Maximum of 2 numbers in Prolog

Question

Comparing 2 numbers: define a predicate great that takes 3 parameters - the 2 numbers that I want to compare them and the output variable that return the greatest number.

Ex.

?- great(X,3,5). 
X=5.

in Prolog language

Do you mean Prolog? Or is there some kind of "bro"-variant of Prolog named "brolog"? :-D — Sergii Dymchenko
@user3527224 have you looked at any documentation at all? Go find a Prolog manual online for GNU Prolog or SWI Prolog and look at the comparison operators. Then think of your predicate great(X,Y,Z) :- ... as meaning X is the greater of Y and Z if.... You can have more than one clause for the predicate. So to make it simple, you could have two: great(Y, Y, Z) :-... which is Y is the greater of Y and Z if... and great(Z, Y, Z). which is Z is the greater of Y and Z if.... — lurker

Nicholas Carey Nicholas Carey · Accepted Answer · 2014-04-14T17:01:05

You really should read the documentation. Maybe even read a good book on the language:

</2 is "less than"
>=/2 is "greater than or equal to"
=</2 is "less than or equal to"

For "equal to", you could use:

=/2 is (roughly) "equal to" (unifiable with)
=:=/2 is arithmetic equality
=\=/2 is arithmetic inequality

I'm sure you can figure things out.

Alternatively, you could avoid Prolog's comparison operators entirely and simply say

gt(X,Y,Z) :- Z is max(X,Y) .

or use a discriminant function to select the desired value:

gt(X,Y,Z) :- D is sign(X-Y) + 1 , nth0(D,[Y,X,X],Z) .

Maximum of 2 numbers in Prolog

1 Answers