====== Common divisor 2 ======
{{tag>math}}
===== Description =====
A pattern-directed program to find the greatest common divisor of a set of numbers.

**Source**:  PROLOG programming for artificial intelligence, 3rd Edition, Harlow, 2001, ISBN 0-201-40375-7.
===== Download =====
Program source code: {{common_divisor_2.pl}}
===== Listing =====
<code prolog>
%  Figure 23.12  A pattern-directed program to find the greatest 
%  common divisor of a set of numbers.

%  The follwing directive is required by some Prologs:

:-  dynamic num/1.

%  Production rules for finding greatest common divisor (Euclid algorithm)

:-  op( 800, xfx, --->).
:-  op( 300, fx, num).

[ num X, num Y, X > Y ]  --->
[ NewX is X - Y, replace( num X, num NewX) ].

[ num X]  --->  [ write( X), stop ].


%  An initial database

num 25.
num 10.   
num 15.   
num 30.
</code>
===== Comments =====