[R] understanding recursive functions

[(Ted Harding)]

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.

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Date: 18-Dec-08                                       Time: 22:51:41
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