Wacek Kusnierczyk wrote:
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??
This is probably due to that in the gram.y file :
case ':':
if (nextchar(':')) {
if (nextchar(':')) {
yylval = install(":::");
return NS_GET_INT;
}
else {
yylval = install("::");
return NS_GET;
}
}
if (nextchar('=')) {
yylval = install(":=");
return LEFT_ASSIGN;
}
yylval = install(":");
return ':';
which gives a meaning to ":=", so that parsing x := 2 makes sense.
> parse( text = "x := 2" )
expression(x := 2)
attr(,"srcfile")
<text>
Romain
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
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--
Romain Francois
Independent R Consultant
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
