function changes argument (PR#9216)

(This report appeared in R-help.  I've sent it separately to the bugs 
list to avoid a long string of cross-postings.)
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.
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.

(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.

______________________________________________
R-devel at r-project.org mailing list
https://stat.ethz.ch/mailman/listinfo/r-devel

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