Solved ProjectEuler/021: php, ruby, c

ProjectEuler/021/desc.yml

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+title: Evaluate the sum of all amicable pairs under 10000.
+url: http://projecteuler.net/problem=21
+
+desc: |
+  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.
+  
+solution: Bruteforce
+
+todo: Implement caching in other solutions
+
+solutions:
+  solve.php:
+    desc: (Uses a cache to find allready calculated values for d()
+    language: php
+  solve.rb:
+    desc: Basic solution
+    language: ruby
+  solve.c:
+    desc: ANSI C solution (Compiled with TCC)
+    language: c

ProjectEuler/021/solve.c

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+#define MAX 10000
+
+#include <stdio.h>
+#include <math.h>
+
+int d( int input) {
+	int sum = 0;
+	int n;
+	for(n=1;n<input;n++) {
+		if(input % n == 0)
+			sum += n;
+	}
+	return sum;
+}
+
+int main( )
+{
+	int result = 0;
+	int number;
+	int d_sum;
+	for(number = 1; number < MAX; number++) {
+		d_sum = d(number);
+		if(number != d_sum && number == d(d_sum)) {
+			result += number;
+		}
+	}
+	printf("%d", result);
+}

ProjectEuler/021/solve.php

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+<?php
+define('MAX',10000);
+
+function d($int) {
+	static $cache;
+	if(isset($cache[$int])) {return $cache[$int]; }
+	$sum = 0;
+	for($n=1;$n<$int;$n++) {
+		if($int % $n == 0)
+			$sum += $n;
+	}
+	$cache[$int] = $sum;
+	return $sum;
+}
+
+$result = 0;
+for($number = 1; $number < MAX; $number++) {
+	$d_sum = d($number);
+	if($number != $d_sum AND $number == d($d_sum)) {
+		$result += $number;
+	}
+}
+echo $result;

ProjectEuler/021/solve.rb

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+MAX = 10000
+
+def d(num) 
+	sum = 0
+	1.upto(num-1) do |d|
+		if(num % d == 0)
+			sum += d
+		end
+	end
+	return sum
+end
+
+result = 0
+Range.new(0,MAX,true).each do |number|
+d_sum = d(number)
+	if(number != d_sum && number == d(d_sum))
+		result += number
+	end
+end
+puts result