Re: ord_select ?

On 13/02/2014, at 12:07 AM, Nicos Angelopoulos wrote:

> 
> Dear all, 
> 
> in library  library(ordsets) I would like to have the  same behaviour as select/3 for lists,
> in that if the element is not in the set the predicate should fail.

select(X, L, R) <->
   append(A, Z, R) & append(A, [X|Z], L).

In the ?X, +L, -R mode, this works perfectly for
ordered sets.  In the +X, -L, +R mode it doesn't.

> ord_del_element/3 is the closest thing as far as i can see, 
> but it does not fail if the element is not in the set.

	That's because x ∉ S ⇒ S \ {x} = S

and S \ {x} is what ord_del_element is supposed to compute.

> 1. am i missing the correct predicate to achieve this?
> 2. would it make sense to include some such like predicate to the library? 

In Quintus Prolog, library(ordsets) provides this predicate.
The actual code is unrolled for speed; the code here is newly
written.

%   ord_selectchk(Item, Set1, Set2)
%   is true when select(Item, Set1, Set2) and Set1, Set2 are
%   both sorted lists without duplicates.  This implementation
%   is only expected to work for Item ground and either Set1 or
%   Set2 ground.  The "chk" suffix is meant to remind you of
%   memberchk/2, which also expects its first argument to be ground.
%   ord_selectchk(X, S, T) =>
%      ord_memberchk(X, S) & \+ ord_memberchk(X, T).

ord_selectchk(Item, [X|Set1], [X|Set2]) :-
    X @< Item,
    !,
    ord_selectchk(Item, Set1, Set2).
ord_selectchk(Item, [Item|Set1], Set1) :-
    (   Set1 == [] -> true
    ;   Set1 = [Y|_] -> Item @< Y
    ).



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