Re: Help with lists.

joonix wrote: 
Greetings,

I have just started learning prolog. I come from a C/C++/Java
background and I am having a very hard time getting my head around this
style of programming.

Can some one please offer me some assistance in creating the following
[snip] 

2) A predicate no-triplicates(List1, List2) that replaces all
occurrences of any element that appears three or more times in List1
(i.e., triplicate members) with a single occurrence of that element to
generate List2. This single occurrence must occupy the position in the
list held by the first occurrence of the triplicate member in List1.
Duplicate occurrences are left unchanged. For instance, if the input
list List1 is [1, 2, 3, 2, 3, 3, 4], then the output list List2 must be
[1, 2, 3, 2, 4].

Thanks in advance,

joo.


Another possibility:

reduce_triples([],[]).

reduce_triples([X|L1],[X|L2]):-
  ( remove(X,L1,LX), remove(X,LX,LXX) ->
      removeall(X,LXX,L0), reduce_triples(L0,L2) ;
    reduce_triples(L1,L2) ).

remove(X,[Y|L1],L2) :-
  ( X==Y -> L2=L1 ;
    remove(X,L1,L0), L2=[Y|L0] ).

removeall(_,[],[]).
removeall(X,[Y|L1],L2) :-
  ( X==Y -> removeall(X,L1,L0), L2=L0 ;
    removeall(X,L1,L0), L2=[Y|L0] ).

test :-
  data(List1),
  reduce_triples(List1,List2),
  write(List2),nl,
  fail.
test.

data([1, 2, 3, 2, 3, 3, 4]).
data([4,3,2,1,4,3,2,4,3,4]).
data([4,5,5,3,2,1,5,3,4,2,4,5,3,5,4]).

?- test.
[1, 2, 3, 2, 4]
[4, 3, 2, 1, 2]
[4, 5, 3, 2, 1, 2]


--
billh
.

Relevant Pages

  • Re: Help with lists.
    ... background and I am having a very hard time getting my head around this style of programming. ... occurrences of any element that appears three or more times in List1 (i.e., triplicate members) with a single occurrence of that element to generate List2. ... This single occurrence must occupy the position in the list held by the first occurrence of the triplicate member in List1. ...
    (comp.lang.prolog)
  • Help with lists.
    ... background and I am having a very hard time getting my head around this ... nested lists beyond the first level. ... (i.e., triplicate members) ... list held by the first occurrence of the triplicate member in List1. ...
    (comp.lang.prolog)