How to count all even numbers in a list

Question

Please help me with how to count even numbers in a list in Prolog. I am a beginner, just started learning Prolog yesterday. I know to count the elements in the list is

Answer 1

If your Prolog system offers clpfd, you can use the meta-predicate tcount/3 in combination with even_truth/2. Here's how:

?- tcount(even_truth, [1,2,3,5,7,9,10], N_even).
N_even = 2.

Both predicates (tcount/3 and even_truth/2) are monotone and preserve logical-purity! This makes them very robust and enables you to always get logically sound answers.

Consider the following more general query:

?- Xs = [_,_,_], tcount(even_truth, Xs, N_even).
Xs = [_A,_B,_C], N_even = 0, _A mod 2 #= 1, _B mod 2 #= 1, _C mod 2 #= 1 ;
Xs = [_A,_B,_C], N_even = 1, _A mod 2 #= 1, _B mod 2 #= 1, _C mod 2 #= 0 ;
Xs = [_A,_B,_C], N_even = 1, _A mod 2 #= 1, _B mod 2 #= 0, _C mod 2 #= 1 ;
Xs = [_A,_B,_C], N_even = 2, _A mod 2 #= 1, _B mod 2 #= 0, _C mod 2 #= 0 ;
Xs = [_A,_B,_C], N_even = 1, _A mod 2 #= 0, _B mod 2 #= 1, _C mod 2 #= 1 ;
Xs = [_A,_B,_C], N_even = 2, _A mod 2 #= 0, _B mod 2 #= 1, _C mod 2 #= 0 ;
Xs = [_A,_B,_C], N_even = 2, _A mod 2 #= 0, _B mod 2 #= 0, _C mod 2 #= 1 ;
Xs = [_A,_B,_C], N_even = 3, _A mod 2 #= 0, _B mod 2 #= 0, _C mod 2 #= 0.