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From: markv@fourier.Princeton.EDU (Mark VandeWettering)
Newsgroups: comp.lang.scheme
Subject: Re: Scheme recursive procedures
Message-ID: <5834@idunno.Princeton.EDU>
Date: 31 Jan 91 15:57:05 GMT
References: <11739@darkstar.ucsc.edu>
Sender: news@idunno.Princeton.EDU
Reply-To: markv@fourier.Princeton.EDU (Mark VandeWettering)
Organization: Princeton University
Lines: 36

In article <11739@darkstar.ucsc.edu> gforce@ucscb.UCSC.EDU (60766000) writes:
>
>Can one of you scheme hackers out there help me with a prog. problem ?
>the version of scheme at my school doesn't have the function "reverse"
>I am trying to write one but so far have only come up with one that is
>recursive but needs to take in two arguments.. CAn someone show me a way
>to do it with only one ?..
>
>Thanx

Sigh!  Someone should work harder to understand what is going on.  If you
can't code reverse, then you can't code LOTS.

But, in true spirit of a spoiler....

Try any one of the following (done off the top of my head):

(define (reverse l) (if (null? l) '() (append (reverse (cdr l)) (car l))))

This is ugly because of the append.  Appending does lots and lots of 
consing.  It is also not tail recursive, so it consumes stack.  First,
let's deal with the tail recursion problem....


(define (reverse-tr l)
  (define (rloop l r)
     (if (null? l)
	 r
	 (rloop (cdr l) (cons (car l) r))))
  (rloop l '()))

By some strange coincidence, we also have gotten rid of the append problem.
This will execute in all reasonable Scheme implementations with a constant
amount of stack, since it is fully tail recursive.
Mark VandeWettering
markv@acm.princeton.edu