Ascend FD-23R Bedienungsanleitung PDF herunterladen (Seite 37)

2.2. Introductory TOY(FD) Examples 25

N queens in a chessboard in such a way that no queen attacks each other. Observe the

use of the directive include to make use of both the FD constraint functions and the

function length/1 (predeﬁned in the ﬁle misc.toy).

include "cflpfd.toy"

include "misc.toy" %%To use length/1

queens :: int -> [int] -> [labelingType] -> bool

queens N L Label = true <==

length L == N,

domain L 1 N,

constrain_all L,

labeling Label L

constrain_all :: [int] -> bool

constrain_all [] = true

constrain_all [X|Xs] = true <==

constrain_between X Xs 1,

constrain_all Xs

constrain_between :: int -> [int] -> int -> bool

constrain_between X [] N = true

constrain_between X [Y|Ys] N = true <==

no_threat X Y N,

N1 == N+1,

constrain_between X Ys N1

no_threat:: int -> int -> int -> bool

no_threat X Y I = true <==

X #\= Y,

X #+ I #\= Y,

X #- I #\= Y

Again, if we compile and load this program, we can solve goals as the following:

TOY(FD)> queens 4 L [ff]

yes

L == [ 2, 4, 1, 3 ]

Elapsed time: 0 ms.

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Ascend FD-23R Bedienungsanleitung Seite 37