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

[Rolf Turner]

On 20/05/17 22:42, Duncan Murdoch wrote:
> 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.

Ah, I see.  I missed the point.

cheers,

Rolf

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