[R] inc function .. more generally, var arguments

[Prof Brian D Ripley]

On Fri, 2 Mar 2001, John Aitchison wrote:

>
> Hi
>
> I'd like to be able to write a simple 'increment' function (like inc in
> pascal or ++ in C++) that takes the argument and increments it
> 'permanently'  ie so that z<-1; inc(z) # no assignment; z  #2
>
> I can see how to do it with a global variable but that is not what I want -
>  I want to modify the object that is the actual argument.. I have looked
> at  assign(), << etc .. I am not sure where to go next.
>
> I thought I could possibly do it by reference to the calling environment
> and tried
>
> > inc
> function(a){ eval.parent(a<-a+1,1) }
> > z<-1
> > inc(z)
> [1] 2
> > z
> [1] 1
> >
> no good
>
> Is what I want to do possible?  (I realize that it is possibly inadvisable
> as there would be no obvious way of knowing from the function
> definition that it modified its arguments)

This is precisely exercise 4.4 of Venables & Ripley (1997).  There is an
answer online at

http://www.stats.ox.ac.uk/pub/MASS/VR2ans.pdf

for S: R could be slightly different.

-- 
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 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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