Re: simple list q's

From: Martin Sondergaard (nobody_at_nowhere.com)
Date: 06/02/04 

Date: Wed, 2 Jun 2004 20:48:25 +0100

You were using "is" the wrong way round.
You used this :

    H mod 2 is 0.

When you use "X is Y", it evaluates an expression Y,
and matches the result with X.
The term X is not evaluated.

So "H mod 2 is 0" makes it match "0" with the term "mod(H, 2)", which fails.

To evaluate "H mod 2" and compare it with 0, you could have used this :

    0 is H mod 2.

Your code would use it like this :

% base case
printeven([]):-
    write('List empty.').
%
% recursive case
printeven([H|T]):-
    0 is H mod 2,
    write(H),write(', '),
    printeven(T).
%
printeven([_|T]):-
    printeven(T). 

--
Martin Sondergaard,
London.