Write a C program to enter any number and check whether the number is Perfect number or not. How to check perfect numbers in C programming using loop. Logic to check perfect number in C program.

**Example**

**Input**

Input any number: 6

**Output**

6 is PERFECT NUMBER

## Required knowledge

Basic C programming, For loop, If else

Must know -

### What is Perfect number?

Perfect number is a positive integer which is equal to the sum of its proper positive divisors.

For example: 6 is the first perfect number

Proper divisors of 6 are 1, 2, 3

Sum of its proper divisors = 1 + 2 + 3 = 6.

Hence 6 is a perfect number.

## Logic to check Perfect number

Below is the step by step descriptive logic to check Perfect number.

- Input a number from user, which is to be checked for perfect number. Store it in some variable say num.
- Initialize another variable to store sum of proper positive divisors, say sum = 0.
- Run a loop from 1 to num/2, incrementing 1 in each iteration. The loop structure is for(i=1; i<=num/2; i++). Here a question might pop in your mind. Why running loop from 1 to num/2, why not till num? Because a number does not have any proper positive divisor greater than num/2.
- Inside the loop if current number i.e. i is proper positive divisor of num, then add it to sum. Which is if(num % i == 0) then, perform sum = sum + i.
- Finally, if the sum of proper positive divisors equals to the original number. Which is if(sum == num) then, the given number is Perfect number otherwise not.

## Program to check perfect number

```
/**
* C program to check whether a number is Perfect number or not
*/
#include <stdio.h>
int main()
{
int i, num, sum = 0;
/* Input a number from user */
printf("Enter any number to check perfect number: ");
scanf("%d", &num);
/* Calculate sum of all proper divisors */
for(i=1; i<num; i++)
{
/* If i is a divisor of num */
if(num%i==0)
{
sum += i;
}
}
/* Check whether the sum of proper divisors is equal to num */
if(sum == num)
{
printf("%d is PERFECT NUMBER", num);
}
else
{
printf("%d is NOT PERFECT NUMBER", num);
}
return 0;
}
```

Output

Enter any number to check perfect number: 6 6 is PERFECT NUMBER

Happy coding 😉

<pre><code> ----Your Source Code---- </code></pre>