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.tur...@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.ban...@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
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