:

  c("a","b")[[c(1,2)]]
  c("a","b")[[TRUE]]

  -Peter Ehlers
> 
> sessionInfo is (for 2.11) :
> R version 2.11.0 Under development (unstable) (2010-02-08 r51108) 
> i386-pc-mingw32 
> 
> locale:
> [1] LC_COLLATE=German_Germany.1252  LC_CTYPE=German_Germany.1252   
> LC_MONETARY=German_Germany.1252
> [4] LC_NUMERIC=C                    LC_TIME=German_Germany.1252    
> 
> attached base packages:
> [1] stats     graphics  grDevices datasets  utils     methods   base
> 
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
> 
> 
-- 
Peter Ehlers
University of Calgary

On 09/02/2010 4:50 AM, g.russell at eos-solutions.com
wrote:
> Full_Name: George Russell
> Version: 2.10.0 and 2.11.0 Under development (unstable) (2010-02-08 r51108)
> OS: Windows
> Submission from: (NULL) (217.111.3.131)
>
>
> > c("a","b")[[c(TRUE,FALSE)]]
> Error in `[[.default`(factor(c("a", "b")), c(TRUE,
FALSE)) :
>   recursive indexing failed at level 1
>   
I don't see that.  I get this:

 >  c("a","b")[[c(TRUE,FALSE)]]
Error in c("a", "b")[[c(TRUE, FALSE)]] :
  recursive indexing failed at level 1

which differs because c("a", "b") is not a factor.
> I find this error message confusing, though after reading the HELP
carefully I
> think I know what is going on. Would not something like "[[ does not
work with
> logical index vectors" be more appropriate?
>   
No, because it sometimes does work with logical index vectors:

 > x <- 1:2
 > x[[TRUE]]
[1] 1

(Here the TRUE is treated as 1.  I think it only works when the logical 
vector contains TRUE values, FALSE will fail, just as x[[0]] fails.)

The problem is that you were asking for the FALSE entry of the TRUE 
entry of the object, and since you had a simple vector, recursive 
indexing fails, and that's what the message says.  I imagine you had 
meant to type c("a", "b")[c(TRUE, FALSE)], but how can R
know you meant
that?

Duncan Murdoch