using ``<-'' in function argument
It's only obvious when someone points it out :) fubar is not created because, in the test x > 3 returned FALSE, which means the cat function doesn't get used, which means the y arg (fubar <- 6) is never required and therefore not evaluated. Evil isn't it ? Michael
On 3 December 2010 20:18, Ivan Calandra <ivan.calandra at uni-hamburg.de> wrote:
See below Le 12/3/2010 06:54, Berwin A Turlach a ?crit :
On Thu, 2 Dec 2010 23:34:02 -0500 David Winsemius<dwinsemius at comcast.net> ?wrote:
[...] Erik is telling you that your use of ncol<-4 got evaluated to 4 and that the name of the resulting object was ignored, howevert the value of the operation was passed on to matrix which used positional matching since "=" was not used.
Sounds like a fair summary of what Erik said, but it is subtly wrong.
R has lazy evaluation of its arguments. ?There is nothing that forces
the assignment to be evaluated and to pass the result into the
function. ?On the contrary, the assignment takes place when the
function evaluates the argument. ?For example:
R> ?rm(list=ls(all=TRUE))
R> ?ls()
character(0)
R> ?foo<- function(x, y){
+ if (x> ?3) cat(y, "\n")
+ x}
R> ?foo(4, bar<- 5)
5
[1] 4
R> ?ls()
[1] "bar" "foo"
R> ?bar
[1] 5
R> ?foo(2, fubar<- 6)
[1] 2
R> ?fubar
Error: object 'fubar' not found
R> ?ls()
[1] "bar" "foo"
Could you explain what's happening here?! In the first case "bar" is created, but in the second "fubar" is not... Why is that? Am I missing something obvious?
Usually the problem facing newbies is that they want to save keystrokes and so use "=" for assignment (also a potential pitfall although not as likely to mess you up as the choice to use the two-keystroke path for argument assignment).
On the contrary, the opposite is also very likely. ?One of my favourite idioms is: ? ? ? ?plot(fm<- lm(y~x, data=some.data)) to (1) fit a model, (2) assign the fitted model to an object and (3) look immediately at diagnostic plots. Students came to me and said that the code in the lab sheet didn't work and they were getting strange error messages "about objects not being found". ?They reassured me that they had typed in exactly what was on the lab sheet. ?Of course, once I got to their computer and looked at their screen, it was clear that they had typed: ? ? ? ?plot(fm = lm(y~x, data=some.data))
It's not much more complicated to type it in two lines, but it's much clearer and safer!
Cheers, ? ? ? ?Berwin ========================== Full address ============================ Berwin A Turlach ? ? ? ? ? ? ? ? ? ? ?Tel.: +61 (8) 6488 3338 (secr) School of Maths and Stats (M019) ? ? ? ? ? ?+61 (8) 6488 3383 (self) The University of Western Australia ? FAX : +61 (8) 6488 1028 35 Stirling Highway Crawley WA 6009 ? ? ? ? ? ? ? ?e-mail: berwin at maths.uwa.edu.au Australia ? ? ? ? ? ? ? ? ? ? ? ?http://www.maths.uwa.edu.au/~berwin
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-- Ivan CALANDRA PhD Student University of Hamburg Biozentrum Grindel und Zoologisches Museum Abt. S?ugetiere Martin-Luther-King-Platz 3 D-20146 Hamburg, GERMANY +49(0)40 42838 6231 ivan.calandra at uni-hamburg.de ********** http://www.for771.uni-bonn.de http://webapp5.rrz.uni-hamburg.de/mammals/eng/1525_8_1.php
______________________________________________ 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.