adding H and T in list

[charlene1225]

I'm new in prolog.
May I ask, if I wanna add the H and Tail in a list separately,how should i write??

for example, i have list [[3,12],[4,5]].
and I want the output become H = 7,T = 17.

 

[joel76]

You don't understand what mean Head and Tail in Prolog.
In Prolog, as in Lisp, you acces to the first element of a list (often named Head) and the rest of this list (often named Tail).

1 ?- L = [1,2,3,4], L = [H | T].
L = [1, 2, 3, 4],
H = 1,
T = [2, 3, 4] ;
false.

So, I think that you want to add the elements of the list column by column.
There are differents ways to do that
First you use an accumulator to memorize the results

% when the list is empty, it'over
% but the results are in a reverse order
add_list1([[],[]], L1, L2) :-
	reverse(L1, L2).

% I add the first elements of the list
% I memorize the result in the middle list
% I carry on with the rest
add_list1([[H1 | T1], [H2 | T2]], L, LF) :-
	H3 is H1 + H2,
	add_list1([T1, T2], [H3 | L], LF).

Call : add_list1([[3,12],[4,5]], [], L).
Another way is to travel through the list and construct the result by the end.

% when there is nothing in the lists
% the result is empty
add_list2([[],[]], []).

% I construct the result list by the end
% I first built the result list with the tails of the first list
% I put the sum of the elements at the head of the result list.
add_list2([[H1 | T1], [H2 | T2]], [H3 | T3]) :-
	add_list2([T1, T2], T3),
	H3 is H1 + H2.

Call : add_list2([[3,12],[4,5]], L).