understanding recursive functions
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 Renaissance Bldg 510, 3233-D Mail Stop RN0320 8-275-3661 cell: (610) 209-8531 [[alternative HTML version deleted]]
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