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

[Rolf Turner]

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.

cheers,

Rolf

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