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Understanding the prologue [lists]

I have to write a program that does this:

?- pLeap(2,5,X,Y).
X = 2,
Y = 3 ;
X = 3,
Y = 4 ;
X = 4,
Y = 5 ;
X = 5,
Y = 5 ;
false.

(gives all pairs X, X + 1 between 2 and 5, plus a special case at the end).

This is supposedly the solution. I don't really understand how this works, can anyone guide me through it?

pLeap(X,X,X,X).
pLeap(L,H,X,Y) :-
        L<H,
        X is L,
        Y is X+1.
pLeap(L,H,X,Y) :-
        L=<H,
        L1 is L+1,
        pLeap(L1,H,X,Y).

I would do it just like this:

pLeap(L,H,X,Y) :-
        X >= L,
        X =< H,
        Y is X+1.

Why isn't it working (ignoring the special case at the end)?

+2


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2 answers


Operators >=

and =<

do not create their own arguments, and you can use them only after the arguments have already been created.

In other words, in this solution, X

and are Y

given values ​​with is

, and operators <

and =<

are used only on L

and H

, whose values ​​are set by the user. (Try this solution pLeap(L,H,2,3)

and you get the same problem as yours.)



In your case, you are trying to use >=

and =<

on X

, which doesn't matter yet, and that's why the interpreter is complaining.

+2


a source


You can use clpfd library for your problem.

:- use_module(library(clpfd)).

pLeap(L,H,X,Y) :-
    X in L..H,
    Y #= min(H, X+1),
    label([X]).


Here's the result:

 ?- pLeap(2,5,X,Y).
X = 2,
Y = 3 ;
X = 3,
Y = 4 ;
X = 4,
Y = 5 ;
X = 5,
Y = 5.
+4


a source






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