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

[Duncan Murdoch]

On 20/05/2017 10:03 PM, Ramnik Bansal wrote:
> In the case of logical operations performed with an operand being a
> character type, the error is:
> "operations are possible only for numeric, logical or complex types"
>
> Now the display of this error message seems to be the OUTCOME of the
> fact that implicit coercion is NOT BEING APPLIED INTERNALLY to
> character operands in the case of logical operations but is applied to
> all other "numeric" type operands. This error message is NOT the
> CAUSE.
>
> Well, why not:
>
> F & "abc" return a FALSE because if F & NA can return  FALSE, the
> logic being that once an operand is FALSE in case of AND operation
> then the output should be False, IRRESPECTIVE of the second operand.
> In that case how does it matter if the second operand  is "abc" or
> 4+5i or NA? The output is deterministically FALSE anyways.
>
> Effectively the point that I want to make is: Treat the character
> operand as NA ( except "F", "FALSE", "T", "TRUE" etc which can be
> treated as FALSE/TRUE) and then apply the logical operations. Or in
> short "Apply implicit coercion on character types as well in case of
> logical operations."

For consistency with that point of view, we'd also want

1 + "1"

to equal 2, as it does in some languages.  And maybe "1" + "1" should 
equal 2.

R doesn't do things like that.  It tries for consistency in the 
primitive operations:  you don't get auto-conversion of strings to 
logical/numeric values.  This forces some inconsistencies between the 
primitive operations and explicit coercions like as.logical(), but we 
think that's a good thing.

Duncan Murdoch

>
>
>
>
> On Sat, May 20, 2017 at 4:15 PM, Rolf Turner <r.turner at auckland.ac.nz> wrote:
>> 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
>>
>> --
>> Technical Editor ANZJS
>> Department of Statistics
>> University of Auckland
>> Phone: +64-9-373-7599 ext. 88276