C#: Prime number in strictly descending decimal digit order

C# Sharp Basic: Exercise-98 with Solution

Primes in Descending Order

Write a C# Sharp program to create and display all prime numbers in strictly descending decimal digit order.

Sample Data:
2, 3, 5, 7, 31, 41, 43, 53, 61, 71, 73, 83, 97, 421, 431.....

Sample Solution:

C# Sharp Code:

using System;
using System.Linq;

namespace exercises
{
    class Program
    {
        // Method to check if a number is prime
        public static bool IsPrime(uint n)
        {
            if (n <= 1)
            {
                return false;
            }

            int ctr = 0;
            for (int i = 1; i <= n; i++)
            {
                if (n % i == 0)
                {
                    ctr++;
                }
                if (ctr > 2)
                {
                    return false;
                }
            }

            return true;
        }

        static void Main(string[] args)
        {
            uint z = 0; // Counter variable for prime numbers
            int nc; // Variable to hold the count of numbers

            // Array of prime numbers
            var p = new uint[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
            var nxt = new uint[128]; // Array for next potential prime numbers

            while (true)
            {
                nc = 0; // Reset the count of numbers

                // Loop through each number in the array of prime numbers
                foreach (var x in p)
                {
                    // Check if the number is prime and display it
                    if (IsPrime(x))
                        Console.Write("{0,8}{1}", x, ++z % 5 == 0 ? "\n" : " ");

                    // Generate next potential prime numbers
                    for (uint y = x * 10, l = x % 10 + y++; y < l; y++)
                        nxt[nc++] = y;
                }

                // Check if there are more than one potential prime numbers
                if (nc > 1)
                {
                    // Resize the prime number array and copy next potential primes
                    Array.Resize(ref p, nc);
                    Array.Copy(nxt, p, nc);
                }
                else
                {
                    // Break the loop if there are no more potential primes
                    break;
                }
            }

            // Output the number of descending primes found
            Console.WriteLine("\n{0} descending primes found", z);
        }
    }
}

Sample Output:

       2        3        5        7       31
      41       43       53       61       71
      73       83       97      421      431
     521      541      631      641      643
     653      743      751      761      821
     853      863      941      953      971
     983     5431     6421     6521     7321
    7541     7621     7643     8431     8521
    8543     8641     8731     8741     8753
    8761     9421     9431     9521     9631
    9643     9721     9743     9851     9871
   75431    76421    76541    76543    86531
   87421    87541    87631    87641    87643
   94321    96431    97651    98321    98543
   98621    98641    98731   764321   865321
  876431   975421   986543   987541   987631
 8764321  8765321  9754321  9875321 97654321

Flowchart:

C# Sharp Code Editor:

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