C#: Check whether a number can be express as sum of two prime numbers

C# Sharp For Loop: Exercise-56 with Solution

Write a program in C# Sharp to check whether a number can be expressed as the sum of two prime numbers.

Sample Solution:-

C# Sharp Code:

using System;  // Importing necessary namespace

public class Exercise56  // Declaration of the Exercise56 class
{  
    public static void Main()  // Main method, entry point of the program
    {
        int n, i, flg1 = 1, flg2 = 1, flg3 = 0, j;  // Declaration of variables

        Console.Write("\n\n");
        Console.Write("Check whether a number can be expressed as the sum of two prime numbers:\n");
        Console.Write("-------------------------------------------------------------------");
        Console.Write("\n\n");	

        Console.Write("Input a positive integer: ");
        n = Convert.ToInt32(Console.ReadLine());  // Taking user input for a positive integer

        // Loop to find and display prime numbers that sum up to the given number 'n'
        for (i = 3; i <= n / 2; i++)
        {     
            // Check if 'i' is a prime number
            flg1 = 1;
            flg2 = 1;

            // Loop to check if 'i' is prime
            for (j = 2; j < i; j++)
            {     
                if (i % j == 0)
                { 
                    flg1 = 0;
                    j = i;
                }
            }

            // Loop to check if 'n-i' is prime
            for (j = 2; j < n - i; j++)
            {     
                if ((n - i) % j == 0)
                { 
                    flg2 = 0;
                    j = n - i;
                }
            }

            // If 'i' and 'n-i' both are prime, display the combination
            if (flg1 == 1 && flg2 == 1)
            { 
                Console.Write("{0} =  {1} + {2}  \n", n, i, n - i); 
                flg3 = 1;
            }
        }

        // If no combination is found, display appropriate message
        if (flg3 == 0)
        {
            Console.Write("\n{0} can not be expressed as the sum of two prime numbers.\n\n", n);
        }    
    }
}

Sample Output:

Check whether a number can be express as sum of two prime numbers:                                          
-------------------------------------------------------------------                                                                                            
Input  a positive integer: 50                                                                               
50 =  3 + 47                                                                                                
50 =  7 + 43                                                                                                
50 =  13 + 37                                                                                               
50 =  19 + 31 

Flowchart:

C# Sharp Code Editor:

Contribute your code and comments through Disqus.

Previous: Write a program in C# Sharp to convert a decimal number to hexadecimal.
Next: Write a program in C# Sharp to print a string in reverse order.