prolog - Excluding all occurrences of the minimum number in a list

Question

As a Prolog newbie, I try to define a predicate filter_min/2 which takes two lists to determine if the second list is the same as the first, but with all occurrences of the minimum number removed.

Sample queries with expected results:

?- filter_min([3,2,7,8], N).
N = [3,7,8].

?- filter_min([3,2,7,8], [3,7,8]).
true.

I tried but I always get the same result: false. I don't know what the problem is. I need help!

Here is my code:

filter_min(X,Y) :-
    X == [],
    write("ERROR: List parameter is empty!"),
    !;
    min_list(X,Z),
    filter(X,Y,Z).

filter([],[],0).
filter([H1|T1],[H2|T2],Z) :-
    + number(H1),
    write("ERROR: List parameter contains a non-number element"),
    !;
    H1 = Z -> H2 is H1, filter(T1,T2,Z);
    filter(T1,T2,Z).

Answer 1

There are a couple of problems with your code:

• filter([],[],0). will not unify when working with any list that does not have 0 as its minimum value, which is not what you want. You want it to unify regardless of the minimum value to end your recursion.
• The way you wrote filter([H1|T1],[H2|T2],Z) and its body will make it so that the two lists always have the same number of elements, when in fact the second one should have at least one less.

A correct implementation of filter/3 would be the following:

filter([],[],_).
filter([H1|T1],L2,Z):-
    + number(H1),
    write("ERROR: List parameter contains a non-number element"),
    !;
    H1 = Z -> filter(T1,T2,Z), L2 = [H1|T2];
    filter(T1,L2,Z).