[R] [FORGED] Re: [FORGED] Logical Operators' inconsistent Behavior

[Duncan Murdoch]

On 20/05/2017 6:39 AM, Rolf Turner wrote:
> On 20/05/17 22:18, Duncan Murdoch wrote:
>> On 20/05/2017 5:53 AM, Martin Maechler wrote:
>>>>>>>> Ramnik Bansal <ramnik.bansal at gmail.com>
>>>>>>>>     on Sat, 20 May 2017 08:52:55 +0530 writes:
>>>
>>>     > Taking this question further.
>>>     > If I use a complex number or a numeric as an operand in logical
>>>     > operations, to me it APPEARS that these two types are first
>>> coerced to
>>>     > LOGICAL internally and then THIS logical output is further used
>>> as the
>>>     > operand.
>>>
>>>     > For eg.
>>>     >> x <- 4+5i; c(x & F, x & T, x | F, x | T)
>>>     > [1] FALSE  TRUE  TRUE  TRUE
>>>
>>>     > This output is consistent with
>>>     >> x <- 4+5i; c(as.logical(x) & F, as.logical(x) & T,
>>> as.logical(x) | F, as.logical(x) | T)
>>>     > [1] FALSE  TRUE  TRUE  TRUE
>>>
>>>     > This consistency makes me draw an on-the-surface conclusion that in
>>>     > the case of logical operations if the operand is not of type
>>> 'logical'
>>>     > it is first coerced into 'logical'.
>>>
>>> That conclusion is wrong as you show below.
>>> Rather, as the error message says,
>>> logical
>>>     "operations are possible only for numeric, logical or complex types"
>>>
>>> Again:
>>>
>>> 1) Logical/Arithmetic  operations "work" with "numeric-like" types,
>>> namely
>>>   numeric, logical or complex, (and numeric = {integer, double})
>>>
>>>   ==> all other types give an error (the one you've cited twice)
>>>
>>> 2) For "numeric-like" types and *logical* operations (&, |, !; plus &&
>>> and ||)
>>>    the equivalent of as.logical() is applied before performing the Op.
>>>
>>> Seems pretty consistent ...
>>> and also according to the principle of "least surprise" (for me at
>>> least).
>>>
>>
>> The surprise is that as.logical("TRUE") returns TRUE, whereas automatic
>> coercion doesn't apply to character strings.  I don't think we should
>> change this, but it is an inconsistency.  (We could perhaps mention it
>> in the ?logical help page.)
>
>
> Actually it *is* mentioned.  From ?logical:
>
>> Character strings c("T", "TRUE", "True", "true") are regarded as
>> true,  c("F", "FALSE", "False", "false") as false, and all others as NA.
>

I meant that the negative part should be mentioned:  this only works 
with an explicit as.logical(), not with implicit coercion.

Duncan Murdoch