CODEWITHSUNDEEP
prolog
user3043278
user3043278

Reputation: 184

Generate list in prolog

I'm try to make a list of the values n,n+1,...2*n-1 for example if I have n=4 ==> [4,5,6,7]

I have managed to write this code but it shows the list of first n elements. Can you help me modify it?

create(N,L):-
   create_list(N,[],L).

create_list(0,L,L).         
create_list(N,R,L):-
   N>0,
   N1=N-1,
   create_list(N1,[N|R],L).

Upvotes: 0

Views: 1345

Answers (2)

repeat
repeat

Reputation: 18726

Here's how you could write a logically pure version of create/2 based on list_from_to/3, defined in my answer to question "Fill list in SWI-Prolog":

create(From,Zs):-
   To #= 2*From-1,
   list_from_to(Zs,From,To).

Here the query you had in your question:

?- create(4,Zs).
Zs = [4,5,6,7] ;
false.

As create/2 is pure, it is versatile and can also be used in different ways:

?- create(N,[4,5,6,7]).
N = 4.

?- create(N,[4,_,_,_]).
N = 4.

Upvotes: 1

CapelliC
CapelliC

Reputation: 60014

You're doing 'in reverse', in a certain sense. Try

create(N,L):-
    M is 2*N,
    create_list(N,M,L).

create_list(N,M,[N|R]):-
    N < M,
    N1 = N+1, % note: check if your Prolog accepts is/2 instead
    create_list(N1,M,R).
create_list(M,M,[]).

Upvotes: 0

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