On Mon, 24 May 2010, Josef Leydold wrote:
Dear Brian and Uwe,
Thanks a lot for the clarification.
I made the naive assumption that numeric constants in R are similar to
those in C.
Two questions still remain:
(1) when I have a function
f<- function(a=1,b=-1) { a+b }
is it safe to use
val <- as.character(deparse(formals(f)$b))
to obtain a string that contains the default value for
argument "b". (Does is also work for other arguments with some
default of arbitrary class?)
But the defualt value is not character, so this cannot in general be
done.
(2) I have seen that packages like gWidget (in function ggenericwidget)
use a statement like
switch(class(formals(f)$b),
numeric = { .... },
character = { .... },
class = { .... }, ....
for automatically processing function arguments.
in the case of "b=-1" this procedure obviously fails.
(I found this behavior of 'formals' while playing around with
packages "gWidgets" and "fgui" from CRAN).
Is there a safe workaround for this problem?
That is, is there a safe function that returns class
"numeric" for an exresion like "-1" or "-Inf"?
Why are you using class() when you seem to mean typeof()?
But the short answer is you appear to be trying to circumvent the
language, and who really cares what the default is? If it is used, it
is evaluated, and then you can simply do typeof(b). And if it is not
used, who cares what it is?
Josef
On Sun, May 23, 2010 at 03:52:00PM +0100, Prof Brian Ripley wrote:
Documented too: from ?NumericConstants
Note that a leading plus or minus is not regarded by the parser as
part of a numeric constant but as a unary operator applied to the
constant.
On Sun, 23 May 2010, Uwe Ligges wrote:
On 23.05.2010 16:14, Josef Leydold wrote:
Hi,
I am a little bit surprised by the following output of
'formals'. Is this the intended behavior?
f<- function(a=1,b=-1) { a+b }
class(formals(f)$a)
[1] "numeric"
class(formals(f)$b)
[1] "call"
Josef
Yes, the arguments have not yet been evaluated, hence -1 is still a language
object.
Try to write
f<- function(a= +1, b= -1) { a+b }
and you will find that this is a fascinating feature.
Uwe Ligges
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Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
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Josef Leydold | WU (Vienna University of Economics and Business)
| Institute for Statistics and Mathematics
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Alles Unglueck kam daher, dass die Denkenden nicht mehr handeln konnten,
und die Handelnden keine Zeit mehr fanden zu denken. (Marlen Haushofer)
--
Brian D. Ripley, rip...@stats.ox.ac.uk
Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/
University of Oxford, Tel: +44 1865 272861 (self)
1 South Parks Road, +44 1865 272866 (PA)
Oxford OX1 3TG, UK Fax: +44 1865 272595
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