On 1/23/2011 12:15 PM, Vitalie S. wrote:
Spencer Graves<spencer.gra...@structuremonitoring.com> writes:
On 1/23/2011 8:50 AM, peter dalgaard wrote:
On Jan 22, 2011, at 21:08 , Vitalie S. wrote:
The only definite argument occurred in the thread against "+" operator
was the lack of commutativity (as if one have to prove algebraic
theorems in R).
I think the real killer was associativity, combined with coercion rules:
Is "x"+1+2 supposed to be equal to "x12" or "x3"?
Excellent: This seems like a good reason to follow Python: Allow "a+b" with a
character vector "a" only if
"b" is also a character vector (or factor?).
This example raises another question: If we allow "a+b" for "a" and "b"
both character vectors (and give an
error if one is numeric), what do we do with factors? If "a" is a factor,
return a factor?
If we define custom %+% as:
`%+%`<- function(a, b){
if(is.character(a) || is.character(b))
paste(as.character(a), as.character(b), sep="")
else
a + b
}
because of higher precedence of %any% operators over binary + we have:
"a" %+% 1 %+% 2
## [1] "a12"
and
str("a" %+% factor(1:2))
## chr [1:2] "a1" "a2"
so if + on characters would behave "as if" having slightly higher priority than
other + operators that might solve reasonably the problem.
Vitalie.
No: 'a' %+% (1 %+%2) != ('a' %+% 1) %+% 2, as Peter Dalgaard noted:
'a3' != 'a12'.
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