[R] modifying arrays within functions

[Bill.Venables at csiro.au]

I can feel a sermon coming on...

I'd like to emphasize Martin's warning below and encourage you to avoid
this kind of construction.  The problem is side effects.  Some R
functions do this (e.g. fix) but everyone understands why.   The real
problem, in my view, is deeper.  The question you ask originally implies
that you don't really want a function, you want a macro.  R does not
work comfortably with macros, it works with functions.  

This in turn suggests to me you are trying to make R work like some
other software environment, presumably one with which you are familiar.
This is very understanble, but nearly always a bad thing.  If you are
new to R, as you say, I would encourage you to find out how R works and
work with it, rather than try to may R work the way you would like it to
do.  You will always be fighting the system unless you do.

Here endeth the sermon.

Bill Venables.


-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Martin Elff
Sent: Wednesday, 6 February 2008 2:23 AM
To: r-help at r-project.org
Subject: Re: [R] modifying arrays within functions

On Tuesday 05 February 2008 (16:51:41), Konrad BLOCHER wrote:
> Hi,
>
> I'm pretty new to R and seem to be having difficulties with writing a
> function that would change an array and keep the change after the
function
> finishes its work.
>
It seems you want this:

X<-array(1,dim=c(2,2))
addition<-function(a){
	X[1,1] <<- X[1,1]+a # not '='
}
addition(1)
X


     [,1] [,2]
[1,]    2    1
[2,]    1    1
                           

Nevertheless I would advise against writing and using functions that
have side effects outside their scope. 
It is in general dangerous.

Best, 

Martin

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