Print Page - NewLisp not Tail Recursive?

Title: NewLisp not Tail Recursive?
Post by: AlexPeake on December 10, 2003, 07:38:26 PM

I Try:

(define (fact x ans)

(if (< x 1)

ans

(fact (- x 1) (* x ans))))

Which is Tail Recursive and yet I still get stack overflow??

Title:
Post by: HPW on December 10, 2003, 10:19:26 PM

Alex,

welcome on the forum.

Lutz did a comment here:

http://www.alh.net/newlisp/phpbb/viewtopic.php?t=128

Lutz comment:

~~Quote~~newLISP does no tail-recursion optimization. Fortunately those tail-recursive routines are also the easiest to re-write them to an iterative pattern.

Title: Tail Recursion
Post by: AlexPeake on December 11, 2003, 03:55:23 PM

Thanks for the "work-around".

Kind of misses the point of using Lisp/Scheme?

Title:
Post by: Lutz on December 12, 2003, 07:05:02 AM

Here is a non-recursive version of factorial:

(define (fact n) (apply mul (sequence 1 n)))

Its is also much faster then the recursive version. You also could use * instaead of mul, but it would flow over to a max int of 2147463647 very fast. the 'mul' does foating point calculation.

I think that introductory books on LISP stress recursion too much, many of the examples typically published in that context are much better solved with an iteration.

I know that the topic recursion versus iteration is a very 'religous' one in the LISP community. newLISP's focus is more on solving 'real world' problems with LISP.

BTW, welcome to our discussion group Alex!

Lutz

Title:
Post by: Sammo on December 12, 2003, 03:22:14 PM

(define (fact n) (apply mul (sequence 1 n)))

(fact 100) --> 9.332621544e+157

(fact 150) --> 5.713383956e+262

(fact 500) --> crash!

Title:
Post by: Lutz on December 12, 2003, 04:00:14 PM

This is due to incorrect handling of INF in the Win32 version and is fixed in the version 7.4.0rc1 posted this morning in:

http://newlisp.org/download/development/

(fact 500) => +INF

Lutz

Title:
Post by: Sammo on December 12, 2003, 04:06:16 PM

Outstanding!

Title:
Post by: Lutz on December 12, 2003, 04:16:43 PM

Thanks Sam, I always joke: "math is the most difficult thing for a computer to do"

Getting all the NaN and over/underflow right on the Win32 version has taken some time. On Linux/BSD and friends it's all handled pretty much automatically by the GCC compiler and you don't have to do much. On Borland C-Builder (which is an excellent compiler in my opinion) it took some time to make it work exactly like on the other platforms.

Lutz

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Forum => newLISP in the real world => Topic started by: AlexPeake on December 10, 2003, 07:38:26 PM