possible bug: NULL equality in lists.

[Charles Dupont]

I was messing around with R and I found an example R behaving oddly:

a <- alist(NULL, "bob", c(3,6,2,3))
a
a == 'NULL'
a == "NULL"
a == 'cat'


If I create a list with a NULL value
  >a <- alist(NULL, "bob", c(3,6,2,3))
  >a
[[1]]
NULL

[[2]]
[1] "bob"

[[3]]
c(3, 6, 2, 3)

and run some tests on 'a', the '== "NULL' test returns TRUE for the NULL
entry in the list 'a'.
  >a == 'NULL'
[1]  TRUE FALSE FALSE
  >a == "NULL"
[1]  TRUE FALSE FALSE
  >a == 'cat'
[1]  FALSE FALSE FALSE

This is consistent for every example of NULL's in a list that I can
think of.

Is this a bug or undocumented correct behavior?

Here is my version output

platform i486-pc-linux-gnu
arch     i486
os       linux-gnu
system   i486, linux-gnu
status
major    2
minor    2.0
year     2005
month    10
day      06
svn rev  35749
language R


Thanks

Charles
--
Charles Dupont Computer System Analyst School of Medicine
                Department of Biostatistics Vanderbilt University

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[Uwe Ligges]

Charles Dupont wrote:

> I was messing around with R and I found an example R behaving oddly:
>
> a <- alist(NULL, "bob", c(3,6,2,3))
> a
> a == 'NULL'
> a == "NULL"
> a == 'cat'
>


Always use is.null() to test on NULL, as in:

   sapply(a, is.null)

Uwe Ligges

> If I create a list with a NULL value
>   >a <- alist(NULL, "bob", c(3,6,2,3))
>   >a
> [[1]]
> NULL
>
> [[2]]
> [1] "bob"
>
> [[3]]
> c(3, 6, 2, 3)
>
> and run some tests on 'a', the '== "NULL' test returns TRUE for the NULL
> entry in the list 'a'.
>   >a == 'NULL'
> [1]  TRUE FALSE FALSE
>   >a == "NULL"
> [1]  TRUE FALSE FALSE
>   >a == 'cat'
> [1]  FALSE FALSE FALSE
>
> This is consistent for every example of NULL's in a list that I can
> think of.
>
> Is this a bug or undocumented correct behavior?
>
> Here is my version output
>
> platform i486-pc-linux-gnu
> arch     i486
> os       linux-gnu
> system   i486, linux-gnu
> status
> major    2
> minor    2.0
> year     2005
> month    10
> day      06
> svn rev  35749
> language R
>
>
> Thanks
>
> Charles

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[Seth Falcon-2]

Uwe Ligges <[hidden email]> writes:

> Charles Dupont wrote:
>
>> I was messing around with R and I found an example R behaving oddly:
>>
>> a <- alist(NULL, "bob", c(3,6,2,3))
>> a
>> a == 'NULL'
>> a == "NULL"
>> a == 'cat'
>>
>
>
> Always use is.null() to test on NULL, as in:

What should I do if I want to check for the string "NULL"?

> a <- list(NULL, "NULL", NA, "NA")

> a == "NULL"
[1]  TRUE  TRUE FALSE FALSE

> a == "NA"
[1] FALSE FALSE  TRUE  TRUE

These are because of as.character:

> as.character(a)
[1] "NULL" "NULL" "NA"   "NA"  

Yet,

> as.character(NULL)
character(0)
> as.character(NA)
[1] NA



+ seth

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[Dimitris Rizopoulos]

I think it'd be more appropriate to use:

sapply(a, "==", "NULL")

or

sapply(a, "==", "NA")

for this case.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: [phone]
Fax: [phone]
Web: http://www.med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm


----- Original Message -----
From: "Seth Falcon" <[hidden email]>
To: <[hidden email]>
Sent: Tuesday, March 07, 2006 4:37 PM
Subject: Re: [Rd] possible bug: NULL equality in lists.

