[Rd] R (development) changes in arith, logic, relop with (0-extent) arrays

[Martin Maechler]

>>>>> Martin Maechler <maechler at stat.math.ethz.ch>
>>>>>     on Tue, 6 Sep 2016 22:26:31 +0200 writes:

    > Yesterday, changes to R's development version were committed, relating
    > to arithmetic, logic ('&' and '|') and
    > comparison/relational ('<', '==') binary operators
    > which in NEWS are described as

    > SIGNIFICANT USER-VISIBLE CHANGES:

    > [.............]

    > • Arithmetic, logic (‘&’, ‘|’) and comparison (aka
    > ‘relational’, e.g., ‘<’, ‘==’) operations with arrays now
    > behave consistently, notably for arrays of length zero.

    > Arithmetic between length-1 arrays and longer non-arrays had
    > silently dropped the array attributes and recycled.  This
    > now gives a warning and will signal an error in the future,
    > as it has always for logic and comparison operations in
    > these cases (e.g., compare ‘matrix(1,1) + 2:3’ and
    > ‘matrix(1,1) < 2:3’).

    > As the above "visually suggests" one could think of the changes
    > falling mainly two groups,
    > 1) <0-extent array>  (op)     <non-array>
    > 2) <1-extent array>  (arith)  <non-array of length != 1>

    > These changes are partly non-back compatible and may break
    > existing code.  We believe that the internal consistency gained
    > from the changes is worth the few places with problems.

    > We expect some package maintainers (10-20, or even more?) need
    > to adapt their code.

    > Case '2)' above mainly results in a new warning, e.g.,

    >> matrix(1,1) + 1:2
    > [1] 2 3
    > Warning message:
    > In matrix(1, 1) + 1:2 :
    > dropping dim() of array of length one.  Will become ERROR
    >> 

    > whereas '1)' gives errors in cases the result silently was a
    > vector of length zero, or also keeps array (dim & dimnames) in
    > cases these were silently dropped.

    > The following is a "heavily" commented  R script showing (all ?)
    > the important cases with changes :

    > ----------------------------------------------------------------------------

    > (m <- cbind(a=1[0], b=2[0]))
    > Lm <- m; storage.mode(Lm) <- "logical"
    > Im <- m; storage.mode(Im) <- "integer"

    > ## 1. -------------------------
    > try( m & NULL ) # in R <= 3.3.x :
    > ## Error in m & NULL :
    > ##  operations are possible only for numeric, logical or complex types
    > ##
    > ## gives 'Lm' in R >= 3.4.0

    > ## 2. -------------------------
    > m + 2:3 ## gave numeric(0), now remains matrix identical to  m
    > Im + 2:3 ## gave integer(0), now remains matrix identical to Im (integer)

    > m > 1      ## gave logical(0), now remains matrix identical to Lm (logical)
    > m > 0.1[0] ##  ditto
    > m > NULL   ##  ditto

    > ## 3. -------------------------
    > mm <- m[,c(1:2,2:1,2)]
    > try( m == mm ) ## now gives error   "non-conformable arrays",
    > ## but gave logical(0) in R <= 3.3.x

    > ## 4. -------------------------
    > str( Im + NULL)  ## gave "num", now gives "int"

    > ## 5. -------------------------
    > ## special case for arithmetic w/ length-1 array
    > (m1 <- matrix(1,1,1, dimnames=list("Ro","col")))
    > (m2 <- matrix(1,2,1, dimnames=list(c("A","B"),"col")))

    > m1 + 1:2  # ->  2:3  but now with warning to  "become ERROR"
    > tools::assertError(m1 & 1:2)# ERR: dims [product 1] do not match the length of object [2]
    > tools::assertError(m1 < 1:2)# ERR:                  (ditto)
    > ##
    > ## non-0-length arrays combined with {NULL or double() or ...} *fail*

    > ### Length-1 arrays:  Arithmetic with |vectors| > 1  treated array as scalar
    > m1 + NULL # gave  numeric(0) in R <= 3.3.x --- still, *but* w/ warning to "be ERROR"
    > try(m1 > NULL)    # gave  logical(0) in R <= 3.3.x --- an *error* now in R >= 3.4.0
    > tools::assertError(m1 & NULL)    # gave and gives error
    > tools::assertError(m1 | double())# ditto
    > ## m2 was slightly different:
    > tools::assertError(m2 + NULL)
    > tools::assertError(m2 & NULL)
    > try(m2 == NULL) ## was logical(0) in R <= 3.3.x; now error as above!

    > ----------------------------------------------------------------------------


    > Note that in R's own  'nls'  sources, there was one case of
    > situation '2)' above, i.e. a  1x1-matrix was used as a "scalar".

    > In such cases, you should explicitly coerce it to a vector,
    > either ("self-explainingly") by  as.vector(.), or as I did in
    > the nls case  by  c(.) :  The latter is much less
    > self-explaining, but nicer to read in mathematical formulae, and
    > currently also more efficient because it is a .Primitive.

    > Please use R-devel with your code, and let us know if you see
    > effects that seem adverse.

I've been slightly surprised (or even "frustrated") by the empty
reaction on our R-devel list to this post.

I would have expected some critique, may be even some praise,
... in any case some sign people are "thinking along" (as we say
in German).

In the mean time, I've actually thought along the one case which
is last above:  The <op>  (binary operation) between a
non-0-length array and a 0-length vector (and NULL which should
be treated like a 0-length vector):

R <= 3.3.1  *is* quite inconsistent with these:


and my proposal above (implemented in R-devel, since Sep.5) would give an
error for all these, but instead, R really could be more lenient here:
A 0-length result is ok, and it should *not* inherit the array
(dim, dimnames), since the array is not of length 0. So instead
of the above [for the very last part only!!], we would aim for
the following. These *all* give an error in current R-devel,
with the exception of 'm1 + NULL' which "only" gives a "bad
warning" :

------------------------

m1 <- matrix(1,1)
m2 <- matrix(1,2)

m1 + NULL #    numeric(0) in R <= 3.3.x ---> OK ?!
m1 > NULL #    logical(0) in R <= 3.3.x ---> OK ?!
try(m1 & NULL)    # ERROR in R <= 3.3.x ---> change to logical(0)  ?!
try(m1 | double())# ERROR in R <= 3.3.x ---> change to logical(0)  ?!
## m2 slightly different:
try(m2 + NULL)  # ERROR in R <= 3.3.x ---> change to double(0)  ?!
try(m2 & NULL)  # ERROR in R <= 3.3.x ---> change to logical(0)  ?!
m2 == NULL # logical(0) in R <= 3.3.x ---> OK ?!

------------------------

This would be slightly more back-compatible than the currently
implemented proposal. Everything else I said remains true, and
I'm pretty sure most changes needed in packages would remain to be done.

Opinions ?



    > In some case where R-devel now gives an error but did not
    > previously, we could contemplate giving another  "warning
    > .... 'to become ERROR'" if there was too much breakage,  though
    > I don't expect that.


    > For the R Core Team,

    > Martin Maechler,
    > ETH Zurich