[Rd] [R] function changes argument (PR#9216)

[Prof Brian Ripley]

I can tell you where the problem is and a workaround:

f <- function(LL) for (ll in names(LL)) LL[[ll]]$txt<-"changed in f"

works.  The problem is that for() is directly exposing the elements of a 
list.

Of course, a more idiomatic construction would be

LL <- lapply(LL, function(x) x$txt <- "changed in f")

which also works.

The question is whether we do wish to change this to make the construction 
work as Andreas appears to intend.  The simplest solution is a 
precautionary duplicate in for(), which is potentially very expensive and 
almost always unneeded.

However, we already have

x <- 1:10
for(i in x) i <- pi
x

which does not change x, so I think the right solution is to make the list 
(LL here) read-only, which can be done cheaply.  That is also AFAICS what 
S does.


On Mon, 11 Sep 2006, murdoch at stats.uwo.ca wrote:

> (This report appeared in R-help.  I've sent it separately to the bugs 
> list to avoid a long string of cross-postings.)
> 
> On 9/11/2006 4:49 AM, Moeltner, Andreas wrote:
> > Dear R-list,
> > 
> > the following function f changes L. I thought, assignments within
> > functions are only local?
> 
> That looks like a bug, still present in R-patched and R-devel.  (I
> haven't got the latest pre-release built yet today, but I expect it's
> there, too.)  Thanks for the report.
> 
> I'll send a copy of this to the bugs list, but I won't be able to
> attempt to fix it.
> 
> Duncan Murdoch
> 
> > 
> > 
> > f<-function(LL)
> > { for (ll in LL)
> >   { ll$txt<-"changed in f"
> >   }
> > }
> > 
> > l<-list(txt="original value")
> > L<-list(l)
> > L[[1]]$txt 
> > f(L)
> > L[[1]]$txt
> > 
> > 
> > gives (using R 2.3.1):
> > 
> > ...
> >> L[[1]]$txt
> > [1] "original value"
> >> f(L)
> >> L[[1]]$txt
> > [1] "changed in f"
> > 
> > Thanks in advance
> > 
> > Andreas
> > 
> > ______________________________________________
> > R-help at stat.math.ethz.ch 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.
> 
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> 

-- 
Brian D. Ripley,                  ripley at 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