> Uwe Ligges <[hidden email]> writes:
>
>> Charles Dupont wrote:
>>
>>> I was messing around with R and I found an example R behaving
>>> oddly:
>>>
>>> a <- alist(NULL, "bob", c(3,6,2,3))
>>> a
>>> a == 'NULL'
>>> a == "NULL"
>>> a == 'cat'
>>>
>>
>>
>> Always use is.null() to test on NULL, as in:
>
> What should I do if I want to check for the string "NULL"?
>
>> a <- list(NULL, "NULL", NA, "NA")
>
>> a == "NULL"
> [1]  TRUE  TRUE FALSE FALSE
>
>> a == "NA"
> [1] FALSE FALSE  TRUE  TRUE
>
> These are because of as.character:
>
>> as.character(a)
> [1] "NULL" "NULL" "NA"   "NA"
>
> Yet,
>
>> as.character(NULL)
> character(0)
>> as.character(NA)
> [1] NA
>
>
>
> + seth
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm

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[Uwe Ligges]

Seth Falcon wrote:

> Uwe Ligges <[hidden email]> writes:
>
>
>>Charles Dupont wrote:
>>
>>
>>>I was messing around with R and I found an example R behaving oddly:
>>>
>>>a <- alist(NULL, "bob", c(3,6,2,3))
>>>a
>>>a == 'NULL'
>>>a == "NULL"
>>>a == 'cat'
>>>
>>
>>
>>Always use is.null() to test on NULL, as in:
>
>
> What should I do if I want to check for the string "NULL"?

These are all dangerous, hence use the "safe" ways:

sapply(a, is.null)
sapply(a, identical, "NULL")
sapply(a, is.na)
sapply(a, identical, "NA")

Best,
Uwe

>
>>a <- list(NULL, "NULL", NA, "NA")
>
>
>>a == "NULL"
>
> [1]  TRUE  TRUE FALSE FALSE
>
>
>>a == "NA"
>
> [1] FALSE FALSE  TRUE  TRUE
>
> These are because of as.character:
>
>
>>as.character(a)
>
> [1] "NULL" "NULL" "NA"   "NA"  
>
> Yet,
>
>
>>as.character(NULL)
>
> character(0)
>
>>as.character(NA)
>
> [1] NA
>
>
> + seth
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel

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[Seth Falcon-2]

Uwe Ligges <[hidden email]> writes:
> These are all dangerous, hence use the "safe" ways:
>
> sapply(a, is.null)
> sapply(a, identical, "NULL")
> sapply(a, is.na)
> sapply(a, identical, "NA")

Point taken, but is the behavior of as.character correct?

as.character(list(NULL))

as.character(NULL)

+ seth

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[Charles Dupont]

Uwe Ligges wrote:

> Seth Falcon wrote:
>
>
>>Uwe Ligges <[hidden email]> writes:
>>
>>
>>
>>>Charles Dupont wrote:
>>>
>>>
>>>
>>>>I was messing around with R and I found an example R behaving oddly:
>>>>
>>>>a <- alist(NULL, "bob", c(3,6,2,3))
>>>>a
>>>>a == 'NULL'
>>>>a == "NULL"
>>>>a == 'cat'
>>>>
>>>
>>>
>>>Always use is.null() to test on NULL, as in:
>>
>>
>>What should I do if I want to check for the string "NULL"?
>
>
> These are all dangerous, hence use the "safe" ways:
>
> sapply(a, is.null)
> sapply(a, identical, "NULL")
> sapply(a, is.na)
> sapply(a, identical, "NA")
>
> Best,
> Uwe

For the NA list problem.  It would be better if 'as.character' when
converting a list when it finds a NA value to set that element to the
string NA value not "NA" the string.


