53 lines
1.3 KiB
Factor
53 lines
1.3 KiB
Factor
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! Copyright (c) 2008 Aaron Schaefer.
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! See http://factorcode.org/license.txt for BSD license.
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USING: kernel math math.parser math.primes sequences ;
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IN: project-euler.037
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! http://projecteuler.net/index.php?section=problems&id=37
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! DESCRIPTION
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! -----------
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! The number 3797 has an interesting property. Being prime itself, it is
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! possible to continuously remove digits from left to right, and remain prime
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! at each stage: 3797, 797, 97, and 7. Similarly we can work from right to
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! left: 3797, 379, 37, and 3.
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! Find the sum of the only eleven primes that are both truncatable from left to
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! right and right to left.
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! NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
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! SOLUTION
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! --------
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<PRIVATE
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: r-trunc? ( n -- ? )
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10 /i dup 0 > [
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dup prime? [ r-trunc? ] [ drop f ] if
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] [
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drop t
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] if ;
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: reverse-digits ( n -- m )
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number>string reverse 10 string>integer ;
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: l-trunc? ( n -- ? )
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reverse-digits 10 /i reverse-digits dup 0 > [
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dup prime? [ l-trunc? ] [ drop f ] if
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] [
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drop t
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] if ;
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PRIVATE>
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: euler037 ( -- answer )
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23 1000000 primes-between [ r-trunc? ] subset [ l-trunc? ] subset sum ;
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! [ euler037 ] 100 ave-time
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! 768 ms run / 9 ms GC ave time - 100 trials
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MAIN: euler037
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