understanding recursive functions

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf Of 
Ted.Harding at manchester.ac.uk
Sent: Thursday, December 18, 2008 2:52 PM
To: r-help at r-project.org
Cc: joseph.g.boyer at gsk.com
Subject: Re: [R] understanding recursive functions

On 18-Dec-08 22:33:28, Jeffrey Horner wrote:

joseph.g.boyer at gsk.com wrote on 12/18/2008 04:22 PM:

I'm trying to understand the use of recursive functions described
on page 45 of An Introduction to R by the R core development team.

A function is a list of expressions, which all get executed with
only the last being assigned to a global variable, right? 
So if a function refers recursively to itself, it should simply
start with the first expression and go from there. At least that
is my understanding of why the example given on page 45 works.

In light of the above, I would appreciate it if someone would
understand why the following example does not work:

q <- function(x,h) {if (x < 2) {x <<- x+1; return(q(x))} else
return(x)}

If x < 1, this should add 1 to x and go back to the beginning of
the if expression, and the final result should be 2. So q(0) should
return 2.
But it returns an error message.

All references to x save one (the assignment with the <<- operator)
are found within the current frame, not by lexical scoping, and
hence is never changed... producing infinite recursion. The 

following

at least fixes your example:

q <- function(x,h) {if (x < 2) {x <<- x+1; x <- x+1; 

return(q(x))} else

return(x)}
ls() # no x in global env just yet
q(-10)
ls()

The following fixes it even more simply (using the same principles):

  q <- function(x,h){
    if (x < 2) {u <- x+1; return(q(u))} else return(x)
  }

Note that "<<-" is not necessary.

Just to test the method more thoroughly:

  q <- function(x,h){
    if (x < 2) {u <- x+h; return(q(u,h))} else return(x)
  }

  q(0,0.3)
# [1] 2.1

Ted.