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# Prime Number Checker

a guest Nov 30th, 2012 47 Never
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1. using System;
2.
4. {
5.     static void Main()
6.     {
7.         Console.Title = "Prime Number Checker";
8.
9.         Console.WriteLine("Please enter a positive integer in the range [1, 100] to\n" +
10.                             "check if it is prime or not:");
11.
12.     Prompt:
14.         byte input;
15.
16.         if (byte.TryParse(inputString, out input) && input <= 100 && input >= 1)
17.         {
18.             if ((input > 10 && input % 2 != 0 && input % 3 != 0 && input % 5 != 0 && input % 7 != 0) || (input == 2 || input == 3 || input == 5 || input == 7))
19.             // If the input is greater than 10 and not divisible by any of the prime numbers in the range [1, sqrt(100)], i.e. {2, 3, 5, 7} then it is prime.
20.             // The only prime numbers less than 10 are 2, 3, 5 and 7.
21.             {
22.                 Console.WriteLine("{0} is prime.", input);
23.                 goto Prompt;
24.             }
25.             else if (input == 1)
26.             {
27.                 Console.WriteLine("1 is neither prime nor composite.");
28.                 goto Prompt;
29.             }
30.             else
31.             {
32.                 Console.WriteLine("{0} is composite.", input);
33.                 goto Prompt;
34.             }
35.         }
36.         else
37.         {
38.             Console.WriteLine("Invalid input. Please enter a positive integer in the\n" +
39.                                 "range [1, 100]:");
40.             Console.WriteLine();
41.             goto Prompt;
42.         }
43.     }
44. }
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