C# Program to Check If a Number is a Perfect Number or Not

C# programs to check if a number is a perfect number or not have been shown here. A perfect number is a positive integer that is equal to the sum of its proper divisors. For example 6 is a perfect number because if we add it's proper divisors (1, 2 and 3), we get 6 i.e. 1 + 2 + 3 = 6. Some of the other perfect numbers are 28, 496, 8128 etc.

Page content(s):

1. Algorithm

2. Pseudocode

3. Time Complexity

4. Program & output

 4.1. Iterative approach

 4.2. Recursive approach

1. Algorithm to check if a number is a perfect number or not

1. Take a number n as input.

2. Set s = 0, x = 1

3. Check if n is divisible by x

4. If step 3 is true perform s = s + x

5. Set x = x + 1

6. Go step 3 until x > n/2

7. If s = n declare "n is a perfect number"

2. Pseudocode to check if a number is a perfect number or not

Input : A positive number n

Output : n is a perfect number or not

1. Procedure isPerfectNumber(n):

2. s := 0

3. x := 1

4. Repeat until x > (n/2)

5. If n mod x = 0:

6. s := s + x

7. If n = s:

8. Return "Perfect number"

9. Else:

10. Return "Not a perfect number"

11. End Procedure

3. Time complexity to check if a number is a perfect number or not

Time Complexity: O(n)

n is the input number

4. Program & Output to check if a number is a perfect number or not

4.1. C# Program & Output to check if a number is a perfect number or not using iteration

Code has been copied

/**************************
     alphabetacoder.com
C# program to check if a
number is a perfect number
***************************/

using System;

namespace PerfectNumber {
    class Program {
        static void Main(string[] args) {
            // declare variables
            int n, i, sum = 0;

            // take input of the number 
            Console.Write("Enter the number: ");
            n = Convert.ToInt32(Console.ReadLine());

            // add all the divisor upto n / 2
            for (i = 1; i <= (n / 2); i++) {
                if (n % i == 0)
                    sum = sum + i;
            }

            // check if the input number 
            // is equal to the sum
            if (n == sum)
                Console.WriteLine(n + " is a perfect number");
            else
                Console.WriteLine(n + " is not a perfect number");

            // wait for user to press any key
            Console.ReadKey();
        }
    }
}

Output

Enter the number: 28

28 is a perfect number

4.2. C# Program & Output to check if a number is a perfect number or not using recursion

Code has been copied

/************************************
	  alphabetacoder.com
C# program to check if a number is 
a perfect number using recursion
*************************************/

using System;

namespace PerfectNumber {
    class Program {
        // recursive function to check if input 
        // number is a perfect number or not
        public static int isPerfect(int n, int i, int sum) {
            if (i > n / 2) // exit condition
                return (sum == n) ? 1 : 0;

            if (n % i == 0) // check divisibility
                sum += i;

            return isPerfect(n, i + 1, sum);
        }

        static void Main(string[] args) {
            // declare variables
            int n;

            // take input of the number 
            Console.Write("Enter the number: ");
            n = Convert.ToInt32(Console.ReadLine());

            // check if the input number is perfect
            // by calling function
            if (isPerfect(n, 1, 0) == 1)
                Console.WriteLine(n + " is a perfect number");
            else
                Console.WriteLine(n + " is not a perfect number");

            // wait for user to press any key
            Console.ReadKey();
        }
    }
}

Output

Enter the number: 6

6 is a perfect number