[R] list of lists, is this element empty

[Boris Steipe]

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.