Initial data commit

Task/Average-loop-length/C-sharp/average-loop-length.cs

@@ -0,0 +1,50 @@
+public class AverageLoopLength {
+	private static int N = 100000;
+	
+	private static double analytical(int n) {
+		double[] factorial = new double[n + 1];
+		double[] powers = new double[n + 1];
+		powers[0] = 1.0;
+		factorial[0] = 1.0;
+		for (int i = 1; i <= n; i++) {
+			factorial[i] = factorial[i - 1] * i;
+			powers[i] = powers[i - 1] * n;
+		}
+		double sum = 0;
+		
+		for (int i = 1; i <= n; i++) {
+			sum += factorial[n] / factorial[n - i] / powers[i];
+		}
+		return sum;
+	}
+
+	private static double average(int n) {
+		Random rnd = new Random();
+		double sum = 0.0;
+		for (int a = 0; a < N; a++) {
+			int[] random = new int[n];
+			for (int i = 0; i < n; i++) {
+				random[i] = rnd.Next(n);
+			}
+			var seen = new HashSet<double>(n);
+			int current = 0;
+			int length = 0;
+			while (seen.Add(current)) {
+				length++;
+				current = random[current];
+			}
+			sum += length;
+		}
+		return sum / N;
+	}
+	
+	public static void Main(string[] args) {
+	Console.WriteLine(" N    average    analytical    (error)");
+	Console.WriteLine("===  =========  ============  =========");
+		for (int i = 1; i <= 20; i++) {
+			var average = AverageLoopLength.average(i);
+			var analytical = AverageLoopLength.analytical(i);
+			Console.WriteLine("{0,3} {1,10:N4} {2,13:N4}  {3,8:N2}%", i, average, analytical, (analytical - average) / analytical * 100);
+		}
+	}
+}