[R] Pass By Value Questions

[Duncan Murdoch]

On 19/08/2010 12:57 PM, lists at jdadesign.net wrote:
> I understand R is a "Pass-By-Value" language. I have a few practical
> questions, however.
>
> I'm dealing with a "large" dataset (~1GB) and so my understanding of the
> nuances of memory usage in R is becoming important.
>
> In an example such as:
> > d <- read.csv("file.csv");
> > n <- apply(d, 1, sum);
> must "d" be copied to another location in memory in order to be used by
> apply? In general, is copying only done when a variable is updated within
> a function?
>   

Generally R only copies when the variable is modified, but its rules for 
detecting this are sometimes overly conservative, so you may get some 
unnecessary copying.  For example,

d[1,1] <- 3

will probably not make a full copy of d when the internal version of 
"[<-" is used, but if you have an R-level version, it probably will.  I 
forget whether the dataframe method is internal or R level. 

In the apply(d, 1, sum) example, it would probably make a copy of each 
row to pass to sum, but never a copy of the whole dataframe/array.
> Would the following example be any different in terms of memory usage?
> > d <- read.csv("file.csv");
> > n <- apply(d[,2:10], 1, sum);
> or can R reference the original "d" object since no changes to the object
> are being made?
>   

This would make a new object containing d[,2:10], and would pass that to 
apply.
> I'm familiar with FF and BigMemory, but are there any packages/tricks
> which allow for passing such objects by reference without having to code
> in C?
>   

Duncan Murdoch