[R] using ``<-'' in function argument

[Ivan Calandra]

See below

Le 12/3/2010 06:54, Berwin A Turlach a écrit :
> 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)
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>
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-- 
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calandra at uni-hamburg.de

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