[R] understanding recursive functions

[Jeffrey Horner]

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:

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()


Jeff
> 
> 
> Joe Boyer
> Statistical Sciences 
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