CODEWITHSUNDEEP
prologswi-prolog
Thomas Briggs
Thomas Briggs

Reputation: 119

Check if a number is between 2 values

I am new to prolog and have am trying to write a program that will do the following tell me if a number is between 2 values I can do the following:

between(L, X, R) :- 
    X > L, X < R.

and doing between(1, 3, 5) works, but I would like it to be able to do between(1, X, 5) and have prolog return all the values in between so in this case X = 2, X = 3, X = 4, I get why my solution doesn't because it needs to be have been initialised, but I cannot think of a solution to this problem, can this type of thing just not be done in prolog?, and help would be great thanks

Upvotes: 1

Views: 1757

Answers (2)

Duda
Duda

Reputation: 3736

In case you don't want to predefine all numbers: let prolog create a list with possible entries and state your X has to be one of them. To understand the code you have to have knowledge about lists in prolog, especially Head and Tail notation of lists.

betweenList(L,R,[]):- 
    L>=R.
betweenList(L,R,[L|Rest]):- 
    L<R,
    LL is L+1,
    betweenList(LL,R,Rest).

between(L, X, R) :- 
    betweenList(L, R, [L| List]),
    member(X, List).

?- between(1,X,5).
X = 2 ;
X = 3 ;
X = 4 ;
false.

betweenList(L,R,List) creates a List of all numbers between L and R, including L (as head element), excluding R. So if you want to generate a List without L, it is the easiest to just call betweenList(L, R, [L| List]) so List will not include L. Now X just has to be a member of List. The member/2 predicate can be easily written as well if you don't want to use the inbuild predicate.

Upvotes: 1

MWB
MWB

Reputation: 12567

One way to approach this:

digit(0).
digit(1).
digit(2).
digit(3).
digit(4).
digit(5).
digit(6).
digit(7).
digit(8).
digit(9).

between(L, X, U) :- digit(L), digit(X), digit(U), L < X, X < U.

Tests:

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

?- between(2, 7, U).
U = 8 ;
U = 9.

Alternatively, you may want to look into Constraint logic programming.

Incidentally, Prolog already has a between/3:

?- between(1, 5, X).
X = 1 ;
X = 2 ;
X = 3 ;
X = 4 ;
X = 5.

although it's "illogical": you can't run it backwards, as the above definition.

Upvotes: 1

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