🚀 go-pugleaf

ulma@coli.uni-sb.de (Ulrich Mayring) writes:

>Hello netters, I have a problem:

>I have a list and need to recursively pick two elements out of it. I need
>every pair of elements in every order, however the order of the pair itself
>is not important. It should also be possible to pick an element and the []
>as a pair, thus guaranteeing that in the end I will have every pair and
>every single element. Then I will test the pair (or single) and if it doesn't
>suit my purpose I want to go back into the recursion to get the next one.

Well, since so many of you have answered, let me introduce the solution I
have finally decided on. Since I am a beginner it may not work in all circum-
stances, but it seems to in mine.

select(X,[X|R]),R).
select(X,[H|T],[H|R]) :- select(X,T,R).
select([],List,List).

Then my predicate goes something like select List A, select List B and then
append List A and List B. The last line in the select predicate appears to
generate the [] only when all other elements have been selected. When I 
have the pairs I test them and if they suit my purpose I append them to my
"Solution List", which is the reason why it is possible to have a pair of
the form (List,[]) represent only List, which has one element.

Thanx again to everybody,


     Ulrich Mayring

       - Also wir waren mit grau eigentlich sehr zufrieden -