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<div class="nb-cell markdown" name="md1">
# Generate n-digit number w/o 1s and 0s in prolog

See https://stackoverflow.com/questions/43352008/generate-n-digit-number-w-o-1s-and-0s-in-prolog
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<div class="nb-cell program" name="p1">
:- use_module(library(clpfd)).

n_digits(N, Ds, Num) :-
        length(Ds, N),
        Ds ins 2..9,
        reverse(Ds, Rs),
        foldl(pow10, Rs, 0-0, _-Num).

pow10(D, Pow0-S0, Pow-S) :-
        Pow #= Pow0 + 1,
        S #= D*10^Pow0 + S0.
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<div class="nb-cell query" name="q1">
n_digits(3, Ds, N).
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<div class="nb-cell query" name="q2">
n_digits(3, Ds, N), label(Ds).
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<div class="nb-cell query" name="q5">
time((n_digits(1000, _Ds, N), label(_Ds))).
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<div class="nb-cell program" name="p2">
n_digits(N, Ds, Num) :-
    length(Ds, N),
    maplist(random_between(0'2, 0'9), Ds),
    number_codes(Num, Ds).
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<div class="nb-cell query" name="q3">
n_digits(3, Ds, N).
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<div class="nb-cell query" name="q4">
time(n_digits(1000, _Ds, N)).
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