1 ! Copyright (c) 2008 Aaron Schaefer.
2 ! See http://factorcode.org/license.txt for BSD license.
3 USING: kernel math math.parser math.primes sequences project-euler.common ;
6 ! http://projecteuler.net/index.php?section=problems&id=37
11 ! The number 3797 has an interesting property. Being prime itself, it is
12 ! possible to continuously remove digits from left to right, and remain prime
13 ! at each stage: 3797, 797, 97, and 7. Similarly we can work from right to
14 ! left: 3797, 379, 37, and 3.
16 ! Find the sum of the only eleven primes that are both truncatable from left to
17 ! right and right to left.
19 ! NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.
29 dup prime? [ r-trunc? ] [ drop f ] if
34 : reverse-digits ( n -- m )
35 number>string reverse string>number ;
38 reverse-digits 10 /i reverse-digits dup 0 > [
39 dup prime? [ l-trunc? ] [ drop f ] if
46 : euler037 ( -- answer )
47 23 1000000 primes-between [ r-trunc? ] filter [ l-trunc? ] filter sum ;
49 ! [ euler037 ] 100 ave-time
50 ! 130 ms ave run time - 6.27 SD (100 trials)