Re: Some strange result

Hi

This is due to heavy use of assert followed by retract on the
currently used predicate hanoiDyn/5.

A solution consists in using another predicate for tabling.

:- dynamic(tabled/5).

hanoiDyn(1,_A,_B,_,1). %--- move _A
to _B

hanoiDyn(N,A,B,C,Moves) :-

tabled(N, A,B,C,Moves), !. % tabled: return the
#moves

hanoiDyn(N,A,B,C,Moves) :-

N>1,

N1 is N-1,

hanoiDyn(N1,A,C,B,Moves1), %--- move
N-1 discs

asserta(tabled(N1,A,C,B,Moves1)),

hanoiDyn(N1,C,B,A,Moves2), %--- move
N-1 discs

retract(tabled(N1,A,C,B,_)),

Moves is Moves1 + Moves2 + 1.

Remark that, when retract is called, only one clause for tabled/5
is present in the database. You could then use retractall which is
faster. Replace retract(tabled(N1,A,C,B,_)), by

retractall(tabled(_,_,_,_,_)),

This should be even faster.

Ramrk also that you can further speed up removing A,B,C from
tabled data (moving N disks always require the same number of move
independently from A,B,C). You thus have a tabled/2 predicate :
tabled(N,Moves).

Finally, you could keep your tabled data until the end of the
predicate and do the retractall only in the solve1/2 predicate.
But this depends on what you try to measure.

Daniel

Le 13/11/2014 18:37, Sylvain Julmy a écrit :

Hi,

at first, excuse me for my English, I'm not very good at...

At the School of Engineering and Architecture of Fribourg, Switzerland,
we have a Prolog course in the thirs year of the Bachelor cursus. In an
exercice, we need to study a programm wo resolve the Hanoi tower problem.

I have create a simple predicat that's simply execute the solve1
predicat (see attachment : hanoi.pl) and the time computation is
increasing and we dont know why.

That's the output when i run my test_solve1 predicat :

| ?- test_solve1(25,10).
25: nbrMoves = 1023.0 6ms
24: nbrMoves = 1023.0 9ms
23: nbrMoves = 1023.0 15ms
22: nbrMoves = 1023.0 18ms
21: nbrMoves = 1023.0 22ms
20: nbrMoves = 1023.0 23ms
19: nbrMoves = 1023.0 25ms
18: nbrMoves = 1023.0 25ms
17: nbrMoves = 1023.0 26ms
16: nbrMoves = 1023.0 27ms
15: nbrMoves = 1023.0 31ms
14: nbrMoves = 1023.0 34ms
13: nbrMoves = 1023.0 168ms
12: nbrMoves = 1023.0 204ms
11: nbrMoves = 1023.0 223ms
10: nbrMoves = 1023.0 251ms
9: nbrMoves = 1023.0 270ms
8: nbrMoves = 1023.0 291ms
7: nbrMoves = 1023.0 313ms
6: nbrMoves = 1023.0 334ms
5: nbrMoves = 1023.0 357ms
4: nbrMoves = 1023.0 381ms
3: nbrMoves = 1023.0 399ms
2: nbrMoves = 1023.0 419ms
1: nbrMoves = 1023.0 441ms

I hope someone could find a good response to this !

Dear,
Sylvain Julmy
School of Engineering and Architecture of Fribourg
Switzerland

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