[R] modifying arrays within functions

[Wittner, Ben, Ph.D.]

In general functions changing global variables as a side effect is dangerous
(i.e., often leads to programming errors and difficult-to-maintain code).

So, if the performance of the following is sufficient, it is what I would
recommend.

addition <- function(X, a) {

  X[1, 1] <- X[1, 1] + a

  X

}

X <- array(1, dim=c(2, 2))

X <- addition(X, a)

Sometimes for performance reasons or when programming an event-driven user
interface you need functions to alter global data as a side effect.
In that case

addition <- function(a) {

  X[1, 1] <<- X[1, 1] + a

}

-Ben

> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
> On Behalf Of Konrad BLOCHER
> Sent: Tuesday, February 05, 2008 10:52 AM
> To: r-help at r-project.org
> Subject: [R] modifying arrays within functions
> 
> 
> 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.
> 
> in other words
> 
> I have an array of 1's    X<-array(1,dim=c(2,2))
> 
> I want to add a number to X[1,1] by means of a function called addition.
> What I am writing is:   addition<-function(a){X[1,1]=X[1,1}+a}
> but it doesn't seem to work
> 
> Any ideas? Please? :)
> 
> Thanks
> 
> KB
> 
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