C program to print perfect numbers between given interval using function

Write a function to print all perfect numbers in a given interval in C programming. How to print all perfect numbers in a given range using functions in C program. C program to print all perfect numbers between 1 to n using functions.

Input lower limit: 1
Input upper limit: 100

Perfect numbers: 6, 28

Required knowledge

Basic C programming, If else, For loop, While loop, Functions

Must know – Program to print all perfect numbers in a given range using loop.

Declare function to print all perfect numbers in given range

1. First give meaningful name to the function. Say printPerfect() will print all perfect numbers in given range. Along with this declare and define one more function int isPerfect(int num); to check perfect number.
2. Next the function must accept two parameters i.e. start and end limit to print perfect numbers in range. Hence, the function declaration should look like printPerfect(int start, int end);
3. Finally the function prints all perfect numbers in given range returning nothing. Hence, return type of the function must be void.

Final function declaration to print all perfect numbers in given range is – void printPerfect(int start, int end);

Program to print perfect numbers in given range using functions

/**
 * C program to print all perfect numbers in given range using function
 */
 
#include <stdio.h>


/* Function declarations */
int isPerfect(int num);
void printPerfect(int start, int end);



int main()
{
    int start, end;
    
    /* Input lower and upper limit to print perfect numbers */
    printf("Enter lower limit to print perfect numbers: ");
    scanf("%d", &start);
    printf("Enter upper limit to print perfect numbers: ");
    scanf("%d", &end);
    
    printf("All perfect numbers between %d to %d are: \n", start, end);
    printPerfect(start, end);
    
    return 0;
}



/**
 * Check whether the given number is perfect or not.
 * Returns 1 if the number is perfect otherwise 0.
 */
int isPerfect(int num)
{
    int i, sum;
    
    /* Finds sum of all proper divisors */
    sum = 0;
    for(i=1; i<num; i++)
    {
        if(num % i == 0)
        {
            sum += i;
        }
    }

    /* 
     * If sum of proper positive divisors equals to given number 
     * then the number is perfect number
     */
    if(sum == num)
        return 1;
    else
        return 0;
}



/**
 * Print all perfect numbers between given range start and end.
 */
void printPerfect(int start, int end)
{
    /* Iterates from start to end */
    while(start <= end)
    {
        if(isPerfect(start))
        {
            printf("%d, ", start);
        }
        
        start++;
    }   
}

Enter lower limit to print perfect numbers: 1
Enter upper limit to print perfect numbers: 10000
All perfect numbers between 1 to 10000 are:
6, 28, 496, 8128,

Happy coding 😉