Getting the minimum value of a list

Question

I am trying to find the minimum value of a list (as a learning experience, so without min).

My approach is the following:

minimo([X], X).
minimo([X,Y|Tail], N):-
    (X > Y, minimo([Y,Tail], Y));
    (X <= Y, minimo([X,Tail], X)).

This gives me the following error:

Syntax error: Operator expected

So my questions are:

• What is causing the syntax error?
• I will try it myself once that is fixed if it actually gives the correct value back, but would this actually be a correct approach?

Thanks in advance.

Answer 1

There are several bugs in your program:

1. as pointed out by Joe Lehmann, there is no '<='/2. It must be '=<'/2.

2. when you call minimo/2 recursively, you construct the lists wrong. Instead of [Y,Tail] use [Y|Tail]. Otherwise, you get a list with a list as second element.

3. you bind the second argument of the recursive call of minimo/2 to Y or X. Instead it must be bound to N. Otherwise, your N will never be instantiated.

You can improve your program further by adding cuts or using if-then-else ('->' + ;):

minimo([X], X) :- !.
minimo([X,Y|Tail], N):-
    ( X > Y ->
        minimo([Y|Tail], N)
    ;
        minimo([X|Tail], N)
    ).