Charles

>
>
>>>a <- list(NULL, "NULL", NA, "NA")
>>
>>
>>>a == "NULL"
>>
>>[1]  TRUE  TRUE FALSE FALSE
>>
>>
>>
>>>a == "NA"
>>
>>[1] FALSE FALSE  TRUE  TRUE
>>
>>These are because of as.character:
>>
>>
>>
>>>as.character(a)
>>
>>[1] "NULL" "NULL" "NA"   "NA"  
>>
>>Yet,
>>
>>
>>
>>>as.character(NULL)
>>
>>character(0)
>>
>>
>>>as.character(NA)
>>
>>[1] NA
>>
>>
>>+ seth
>>
>>______________________________________________
>>[hidden email] mailing list
>>https://stat.ethz.ch/mailman/listinfo/r-devel
>
>
> ______________________________________________
> [hidden email] mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>

--
Charles Dupont Computer System Analyst School of Medicine
                Department of Biostatistics Vanderbilt University

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[Uwe Ligges]

Seth Falcon wrote:

> Uwe Ligges <[hidden email]> writes:
>
>>These are all dangerous, hence use the "safe" ways:
>>
>>sapply(a, is.null)
>>sapply(a, identical, "NULL")
>>sapply(a, is.na)
>>sapply(a, identical, "NA")
>
>
> Point taken, but is the behavior of as.character correct?
>
> as.character(list(NULL))
>
> as.character(NULL)

I thought about it quite a while. I think the current bahaviour is quite
OK. What should as.character do in the follwing case:
   as.character(list("A", NA, NULL))
?

Note that it only can return a character vector...!

So, should it return a character vector of length 2? That's a bad idea,
if the length is reduced.

Moreover, as.character() does not get a NA or a NULL object for coercion
but an element of type list that itself conatins NA or NULL...

So if you want to convert a list to character and *keep* NA/NULL values,
you can only say:

  lapply(a, as.character)

Uwe






> + seth
>
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[Seth Falcon-2]

Uwe Ligges <[hidden email]> writes:

>> Point taken, but is the behavior of as.character correct?
>> as.character(list(NULL))
>> as.character(NULL)
>
>
> I thought about it quite a while. I think the current bahaviour is
> quite OK. What should as.character do in the follwing case:
>    as.character(list("A", NA, NULL))
> ?
>
> Note that it only can return a character vector...!
>
> So, should it return a character vector of length 2? That's a bad
> idea, if the length is reduced.

But consistent with vectorizing a list using unlist:

unlist(list(NULL, NULL, "a"))
[1] "a"

> Moreover, as.character() does not get a NA or a NULL object for
> coercion but an element of type list that itself conatins NA or
> NULL...

In the case of NA, I think converting to "NA" should be a last
resort.  Since NA is a perfectly valid element of a character vector,
it would seem to be a better choice.

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[Uwe Ligges]

Seth Falcon wrote:

> Uwe Ligges <[hidden email]> writes:
>
>>>Point taken, but is the behavior of as.character correct?
>>>as.character(list(NULL))
>>>as.character(NULL)
>>
>>
>>I thought about it quite a while. I think the current bahaviour is
>>quite OK. What should as.character do in the follwing case:
>>   as.character(list("A", NA, NULL))
>>?
>>
>>Note that it only can return a character vector...!
>>
>>So, should it return a character vector of length 2? That's a bad
>>idea, if the length is reduced.
>
>
> But consistent with vectorizing a list using unlist:
>
> unlist(list(NULL, NULL, "a"))
> [1] "a"

Seth, as.character() does NOT unlist anything, it converts the *list*
objects to character, the one element case might be a to trivial
example, instead consider:


 > as.character(list(c("a", "b"), NULL, c(NA, NULL, 2)))
[1] "c(\"a\", \"b\")" "NULL"            "c(NA, 2)"

I think you simply should not apply as.character on a list as a whole in
order to get what you are expecting ...

Best,
Uwe




>>Moreover, as.character() does not get a NA or a NULL object for
>>coercion but an element of type list that itself conatins NA or
>>NULL...
>
>
> In the case of NA, I think converting to "NA" should be a last
> resort.  Since NA is a perfectly valid element of a character vector,
> it would seem to be a better choice.



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