exists function on list objects gives always a FALSE

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

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

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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

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

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

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

  

is.null is unhelpful here, in that lists can contain NULL as a named
element, and retrieving a non-existent element returns NULL:

    foo = list(bar=NULL)
    is.null(foo$bar)
    # TRUE
    is.null(foo$foo)
    # TRUE

i must admit i find it surprising that ?'$' does not appropriately
explain what happens if a list is indexed with a name not included in
the list's names.  the closest is

" When extracting, a numerical, logical or character 'NA' index
     picks an unknown element and so returns 'NA' in the corresponding
     element of a logical, integer, numeric, complex or character
     result, and 'NULL' for a list. "

but it's valid for NAs in the index, and

"  If no match is found then 'NULL' is returned. "

but it's in the section on environments.


vQ

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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
  

kills indeed:

    foo = list(bar=1)

    with(foo, bar)
    # 1

    foo$bar = NULL
    with(foo, bar)
    # error: object 'bar' not found

   > xxx <- list(hello=NULL)
   > names(xxx)

   [1] "hello"                    # 1 name -- constructing with NULL-valued
slot
  

but:

    # cleanup -- don't do it in mission critical session
    rm(list=ls())

    foo
    # error: object 'foo' not found

    foo = NULL
    foo
    # NULL

that is, foo$bar = NULL kills bar within foo (even though NULL is a
valid component of lists), but foo = NULL does *not* kill foo.

Welcome to R!
  

... and its zemanticks.

vQ