R help - May 2009 - exists function on list objects gives always a FALSE

Dear R-users,

in a minimal example exists() gives FALSE on an object which obviously does
exist. How can I check on that list object anyway else, please?
> SmoothData <- list(exists=TRUE, span=0.001)
> SmoothData
$exists
[1] TRUE

$span
[1] 0.001
> exists("SmoothData")
TRUE
> exists("SmoothData$span")
FALSE
> exists("SmoothData[[2]]")
FALSE

Thank you for any opinion regarding this topic.
Zroutik

	[[alternative HTML version deleted]]

SmoothData$span is not an object which can be checked by exists(), but
part of an object which can be checked by is.null().

On Wed, May 20, 2009 at 12:07 AM, ?rout?k <zroutik at gmail.com>
wrote:
> Dear R-users,
>
> in a minimal example exists() gives FALSE on an object which obviously does
> exist. How can I check on that list object anyway else, please?
>
>> SmoothData <- list(exists=TRUE, span=0.001)
>> SmoothData
> $exists
> [1] TRUE
>
> $span
> [1] 0.001
>
>> exists("SmoothData")
> TRUE
>
>> exists("SmoothData$span")
> FALSE
>
>> exists("SmoothData[[2]]")
> FALSE
>
> Thank you for any opinion regarding this topic.
> Zroutik
>
> ? ? ? ?[[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> 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.
>

?rout?k wrote:
>
>> SmoothData <- list(exists=TRUE, span=0.001)
>> SmoothData
>>     
> $exists
> [1] TRUE
>
> $span
> [1] 0.001
>
>   
>> exists("SmoothData")
>>     
> TRUE
>
>   
>> exists("SmoothData$span")
>>     
> FALSE
>
>   
    'SmoothData$span' = 'foo'
    exists("SmoothData$span")
    # TRUE
>> exists("SmoothData[[2]]")
>>     
    'SmoothData[[2]]' = 'bar'
    exists("SmoothData[[2]]")
    # TRUE


the problem in your case is that you have an object named 'SmoothData'
with a nested component named 'span', but you're testing for the
existence of an object named 'SmoothData$span'. 

as shown in a recent post, one attempt to do your task would be

    exists('SmoothData') && 'span' %in%
names(SmoothData)
    # TRUE

vQ

On 5/19/2009 12:07 PM, ?rout?k wrote:
> Dear R-users,
> 
> in a minimal example exists() gives FALSE on an object which obviously does
> exist. How can I check on that list object anyway else, please?
> 
>> SmoothData <- list(exists=TRUE, span=0.001)
>> SmoothData
> $exists
> [1] TRUE
> 
> $span
> [1] 0.001
> 
>> exists("SmoothData")
> TRUE
> 
>> exists("SmoothData$span")
> FALSE
> 
>> exists("SmoothData[[2]]")
> FALSE
> 
> Thank you for any opinion regarding this topic.
There is no variable with name "SmoothData$span", there is an element
of
SmoothData with name "span".

To test for that, the safest test is probably

"span" %in% names(SmoothData)

but a common convention is to use

is.null(SmoothData$span)

because NULL elements are rare in lists.

Duncan Murdoch

?rout?k wrote:
> Dear R-users,
>
> in a minimal example exists() gives FALSE on an object which obviously does
> exist. How can I check on that list object anyway else, please?
>
>   
>> SmoothData <- list(exists=TRUE, span=0.001)
>> SmoothData
>>     
> $exists
> [1] TRUE
>
> $span
> [1] 0.001
>
>   
>> exists("SmoothData")
>>     
> TRUE
>   
>> exists("SmoothData$span")
>>     
> FALSE
>   
This checks for existance of an object called "SmoothData$span", as in
:

`SmoothData$span` <- 1:10
exists("SmoothData$span")

You can do:

is.list( SmoothData ) && !is.null(names(SmoothData)) &&
"span" %in%
names(SmoothData)
>   
>> exists("SmoothData[[2]]")
>>     
> FALSE
>   
Similarly:

`SmoothData[[2]]` <- 1
exists("SmoothData[[2]]")

You can do:

is.list( SmoothData ) && length(SmoothData) > 1
> Thank you for any opinion regarding this topic.
> Zroutik

-- 
Romain Francois
Independent R Consultant
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr

On Tue, May 19, 2009 at 12:07 PM, Žroutík <zroutik@gmail.com> wrote:
> > SmoothData <- list(exists=TRUE, span=0.001)
> > exists("SmoothData$span")
> FALSE
>
As others have said, this just checks for the existence of a variable with
the (strange) name "SmoothData$span".

In some sense, in R semantics, xxx$yyy *always* exists if xxx is a list (or
other recursive object):

     > xxx <- list()
     > xxx$hello
     NULL

You might think that you can check names(xxx) to see if the slot has been
explicitly set, but it depends on *how* you have explicitly set the slot to
NULL:

   > xxx$hello <- 3
   > xxx$hello <- NULL
   > names(xxx)
   character(0)                  # no names -- assigning to NULL kills slot
   > xxx <- list(hello=NULL)
   > names(xxx)
   [1] "hello"                    # 1 name -- constructing with
NULL-valued
slot

Welcome to R!

            -s

	[[alternative HTML version deleted]]