Basic Operators

To manipulate the values we learned from the facts part, now we need to learn some fundamental basic operators to do the job.

Comparison Operators

Symbol	Meaning
==	Equals, assignment
<	Less than
<=	Less than or equal
>	Greater than
>=	Greater than or equal
!=	Not equal

Binary Operators

Symbol	Meaning
+	Add
-	Subtract
*	Multiply
/	Divide
%	Remainder of integer division
^	Raise to power
++	String concatenation

TIP

Logic Programming is declarative, and thus there is no difference between equality and assignment, operator == stands for both.

Example 1

Grandparent(a, d) :- Parent(a, b), Parent(c, d), b == c;

Example 2

Grandparent(x, y) :-
    Parent(a, b), Parent(b, c), x == a, y == c;

Assignment can be used to store results of intermediate computation. Example: Compute volumes and mass of metal cubes.

Example 3

MetalCube("cube1", 10, 15);
MetalCube("cube2", 20, 8);

CubeInfo(cube_id, volume, mass) :-
    MetalCube(cube_id, side, density),
    volume == side * side * side,
    mass == volume * density;