[R] understanding recursive functions

[Nordlund, Dan (DSHS/RDA)]

> -----Original Message-----
> From: r-help-bounces at r-project.org 
> [mailto:r-help-bounces at r-project.org] On Behalf Of Nordlund, 
> Dan (DSHS/RDA)
> Sent: Thursday, December 18, 2008 5:00 PM
> To: r-help at r-project.org
> Subject: Re: [R] 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.
> > 
> 
> I am by no means an expert in programming so I will defer to 
> the experts. But assuming the OP was not intentionally trying 
> to assign in a parent environment, couldn't the above 
> function be written as
> 
> q <- function(x,h){if (x < 2) q(x+h,h) else return(x)

Sorry, I forgot the final '}' .  Or I think you can eliminate the leading '{' and final '}' in this case.

 q <- function(x,h) {if (x < 2) q(x+h,h) else return(x)}
>
> Hope this is helpful,
> 
> Dan
> 
> Daniel J. Nordlund
> Washington State Department of Social and Health Services
> Planning, Performance, and Accountability
> Research and Data Analysis Division
> Olympia, WA  98504-5204
>  
>  
> 
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