Is there a way to do size comparison? - prolog

Question

I have recently discovered the language Prolog and have been doing exercises on its basics. I am currently creating a database on animal classes like mammals, birds and reptiles, I want to expand the database by having a size comparison within the animals but not sure how.
Here is my database.
warm_blooded(bat).
warm_blooded(penguin).
cold_blooded(crocodile).
has_fur(bat).
has_feathers(penguin).
has_scales(crocodile).
gives_birth_live(bat).
lays_eggs(penguin).
lays_eggs(crocodile).
produces_milk(bat).
has_lungs(crocodile).
has_lungs(bat).
has_lungs(penguin).
%% if the being belongs to the mammalai class ,mammalia being the scientific word for mammal
mammalia(X) :-
warm_blooded(X),
produces_milk(X),
(
has_fur(X)
;
gives_birth_live(X)
),
format('~w ~s mammal ~n', [X, "is a"]).
%% if the being belongs to the aves class aves being the scientific word for bird
aves(X) :-
warm_blooded(X),
has_feathers(X),
lays_eggs(X),
has_lungs(X),
format('~w ~s bird ~n', [X, "is a"]).
%% if the being belongs to the reptillia class(reptillia being the scientific word for reptile
reptillia(X) :-
cold_blooded(X),
lays_eggs(X),
has_scales(X),
has_lungs(X),
format('~w ~s reptile ~n', [X, "is a"]).
I've tried adding sizes within the parameters but I keep getting compilation errors. I want to have an output wherein the user is able to determine which animal is bigger when compared with each other.

Answer 1

A simple an effective way is to just associate a size fact with each animal.
size(bat,1).
size(penguin,2).
size(crocodile,3).
Then add one predicate with two clauses to chose the larger of the two animals.
larger(A,B,A) :-
size(A,S1),
size(B,S2),
S1 > S2.
larger(A,B,B) :-
size(A,S1),
size(B,S2),
S2 >= S1.
Examples:
?- larger(penguin,crocodile,X).
X = crocodile.
?- larger(penguin,bat,X).
X = penguin ;
false.
?- larger(bat,bat,X).
X = bat.
Note that for examples where the the second animal is smaller, it tries the first clause and succeeds, but then has a choice point and so tries the second clause and fails. This is the pure solution.
If you want to use a cut to avoid the choice point, which is impure, you can do the following
larger_2(A,B,A) :-
size(A,S1),
size(B,S2),
S1 > S2,
!.
larger_2(A,B,B) :-
size(A,S1),
size(B,S2),
S2 >= S1,
!.
Examples:
?- larger_2(penguin,crocodile,X).
X = crocodile.
?- larger_2(penguin,bat,X).
X = penguin.
?- larger_2(bat,bat,X).
X = bat.
Another way as noted by Daniel Lyons is to use ->/2
larger_3(A,B,Larger) :-
size(A,SA),
size(B,SB),
(
SA > SB
->
Larger = A
;
Larger = B
).
This variation is not one operator of just ->/2 but a combination of both ->/2 and ;2.
This also does not leave a choice point and is impure because it too uses a cut (!). Using listing/1 we can see the implementation in Prolog.
?- listing('->'/2).
:- meta_predicate 0->0.
system:A->B :-
call(( A
-> B
)).
true.
?- listing(;/2).
:- meta_predicate 0;0.
system:A:B;A:C :- !,
call(A:(B;C)).
system:A:B;C:D :-
call(A:(B;C:D)).
true.
Notice the cut !.
How the two operators work together is noted in the SWI-Prolog documentation.
The combination ;/2 and ->/2 acts as if defined as:
If -> Then; _Else :- If, !, Then.
If -> _Then; Else :- !, Else.
If -> Then :- If, !, Then.
One other point to note about the use of ->/2 with ;/2 is that the syntactic layout among many Prolog programmers is to use () with the combination and offset the operators ->/2 and ;2 so that the ; stands out.
(
% condition
->
% true
;
% false
)
When a ; is used as an OR operator and not offset the ; is often overlooked in doing a quick scan of the source code as it is seen as a comma , instead of a ;.
Also note the absence of . or , after
SA > SB
and
Larger = A
and
Larger = B
but at the end an operator is needed,
).