I'm trying to create a list in Prolog (SWI Prolog) and check which numbers are powers of 2 and second find how many times a specific number is in the list (in this example I'm trying to find how many times the number 3 is in the list). For a example, if you ask
?- check([0,2,3,-5,-2,1,8,7,4], MULT2, THREE).
you should see
MULT2=[2,8,4]
THREE=1
My first try to find a solution is to search the list with head and doing head mod 2 = 0 to find all numbers which are powers of 2, but something went wrong and I only get "false" as an answer.
Here's how you can find the "powers of two" in logically-pure way!
Using sicstus-prolog 4.3.5, library(reif)
and library(clpz)
:
:- use_module([library(reif), library(clpz)]). power_of_two_t(I, T) :- L #= min(I,1), M #= I /\ (I-1), call((L = 1, M = 0), T). % using (=)/3 and (',')/3 of library(reif)
Sample query1 using meta-predicate tfilter/3
in combination with power_of_two_t/2
:
?- tfilter(power_of_two_t, [0,2,3,-5,-2,1,8,7,4], Ps).
Ps = [2,1,8,4]. % succeeds deterministically
Here's a more general query suggested by a comment:
?- tfilter(power_of_two_t, [X], Ps).
Ps = [X], 0#=X/\_A, _A+1#=X, X in 1..sup, _A in 0..sup
; Ps = [], dif(_A,0), _A#=X/\_B, _B+1#=X, X in 1..sup, _B in 0..sup
; Ps = [], dif(_A,1), _A#=min(X,1), _B#=X/\_C, _C+1#=X, X#>=_A, _A in inf..1.
Footnote 1: The answer sequences shown above were brushed up to indicate the determinism of calls.
Footnote 2: To reproduce the results use call_det/2
which is defined like this:
call_det(G_0, Det) :- call_cleanup(G_0, Flag = set), ( nonvar(Flag) -> Det = true ; Det = false ).