Project Euler : Problem 021

Project Euler problems in Oforth . Feel free to post your own code.

Project Euler : Problem 021

Let d(n) be defined as the sum of proper divisors of n (numbers less than n which divide evenly into n).
If d(a) = b and d(b) = a, where a ≠ b, then a and b are an amicable pair and each of a and b are called amicable numbers.

For example, the proper divisors of 220 are 1, 2, 4, 5, 10, 11, 20, 22, 44, 55 and 110; therefore d(220) = 284. The proper divisors of 284 are 1, 2, 4, 71 and 142; so d(284) = 220.

Evaluate the sum of all the amicable numbers under 10000.

Code: Select all
Integer method: properDivs { #[ self swap mod not ] self 2 / seq filter }

: amicables(n)   // ( n -- aList ) : Returns list for pairs of amicables under n.
{
| i j |
ListBuffer new
n loop: i [
i properDivs sum dup ->j i <= ifTrue: [ continue ]
j properDivs sum i <> ifTrue: [ continue ]
[ i, j ] over add
]
}

: pe021 { 0 10000 amicables apply(#[ sum + ]) }
Franck

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Joined: 29 Oct 2014 19:01 