R devel - Feb 2003 - nested assignment (PR#2575)

> A <- list(B=1, C=2, D=3)		# A list of 3 elements
> attach(A)				# Attach it to the search list
> assign("D", 4, pos="A")		# Assign a new value to
'D', inside 'A'
> ls()
  D					# Looks good ...
> D
  4					# Looks good ...
  
> A
  $B
  [1] 1

  $C
  [1] 1

  $D
  [1] 3					# Huh?


*Both* values of 'D' somehow live in 'A':
> detach("A")
> attach("A")
> D
  3					# Now the changed value is gone


I'd greatly appreciate if you'd let me know if I'm just doing
something
silly; my email is larry@eye-hand.wustl.edu
But even if I *am* doing something silly, perhaps there's something in
the documentation that might be changed to prevent someone else from
making the same error.

Thanks!

larry
larry@eye-hand.wustl.edu



--please do not edit the information below--

Version:
 platform = i686-pc-linux-gnu
 arch = i686
 os = linux-gnu
 system = i686, linux-gnu
 status = 
 major = 1
 minor = 6.1
 year = 2002
 month = 11
 day = 01
 language = R

Search Path:
 .GlobalEnv, A, package:ctest, Autoloads, package:base

larry@cognition.wustl.edu writes:
> > A <- list(B=1, C=2, D=3)		# A list of 3 elements
> > attach(A)				# Attach it to the search list
> 
> > assign("D", 4, pos="A")		# Assign a new value to
'D', inside 'A'
> 
> 
> > ls()
>   D					# Looks good ...
> 
> > D
>   4					# Looks good ...
>   
> > A
>   $B
>   [1] 1
> 
>   $C
>   [1] 1
> 
>   $D
>   [1] 3					# Huh?
> 
> 
> *Both* values of 'D' somehow live in 'A':
> 
> > detach("A")
> > attach("A")
> > D
>   3					# Now the changed value is gone
The explanation is that attach() attaches a *copy* of the original
list (a virtual copy, physical copying only happens if you attempt to
change one version). The help page should probably have something on
this. 

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907