[R] list of lists, is this element empty

[Bert Gunter]

Boris et. al:

Indeed, corner cases are a bear, which is why it is incumbent on any
OP to precisely define what they mean by, say, "missing",
"null","empty", etc.

Here is an evil example to illustrate the sorts of nastiness that can occur:

> z <- list(a=NULL, b=list(), c=NA)

> with(z,{
+ c(identical(a,b),
+ identical(a,c),
+ identical(b,c)
+ )
+ })
[1] FALSE FALSE FALSE

## OK, none of these three are "the same" in the sense of identical().
But ...

> outer(z,z,identical)
Error in outer(z, z, identical) :
  dims [product 9] do not match the length of object [1]

## outer gets completely flummoxed, as it should!

> expand.grid(z,z)
  Var1 Var2
1 NULL NULL
2 NULL NULL
3   NA NULL
4 NULL NULL
5 NULL NULL
6   NA NULL
7 NULL   NA
8 NULL   NA
9   NA   NA

## and expand.grid gets confused, as it probably should.

:-)

Cheers,
Bert

Bert Gunter
Genentech Nonclinical Biostatistics
(650) 467-7374

"Data is not information. Information is not knowledge. And knowledge
is certainly not wisdom."
Clifford Stoll




On Sat, Dec 20, 2014 at 10:57 AM, Boris Steipe <boris.steipe at utoronto.ca> wrote:
> This can be tricky, because depending on what the missing object is, you can get either NULL, NA, or an error. Moreover is.na() behaves differently when evaluated on its own, or as the condition of an if() statement. Here is a function that may make life easier. The goal is NOT to have to pass extra arguments.
>
> - I use try() and return FALSE if the evaluation returns an error.
>   This applies to objects that are not found, incorrect syntax etc.
> - List elements that don't exist are NULL and return FALSE.
> - If any elements are NA, return FALSE. This handles out-of-bounds
>   elements AND out-of-bounds slices on vectors. But it would also
>   trip on valid vectors that contain an NA. I can't think of a good
>   way to distinguish these two cases right now. The "best" way for
>   this depends on the context.
>
> I think I am handling the most obvious special cases - though I do expect this can be improved.
>
>
> is.valid <- function(x,
>                      na.ignore = FALSE,
>                      null.ignore=FALSE) {
>
>     # errors are always FALSE
>     if (class(try(x, silent=TRUE)) == "try-error") return(FALSE)
>
>     # NULL is FALSE except if ignored
>     if (is.null(x)) {
>         if (!null.ignore) return(FALSE)
>         return(TRUE)
>     }
>
>     # If all elments are NA, return FALSE except if ignored;
>     if (any(is.na(x))) {
>         if (!na.ignore) return(FALSE)
>         return(TRUE)
>     }
>
>     # Everything else is TRUE
>     return(TRUE)
> }
>
>
>
> # Test cases
> is.valid(1)             # TRUE: valid numeric constant
> is.valid(FALSE)         # TRUE: valid boolean constant
> is.valid(nonSuch)       # FALSE: object doesn't exist
>
> x <- 1:5;
> is.valid(x)             # TRUE: existing variable
> is.valid(x[4])          # TRUE: vector element
> is.valid(x[8])          # FALSE: out of bounds: NA
> is.valid(x[5:6])        # FALSE: partially out of bounds: (5, NA)
>
> is.valid(x[8], na.ignore=TRUE)  # TRUE
>
> x[3] <- NA
> is.valid(x)             # FALSE: no element can be NA
> is.valid(x, na.ignore=TRUE)  # TRUE
>
>
> m <- matrix(1:9,nrow=3, ncol=3)
> is.valid(m[2,2])        # TRUE
> is.valid(m[2,4])        # FALSE: subscript out of bounds
> is.valid(m[2,2,2])      # FALSE: incorrect n of dimensions
>
> l <- list(first=7, letters, NULL)
> is.valid(l[["first"]])  # TRUE: existing list elements
> is.valid(l[[4]])        # FALSE: list element does not exist
> is.valid(l[[2]][27])    # FALSE: out of bounds on existing element
> is.valid(l$first)       # TRUE:
> is.valid(l$second)      # FALSE: non-existent element: NULL
> is.valid(l$second, null.ignore=TRUE)  # TRUE
>
>
> Cheers,
> B.
>
>
>
> On Dec 20, 2014, at 11:20 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
>
>> Hello,
>>
>> Your list seems to have only 2 elements. You can check this with
>>
>> length(x)
>>
>> Or you can try
>>
>> lapply(x, is.null)
>>
>> Hope this helps,
>>
>> Rui Barradas
>>
>> Em 20-12-2014 15:58, Ragia Ibrahim escreveu:
>>> Hello,
>>> Kindly I have a list of lists as follow
>>> x
>>> [[1]]
>>> [1] 7
>>>
>>> [[2]]
>>> [1] 3 4 5
>>>
>>> as showen x[[3]] does not have a value and it has NULL, how can I check on this
>>> how to test if x[[3]] is empty.
>>>
>>> thanks in advance
>>> Ragia
>>>
>>>      [[alternative HTML version deleted]]
>>>
>>> ______________________________________________
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>>>
>>
>> ______________________________________________
>> 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.
>
> ______________________________________________
> 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.