Re: ord_select ?

Dear Michael, all,


On Wed, 12 Feb 2014 08:07:19 -0700
Michael Hendricks <[email protected]> wrote:

> On Wed, Feb 12, 2014 at 4:07 AM, Nicos Angelopoulos <
> [email protected]> wrote:
> 
> > 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.
> > ...
> > 1. am i missing the correct predicate to achieve this ?
> >
> 
> Since ordsets are implemented as lists, you should be able to use select/3
> directly.
> 

	this doesn't  take advantage of the fact that the list is a set
        writing the predicate myself is not the issue
> 
> > 2. would it make sense to include some such like predicate to the library?
> >
> 
> That seems reasonable to me.  For large ordsets, a specialized version of
> select/3 could get better performance by utilizing the sort order.
> 


the (one) impementation is pretty simple,  (see end of message).
point (a) is whether it is a useful enough addition to the library- provided it doesn't already exist
and (b) what should the name be

ord_select/3 
or something along the the lines of ord_del_element_?/3  ord_sel_element/3 ?


There is also the issue of whether the doc for  ord_del_element/3 
should explicitly mention that the predicate does not fail in that scenario.
The manual does say that
	This is the same as ord_subtract(Set, [Element], NewSet).
which is certainly a warning in the right direction.

Regards,

Nicos Angelopoulos
---
http://stoics.org.uk/~nicos/




%% ord_select(+Set, +Element, -Rem).
%
%  Succeeds iff Elem is in Set and Rem is the set minus this Element.
%  Very similar to ord_del_element/3 but fails if Element is not in the Set.
%
% ==
%  ord_select([a,b,c], a, BC).
%  BC = [b, c].
%  
%  ?- ord_select([a,b,c], d, ABC).
%  false.
%  
%  ?- ord_del_element([a,b,c], d, ABC).  
%  ABC = [a, b, c].
% ==
%
%  @version  0.1 2014/02/12
% 
ord_select([H|T], V, Rem) :-
	compare(Rel, H, V),
	ord_select_rel(Rel, H, T, V, Rem).

ord_select_rel(<, H, T, V, [H|Rem]) :-
	ord_select(T, V, Rem ).
% ord_select_rel(>, H, T, V, [H|Rem]) :- fail.
ord_select_rel(=, _H, T, _V, T).