Prolog: Arithmetic Operators

Prolog provides ‘is’ keyword to evaluate arithmetic expressions.

Syntax

X is <Some Arithmetic Expression>

1 ?- X is 2 + 3.
X = 5.

2 ?- Y is 2 - 3.
Y = -1.

3 ?- Z is 2 * 3.
Z = 6.

4 ?- W is 2 / 3.
W = 0.6666666666666666.

You can use parenthesis, to avoid the problems with operators precedence and associativity.

5 ?- X1 is 2 * (3/ (4 * 2)).

X1 = 0.75.

Apart from the arithmetic operators, Prolog provided below comparison operators.

Operator	Example	Description
>	X > Y	Return true, if X is greater than Y, else false
<	X < Y	Return true, if X is less than Y, else false
>=	X >= Y	Return true, if X is greater than or equal to Y, else false
=<	X =< Y	Return true, if X is less than or equal to Y, else false

Note: Unlike other languages, less than or equal to is not written as <=, it is written as =< in prolog.

8 ?- 10 > 11.
false.

8 ?- 10 < 11.
true.

9 ?- 10 >= 11.
false.

10 ?- 10 =< 11.
true.

You can even combine multiple operations in one line.

11 ?- X is 2 + 10, Y is 100, X > Y.
false.

Previous                                                 Next                                                 Home