[Rd] Illegal Logical Values

[Martyn Plummer]

On Fri, 2017-10-20 at 14:01 +0000, brodie gaslam via R-devel wrote:
> I'm wondering if WRE Section 5.2 should be a little more explicit
> about misuse of integer values other than NA, 0, and 1 in LGLSXPs. 
> I'm thinking of this passage:
> 
> > Logical values are sent as 0 (FALSE), 1 (TRUE) or INT_MIN =
> > -2147483648 (NA, but only if NAOK is true), and the compiled code
> > should return one of these three values. (Non-zero values other
> > than INT_MIN are mapped to TRUE.) 
> 
> The parenthetical seems to suggest that something like 'LOGICAL(x)[0]
> = 2;' will be treated as TRUE, which it sometimes is, and sometimes
> isn't:

The title of Section 5.2 is "Interface functions .C and .Fortran" and
the text above refers to those interfaces. It explains how logical
vectors are mapped to C integer arrays on entry and back again on exit.

This does work as advertised. Here is a simple example. File
"nottrue.c" contains the text

void nottrue(int *x)
{
   x[0] = 2;
}

This is compiled with "R CMD SHLIB nottrue.c" to created the shared
object "nottrue.so"

> dyn.load("nottrue.so")
> a <- .C("nottrue", x=integer(1))
> a
$x
[1] 2

> a <- .C("nottrue", x=logical(1))
> a
$x
[1] TRUE

> isTRUE(a$x)
[1] TRUE
> as.integer(a)
[1] 1

So for a logical argument, the integer value 2 is mapped back to a
valid value on return.

> not.true <- inline::cfunction(body='
>   SEXP res = allocVector(LGLSXP, 1);
>   LOGICAL(res)[0] = 2;
>   return res;'
> )()
> not.true
> ## [1] TRUE
> not.true == TRUE
> ## [1] FALSE
> not.true[1] == TRUE  # due to scalar subset handling
> ## [1] TRUE
> not.true == 2L
> ## [1] TRUE

In your last example, not.true is coerced to integer (as explain in the
help for ("==") and its integer value of 2 is recovered.

> Perhaps a more explicit warning that using anything other than 0, 1,
> or NA is undefined behavior is warranted?  Obviously people should
> know better than to expect correct behavior, but the fact that the
> behavior is correct in some cases (e.g. printing, scalar subsetting)
> might be confusing.

Yes if people are tripping up on this then we could clarify that the
.Call interface does not remap logical vectors on exit. Hence 
assignment of any value other than 0, 1 or INT_MIN to the elements of a
logical vector may cause unexpected behaviour when this vector is
returned to R.

Martyn

> >
> Best,
> B.rodie.
> 
> 	[[alternative HTML version deleted]]
> 
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