# C Program to Print all Perfect Numbers Between 1 to N

Write a C program to print all perfect numbers between 1 to N using for loop.

#### Required Knowledge

A perfect number is a positive number, that is equal to sum of its positive divisor.
For Example : 6 is a perfect number as positive factors of 6 are 1, 2 and 3 whose sum is 6.

## C program to print all perfect numbers between 1 to N using for loop

```#include<stdio.h>
#include<conio.h>

int main () {
int num, i, divSum, counter;

printf("Enter a number\n");
scanf("%d", &num);

/* Check for every number between 1 to num,
whether it is perfect number or not */
for(counter = 1; counter <= num; counter++){
/* Find all divisors of a number between
1 to num/2 and add it to divSum */
divSum = 0;
for (i = 1; i <= counter/2; i++) {
if (counter % i == 0) {
divSum += i;
}
}

/* Check if Divisor sum is equal to input number */
if (divSum == counter)
printf("%d is a Perfect Number\n", counter);

}

getch();
return 0;
}
```

Output
```Enter a number
500
6 is a Perfect Number
28 is a Perfect Number
496 is a Perfect Number
```
Write a C program to find product of digits of a number using while loop.
Wap in C to multiply the digits of a number.

#### Required Knowledge

To multiply digits of a number we have to remove one digit at a time, we can use '/' division and '%' modulus operator. Number%10 will give the least significant digit of the number, we will use it to get one digit of number at a time. To remove last least significant digit from number we will divide number by 10.

Product of digits of 2534 = 2 x 5 x 3 x 4 = 120

Algorithm to find product of digits of a number
• Get least significant digit of number (number%10) and multiply it to the product variable.
• Remove least significant digit form number (number = number/10).
• Repeat above two steps, till number is not equal to zero.

## C program to find sum of all even numbers between 1 to N using for loop

```#include <stdio.h>
#include <conio.h>

int main() {
int num, temp;
long productOfDigit = 1;

/*
* Take a number as input from user
*/
printf("Enter a Number\n");
scanf("%d", &num);
temp = num;

while(num != 0){
/* get the least significant digit(last digit)
of number and multiply it to productofDigit */
productOfDigit *= num % 10;
/* remove least significant digit(last digit)
form number */
num = num/10;
}

printf("Product of digits of %d = %ld", temp, productOfDigit);

getch();
return 0;
}
```

Output
```Enter a Number
2436
Product of digits of 2436 = 144
```
```Enter a Number
2222
Product of digits of 2436 = 16
```

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