[R] Potential Error/Bug?: addition of NULL during arithmetic

[Prof Brian Ripley]

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
>
> ______________________________________________
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> 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