Help alternating two lists

[badkarma082]

I must develop a function that will accept 3 arguments.
The first two arguments must be a lists.
When either argument is not a list, join_alternate shall display an error message and terminate its processing.
When both arguments are lists, then alternate will produce a new list as its result, as follows:
?- alternate([1,2,3], [a,b,c], R)
R = [1,a,2,b,3,c]
?- alternate([1,2], [a,b,c], R)
R = [1,a,2,b,c]
?- alternate([1], [a,b,c], R)
R = [1,a,b,c]

I would appreciate any help. I just cant figure it out thanks!

 

[joel76]

You must examine all the possibilities :

alternate(X, Y, R)
X is not a list, or Y is not a list, you stop with an error message


Now the cases of end of recursion
alternate([], Y, R)
alternate(X, [], R)

Now the general case
alternate([H1|T1], [H2|T2], R)

 

[badkarma082]

Still don't really get it, but thanks. I guess learning prolog in 24 hours is not 4 me.

 

[joel76]

So, here is the code in SWI-Prolog :

alternate(X, Y, _R) :-
	(\+is_list(X); \+is_list(Y)),
	!,
	write('bad args'), 
	nl.


alternate([], R, R).

alternate(R, [], R).

% in the general case, we add H1 and H2 at the list got
% from the lists T1 and T2
alternate([H1 | T1], [H2 | T2], [H1, H2 | R]) :-
	alternate(T1, T2, R).

is_list/1 is a predicate which succeed if is arg is a list. \+ is_list succeed when is_list fails.

 

[badkarma082]

Wow man you made my day. Thankss!! Now i know this i a little too much, but what if I wanted, to specify which is argument is not a list eg. "list 1 is not valid" or "i list2 is not valid".

 

[joel76]

Well, I think you can do it alone !

 

[badkarma082]

Jaja thanks man, I really appreciate your help. have a good one.