Berwin A Turlach wrote:
> G'day Carl,
>
> On Mon, 23 Mar 2009 20:11:19 -0400
> Carl Witthoft <c...@witthoft.com> wrote:
>
>
>
>> But seriously: can someone explain to me what's going on in the
>> rvalues.r code? I tried a simple experiment, replacing ":=" with a
>> "colec" in the code, and of course the line
>>
>> c(df1, df2) colec list(4:8, 9:13)
>>
>>
>> just gives me a "syntax error" response. Clearly I need a pointer
>> to some documentation about how the colon and equals sign get
>> "special treatment somewhere inside R.
>>
>
> Not sure why := gets a special treatment,
yet another bug??
> perhaps because it is not a
> valid name and, hence, the parser deduces that it is an operator?
>
>
well, you can't do it with, e.g., ':==', because you'd get a *syntactic*
error (while ':=' gives a semantic error):
a :== 1
# syntactic error: unexpected '=' in ':='
':==' = function(a, b) NULL
a :== 1
# syntactic error again, of course
it's indeed surprising that it works with ':=' but not with, e.g., ':>'.
and in cases like ':-' you'd in fact use two operators (so an
'overloading' won't work for these):
':-' = function(a, b) a - if(a > b) b else 0
2 :- 1
# 2 1 0 -1
# not 1
it's interesting to note that
a :< b
# error: unexpected '<' in ':<'
will tell you what's unexpected, while
a :% b
# error: unexpected input in 'a :% b'
a :_ b
# error: unexpected input in 'a :_'
will leave you wondering what's wrong there.
vQ
______________________________________________
R-help@r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
