If statement generates two outputs

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

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Romain Francois
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http://romainfrancois.blog.free.fr

If statement generates two outputs

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