[R] If statement generates two outputs

[Romain Francois]

Wacek Kusnierczyk wrote:
> Berwin A Turlach wrote:
>   
>> G'day Carl,
>>
>> On Mon, 23 Mar 2009 20:11:19 -0400
>> Carl Witthoft <carl at 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
>
> ______________________________________________
> R-help at 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.
>
>
>   


-- 
Romain Francois
Independent R Consultant
+33(0) 6 28 91 30 30
http://romainfrancois.blog.free.fr