p :- a, b.
p :- c.The eqivalent of this code in boolean algebra is:
(a and b) or c
Now let's add a cut:
p :- a,!,b.
p :- c.The eqivalent of this code in boolean algebra is:
(a and b) or (not-a and c)
The following code represents a maze in Prolog:
% Walls are defined like so
mazeWall(0,0).
mazeWall(0,1).
% ...
% ...
mazeWall(10,12).
% Start and end points of the maze
mazeStartPos(0,1).
mazeEndPos(0,2).
% Dimensions of the maze
mazeDimensions(10,12).
% Given a row and column, return a char
% 'S' for start
% 'E' for end
% '*' for wall
mazeElement(R,C,'S') :- mazeStartPos(R,C), !.
mazeElement(R,C,'E') :- mazeEndPos(R,C), !.
mazeElement(R,C,'*') :- mazeWall(R,C), !.
mazeElement(_,_,' ').
% Helper function to detect when we should print a newline
mazeNewLine(C) :- mazeDimensions(_,C), nl.
% Print the entire maze
printMaze :-
mazeDimensions(Rows, Cols),
between(0, Rows, Row),
between(0, Cols, Col),
mazeElement(Row, Col, Appearance),
write(Appearance),
mazeNewLine(Col),
fail.Constraint Logic Programming over Finite Domains
The following query returns an error:
?- 7 is X+4.
is does not support equations with variables.
There is a library for Prolog called CLP(FD) that adds an #= operator. It allows us to use variables in equations. It does not work with floating-point numbers.
?- X #= 3+4.
X = 7.
?- X #< 3.
X in inf..2.
?- X #< 3, X #> -5.
X in -4..2.
If we want values returned one at a time, we can use label.
label takes in a list of CLPFD variables.
?- X #< 3, X #> -5, label([X]).
X = -4.
X = -3.
X = -2.
...
\/ is an OR symbol
?- X in 3 \/ 50, label([X]).
X = 3.
X = 50.
// is for integer division
?- X // 2 #= 5.
X in 10..11.
abs for absolute value
?- 3 #= abs(X)
X in 3\/-3.
midpoint(X,Y) :- midpoint #= (X+Y)//2.A farmer has some chickens and some cows, for a total of 30 animals. The animals have total of 74 legs. How many chickens and how many cows does the farmer have?
farm(Cows,Chickens) :-
(4*Cows)+(2*Chickens) #= 74,
Cows + Chickens #= 30,
Cows #> 0, Chickens #> 0,
label([Cows, Chickens]).fib(0,0).
fib(1,1).
fib(X,Y) :-
X#>1, A1#=X-1,
A2#=X-2, fib(A1,P1), fib(A2,P2),
Y #= P1 + P2.[X,Y,Z] ins 1..3, label([X,Y,Z]).The above returns all permutations of three numbers within the range 1 to 3.
[X,Y,Z] ins 1..3, all_different([X,Y,Z]), label([X,Y,Z]).The above returns all combinations of three unique number within the range 1 to 3.
Regions that share a border cannot be the same color.
Let's represent each color with an integer
% 0 = orange, 1 = red, 2 = blue, 3 = purple
region(Rs) :-
Rs = [A,B,C,D,E,F],
Rs ins 0..3,
A #\= B, A #\= C, A #\= D, A #\= F,
B #\= C, B #= D,
C #\= E, C #\= D,
D #\= E, D #\= F,
E #\= F,
label(Rs).#school/f19/proglang-f19