possible bug: NULL equality in lists.

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 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:

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

a == "NULL"

a == "NA"

as.character(a)

as.character(NULL)

as.character(NA)

Uwe Ligges <ligges at statistik.uni-dortmund.de> 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

Uwe Ligges <ligges at statistik.uni-dortmund.de> 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

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")

Seth Falcon wrote:

Uwe Ligges <ligges at statistik.uni-dortmund.de> 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

Uwe Ligges <ligges at statistik.uni-dortmund.de> 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

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...

Uwe Ligges <ligges at statistik.uni-dortmund.de> 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.