39 lines
1.1 KiB
Factor
39 lines
1.1 KiB
Factor
! Copyright (c) 2007 Aaron Schaefer.
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! See http://factorcode.org/license.txt for BSD license.
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USING: combinators.short-circuit kernel math math.functions
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math.ranges namespaces project-euler.common sequences ;
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IN: project-euler.021
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! http://projecteuler.net/index.php?section=problems&id=21
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! DESCRIPTION
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! -----------
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! Let d(n) be defined as the sum of proper divisors of n (numbers less than n
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! which divide evenly into n).
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! If d(a) = b and d(b) = a, where a != b, then a and b are an amicable pair and
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! each of a and b are called amicable numbers.
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! For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44,
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! 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4,
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! 71 and 142; so d(284) = 220.
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! Evaluate the sum of all the amicable numbers under 10000.
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! SOLUTION
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! --------
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: amicable? ( n -- ? )
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dup sum-proper-divisors
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{ [ = not ] [ sum-proper-divisors = ] } 2&& ;
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: euler021 ( -- answer )
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10000 [1,b] [ dup amicable? [ drop 0 ] unless ] map-sum ;
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! [ euler021 ] 100 ave-time
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! 335 ms ave run time - 18.63 SD (100 trials)
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SOLUTION: euler021
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