CODEWITHSUNDEEP
listprolog
user3546630
user3546630

Reputation: 85

How to make list out of objects in Prolog?

I can add a single object to a list with this code and query:

makelist(A, B, C, X) :- append([1, 2, 3],A, X).

?- makelist(a, b, c, X).
X = [1, 2, 3|a].

However rather than the usual separator comma (,) there's a vertical line separator (|) and I cannot add another object to the same list:

makelist(A, B, C, X) :- append([1, 2, 3],A, X), append(X,B, X).

?- makelist(a, b, c, X).
false.

Upvotes: 2

Views: 363

Answers (2)

false
false

Reputation: 10102

There are several misunderstandings. First, lists and their elements are confused which leads to the "dotted pair" that is |a]. Then, it seems that the role of variables is not clear to you.

In the goal append([1,2,3],A,X) both A and X are supposed to be lists. However, you set A to a which is not a list. The problem behind is that append/3 accepts any term for the second argument. To see this, just look at its answers:

?- append([1,2,3],A,X).
X = [1,2,3|A].

So the A can be anything, but it should be rather only a list.

?- A = a, append([1,2,3],A,X).
   A = a, X = [1,2,3|a].

Note that append/3 insists that the first argument is a list (or a partial list). Then once you have a bad list, you can no longer append it further. That is:

?- A = a, append([1,2,3],A,X), append(X, _, _).
   false.

Note that I did not use your definition literally. Instead, I replaced certain arguments by _ since they cannot help.

The second problem stems from the goal append(X, B, X). This goal, can only be true if B = []. Let's try this out:

?- append(X, B, X).
   X = [], B = []
;  X = [_A], B = []
;  X = [_A,_B], B = []
;  X = [_A,_B,_C], B = []
;  ... .

And:

?- B = [_|_], append(X, B, X).
   loops.

While there are many answers for X, B is always [].

Upvotes: 4

Sergii Dymchenko
Sergii Dymchenko

Reputation: 7209

append appends lists. So you want to make a list with a single element to append it to another list:

makelist(A, B, C, X) :- append([1, 2, 3], [A], X). % What's the purpose of B and C, btw?

Also you can add items to the beginning of a list more effectively:

makelist(A, B, C, [A | [1, 2, 3]]).

Upvotes: 0

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