R help - Aug 2008 - Potential Error/Bug?: addition of NULL during arithmetic

I encountered an error that does not make sense to me given my reading 
of the documentation and does not appear to be referenced in the list 
archives or online. The error occurred when a function received a NULL 
value rather than a numeric value as a parameter and then attempted to 
use the NULL value in the calculateion. The error: "Error during wrapup: 
nothing to replace with" can be easily recreated with the following code:
> 1 + NULL  # note that the opperation succeeds and returns a numeric vector
with dim(NULL)
numeric(0)
> bar <- 1
> bar
[1] 1
> foo <- 1 + NULL
> foo
numeric(0)
> bar <- bar + foo # note that here the assignment operation succeeds and
1 + (1 + NULL) -> numeric(0)
> bar
numeric(0)
> bar <- c(1, 1) # however if the assignment is into a vector
> bar[1] <- bar[1] + foo # note that the mathematical operation is
identical, but the assignment fails
Error during wrapup: nothing to replace with

If this is the intended behavior, a more informative error message (e.g. 
'attempt to assign NULL to vector element') would be useful. If it is 
not the intended behavior, should I log this as a bug?

-eric
>sessionInfo()
R version 2.7.1 (2008-06-23) 
powerpc-apple-darwin8.10.1 

locale:
en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

On Tue, 26 Aug 2008, Eric DeWitt wrote:
> I encountered an error that does not make sense to me given my reading
> of the documentation and does not appear to be referenced in the list
> archives or online. The error occurred when a function received a NULL
> value rather than a numeric value as a parameter and then attempted to
> use the NULL value in the calculateion. The error: "Error during
wrapup:
> nothing to replace with" can be easily recreated with the following
code:
>
>> 1 + NULL  # note that the opperation succeeds and returns a numeric
vector with dim(NULL)
> numeric(0)
As documented, of course.
>> bar <- 1
>> bar
> [1] 1
>> foo <- 1 + NULL
>> foo
> numeric(0)
>
>> bar <- bar + foo # note that here the assignment operation succeeds
and 1 + (1 + NULL) -> numeric(0)
>> bar
> numeric(0)
>
>> bar <- c(1, 1) # however if the assignment is into a vector
>> bar[1] <- bar[1] + foo # note that the mathematical operation is
identical, but the assignment fails
> Error during wrapup: nothing to replace with
That's the trouble, this is not the same operation at all.  This is 
replacing one element of a vector, not naming the result 'bar'.
> If this is the intended behavior, a more informative error message (e.g.
> 'attempt to assign NULL to vector element') would be useful. If it
is
> not the intended behavior, should I log this as a bug?
It is the intended and documented behaviour. You tried to set element 1 of 
bar to bar[1] + foo, which is numeric(0) (by the documented recycling 
rules).  The error message: 'nothing to replace with' seems very 
informative to me.  If you put some effort into understanding your own 
errors by doing things step by step, as in
> bar <- c(1, 1)
> foo <- bar[1] + foo
> bar[1] <- foo
Error in bar[1] <- foo : nothing to replace with
> foo
numeric(0)

you might learn more.

I don't know how you got an 'Error during wrapup': that is not 
reproducible (it might be a function of using a console, if you did).
>
> -eric
>
>> sessionInfo()
> R version 2.7.1 (2008-06-23)
> powerpc-apple-darwin8.10.1
>
> locale:
> en_US.UTF-8/en_US.UTF-8/C/C/en_US.UTF-8/en_US.UTF-8
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> ______________________________________________
> 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.
>
-- 
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