[Rd] unlist errors on a nested list of empty lists

[Steven Nydick]

It also does the same thing if the factor is not on the first level of the
list, which seems to be due to the fact that the islistfactor is recursive,
but if a list is a list-factor, the first level lists are coerced into
character strings.

> x <- list(list(factor(LETTERS[1])))
> unlist(x)
Error in as.character.factor(x) : malformed factor

However, if one of the factors is at the top level, and one is nested, then
the result is:

> x <- list(list(factor(LETTERS[1])), factor(LETTERS[2]))
> unlist(x)

[1] <NA> B
Levels: B

... which does not seem to me to be desired behavior.


On Tue, May 8, 2018 at 2:22 PM Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

> On 08/05/2018 2:58 PM, Duncan Murdoch wrote:
> > On 08/05/2018 1:48 PM, Steven Nydick wrote:
> >> Reproducible example:
> >>
> >> x <- list(list(list(), list()))
> >> unlist(x)
> >>
> >> *> Error in as.character.factor(x) : malformed factor*
> >
> > The error comes from the line
> >
> > structure(res, levels = lv, names = nm, class = "factor")
> >
> > which is called because unlist() thinks that some entry is a factor,
> > with NULL levels and NULL names.  It's not legal for a factor to have
> > NULL levels.  Probably it should never get here; the earlier test
> >
> > if (.Internal(islistfactor(x, recursive))) {
> >
> > should have been false, and then the result would have been
> >
> > .Internal(unlist(x, recursive, use.names))
> >
> > (with both recursive and use.names being TRUE), which returns NULL.
>
> And the problem is in the islistfactor function in src/main/apply.c,
> which looks like this:
>
> static Rboolean islistfactor(SEXP X)
> {
>      int i, n = length(X);
>
>      switch(TYPEOF(X)) {
>      case VECSXP:
>      case EXPRSXP:
>          if(n == 0) return NA_LOGICAL;
>          for(i = 0; i < LENGTH(X); i++)
>              if(!islistfactor(VECTOR_ELT(X, i))) return FALSE;
>          return TRUE;
>          break;
>      }
>      return isFactor(X);
> }
>
> One of those deeply nested lists is length 0, so at the lowest level it
> returns NA_LOGICAL.  But then it does C-style logical testing on the
> results.  I think to C NA_LOGICAL counts as true, so at the next level
> up we get the wrong answer.
>
> A fix would be to rewrite it like this:
>
> static Rboolean islistfactor(SEXP X)
> {
>      int i, n = length(X);
>      Rboolean result = NA_LOGICAL, childresult;
>      switch(TYPEOF(X)) {
>      case VECSXP:
>      case EXPRSXP:
>          for(i = 0; i < LENGTH(X); i++) {
>              childresult = islistfactor(VECTOR_ELT(X, i));
>              if(childresult == FALSE) return FALSE;
>              else if(childresult == TRUE) result = TRUE;
>          }
>          return result;
>          break;
>      }
>      return isFactor(X);
> }
>


-- 
Steven Nydick
PhD, Quantitative Psychology
M.A., Psychology
M.S., Statistics
--
"Beware of the man who works hard to learn something, learns it, and finds
himself no wiser than before, Bokonon tells us. He is full of murderous
resentment of people who are ignorant without having come by their
ignorance the hard way."
-Kurt Vonnegut

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