[R] recursion and lexical scoping

[Thomas Lumley]

On Tue, 21 Mar 2000, Denis White wrote:

> To assign values to a matrix in a recursive function,
> must the matrix be passed as argument and returned
> as value?

It doesn't _have_ to be. It depends on a compromise between slowness and
incomprehensibility.

You can use <<- to assign to a 'global' variable: eg
R> a<-matrix(0,ncol=5,nrow=10)
R> f
function(n) {
if (n==1) {
a<<-a+1
return(1)
}
else return(f(n-1)+f(n-1))
}

but it can get messy writing recursive functions with side-effects.

If you declare the matrix to be the correct size and only assign to
elements of it then I don't think it will be unnecessarily duplicated, so
the loss of efficiency won't be too bad.

	-thomas

Thomas Lumley
Assistant Professor, Biostatistics
University of Washington, Seattle

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