[R] exists function on list objects gives always a FALSE

[Duncan Murdoch]

On 5/19/2009 12:07 PM, Žroutík 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