Question

How to sum all odd positioned elements in a list

example [1,2,3,4,5,6,7,8,9] = 25

odd([],0].
odd([Z],Z).
odd([X,Y|T], Sum+1):- odd(T,Sum).

but it return me 1+3+5+7+9.

Answer 1

What you construct when you write Sum+1 is a term with functor '+'/2 and arguments Sum and 1.

In Prolog, when you want to calculate a sum, you need to use the predicate is/2.

In your code, you should also add cuts to remove unnecessary choicepoints, and add X to the rest of the sum, not 1:

odd([],0) :- !.
odd([Z],Z) :- !.
odd([X,_|T],Sum):- odd(T,Sum0), Sum is Sum0+X.

Using an accumulator would allow you to make the code tail-recursive…