Path: utzoo!attcan!uunet!willett!ForthNet
From: ForthNet@willett.UUCP (ForthNet articles from GEnie)
Newsgroups: comp.lang.forth
Subject: PUZZLES AND PROBLEMS
Message-ID: <1279.UUL1.3#5129@willett.UUCP>
Date: 5 Jul 90 02:23:46 GMT
Organization: String, Scotch tape, and Paperclips.  (in Pgh, PA)
Lines: 44

Category 3,  Topic 35
Message 123       Tue Jul 03, 1990
W.BADEN1 [Wil]               at 22:28 PDT
 
I've changed my mind.  Here's Trotter's algorithm in Forth.

 10 CONSTANT Maxperm
 CREATE P   Maxperm ALLOT   1 P C!
 CREATE D   Maxperm ALLOT
 : perm ( n -- i)
    P C@ 
    IF
       0 DO (  )
          0 P I + C!   1 D I + C!
       LOOP
       0 EXIT
    THEN
    0 ( n k) 1 ROT 1-
    DO ( k)
       D I + C@ P I + C+!
       P I + C@ ( k q) DUP 
       IF
          DUP I > 0=
             IF   + UNLOOP EXIT   THEN
          DROP ( k)
          -1 D I + C!
       ELSE ( k q) DROP ( k)
          1 D I + C!
          1+
       THEN
    -1 +LOOP
    1 P C!
    1+ ;

If you lack : C+! ( n a -- ) DUP C@ +under C! ; 

This was easy to code from the C.  And I find the Forth easy to read.  But
discovering the algorithm in Forth without benefit of a higher level language
would have been difficult.

Procedamus in pace.
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