Retrieves the ith term in a term vector.

tv_get(+Vector, +Index, ?Term)

tv_get/3 retrieves a copy of the term at index Index in the term vector Vector and unifies it with Term. Index has to be an integer in the range 1 .. N, where N is the size of the term vector (see tv_size/2).

Arguments

Vector                  object (term vector)
Index                   integer
Term                    term

Examples

% find all prime numbers between 1 and N
prime_sieve(N, Primes) :-
    % create a term vector
    tv_create(Vector),
    % set it's size to N and fill all elements to 'prime'
    init_sieve(Vector, N),
    % set all non prime numbers to 'not_prime'
    compute_primes(Vector, N),
    % collect all elements still marked with 'prime'
    collect_primes(Vector, N, Primes).

init_sieve(Vector, N) :-
    % set the vector size to N
    tv_setsize(Vector, N),
    % set all vector elements to 'prime'
    for(1, I, N),
        tv_set(Vector, I, prime),
    fail.
init_sieve(_, _).

compute_primes(Vector, N) :-
    for(2, I, N),
        % if I is still marked as prime ...
        tv_get(Vector, I, prime),
        % ... mark all multiples of I as 'not_prime'
        filter_primes(Vector, I, I, N),
    fail.
compute_primes(_, _).

filter_primes(Vector, I, Step, N) :-
    J is I+Step, J =< N, !,
    % mark all multiples of 'Step' as 'not_prime'
    tv_set(Vector, J, not_prime),
    filter_primes(Vector, J, Step, N).
filter_primes(_, _, _, _).

collect_primes(Vector, N, Primes) :-
    % collect all elements still marked as 'prime'
    findall(I, isprime(Vector,I,N), Primes).

isprime(Vector, I, N) :-
    for(1,I,N),
    tv_get(Vector,I,prime).

Standard

This predicate is not part of the ISO-Prolog Standard.

See also