[Rd] Recent changes to as.complex(NA_real_)

[Martin Maechler]

>>>>> Mikael Jagan 
>>>>>     on Thu, 21 Sep 2023 00:47:39 -0400 writes:

    > Revisiting this thread from April:

    >      https://stat.ethz.ch/pipermail/r-devel/2023-April/082545.html

    > where the decision (not yet backported) was made for
    > as.complex(NA_real_) to give NA_complex_ instead of
    > complex(r=NA_real_, i=0), to be consistent with
    > help("as.complex") and as.complex(NA) and as.complex(NA_integer_).

    > Was any consideration given to the alternative?  
    > That is, to changing as.complex(NA) and as.complex(NA_integer_) to
    > give complex(r=NA_real_, i=0), consistent with
    > as.complex(NA_real_), then amending help("as.complex")
    > accordingly?

Hmm, as, from R-core, mostly I was involved, I admit to say "no",
to my knowledge the (above) alternative wasn't considered.

  > The principle that
  > Im(as.complex(<real=(double|integer|logical)>)) should be zero
  > is quite fundamental, in my view, hence the "new" behaviour 
  > seems to really violate the principle of least surprise ...

of course "least surprise"  is somewhat subjective.  Still,
I clearly agree that the above would be one desirable property.

I think that any solution will lead to *some* surprise for some
cases, I think primarily because there are *many* different
values z  for which  is.na(z)  is true,  and in any case
NA_complex_  is only of the many.

I also agree with Mikael that we should reconsider the issue
that was raised by Davis Vaughan here ("on R-devel") last April.

    > Another (but maybe weaker) argument is that
    > double->complex coercions happen more often than
    > logical->complex and integer->complex ones.  Changing the
    > behaviour of the more frequently performed coercion is
    > more likely to affect code "out there".

    > Yet another argument is that one expects

    >      identical(as.complex(NA_real_), NA_real_ + (0+0i))

    > to be TRUE, i.e., that coercing from double to complex is
    > equivalent to adding a complex zero.  The new behaviour
    > makes the above FALSE, since NA_real_ + (0+0i) gives
    > complex(r=NA_real_, i=0).

No!  --- To my own surprise (!) --- in current R-devel the above is TRUE, 
and
      NA_real_ + (0+0i)  , the same as
      NA_real_ + 0i      , really gives  complex(r=NA, i=NA) :

Using showC() from ?complex

  showC <- function(z) noquote(sprintf("(R = %g, I = %g)", Re(z), Im(z)))

we see (in R-devel) quite consistently

> showC(NA_real_ + 0i)
[1] (R = NA, I = NA)
> showC(NA       + 0i)  # NA is 'logical'
[1] (R = NA, I = NA)
> 

where as in R 4.3.1 and "R-patched" -- *in*consistently

> showC(NA_real_ + 0i)
[1] (R = NA, I = 0)
> showC(NA + 0i)
[1] (R = NA, I = NA)
> 

.... and honestly, I do not see *where* (and when) we changed
the underlying code (in arithmetic.c !?)  in R-devel to *also*
produce  NA_complex_  in such complex *arithmetic*


    > Having said that, one might also (but more naively) expect

    >     identical(as.complex(as.double(NA_complex_)), NA_complex_)

    > to be TRUE.  

as in current R-devel

    > Under my proposal it continues to be FALSE.

as in "R-release"

    > Well, I'd prefer if it gave FALSE with a warning
    > "imaginary parts discarded in coercion", but it seems that
    > as.double(complex(r=a, i=b)) never warns when either of
    > 'a' and 'b' is NA_real_ or NaN, even where "information"
    > {nonzero 'b'} is clearly lost ...

The question of *warning* here is related indeed, but I think
we should try to look at it only *secondary* to your first
proposal.

    > Whatever decision is made about as.complex(NA_real_),
    > maybe these points should be weighed before it becomes part of
    > R-release ...

    > Mikael

Indeed.

Can we please get other opinions / ideas here?

Thank you in advance for your thoughts!
Martin

--- 

PS: 

 Our *print()*ing  of complex NA's ("NA" here meaning NA or NaN)
 is also unsatisfactory, e.g. in the case where all entries of a
 vector are NA in the sense of is.na(.), but their
 Re() and Im() are not all NA:
 
  showC <- function(z) noquote(sprintf("(R = %g, I = %g)", Re(z), Im(z)))
  z <- complex(, c(11, NA, NA), c(NA, 99, NA))
  z
  showC(z)

gives

  > z
  [1] NA NA NA
  > showC(z)
  [1] (R = 11, I = NA) (R = NA, I = 99) (R = NA, I = NA)

but that (printing of complex) *is* another issue,
in which we have the re-opened bugzilla PR#16752
    ==>   https://bugs.r-project.org/show_bug.cgi?id=16752

on which we also worked during the R Sprint in Warwick three
weeks ago, and where I want to commit changes in any case {but
think we should change even a bit more than we got to during the
Sprint}.