C program to determine whether a given number is a perfect number or not

In this article, we will learn how to create a program in C that will ask the user to enter any number (at run-time) as input to check whether the given number is a perfect number or not. A perfect number is a number in which a factor's sum is equal to the number itself.

Check if the given number is a perfect number in C

The question is, "Write a program in C that checks whether a given number is a perfect number or not." The program given below is its answer. 6 is a perfect number because 1, 2, and 3 are its three factors, and after summing them up, you will get the same number itself.

#include<stdio.h>
#include<conio.h>
int main()
{
    int num, sum=0, i;
    printf("Enter any number: ");
    scanf("%d", &num);
    for(i=1; i<num; i++)
    {
        if(num%i == 0)
            sum = sum + i;
    }
    if(num == sum)
        printf("\nIt's a Perfect Number.");
    else
        printf("\nIt's not a Perfect Number.");
    getch();
    return 0;
}

The above program was built and run in the Code::Blocks IDE; here is the output. This is the first snapshot of the sample run:

c program check perfect number

Supply any number, say 6, and press the ENTER key to see if the given number is a perfect number or not. This is the second snapshot of the sample run:

c program perfect number or not

Let's check for another number, say 24. Here is the sample run for case 24 (as input):

check perfect number or not

Because the factors of 24 are 1, 2, 3, 4, 6, 8, 12, finding the sum of all these factors, 1+2+3+4+6+8+12, equals 36, which is not equal to the number itself (24 here).Therefore, according to the definition of a perfect number given here, 24 is not a perfect number.

The following are some of the main steps in the above program:

Print all perfect numbers in the given range in C

Here is another program on perfect numbers. This program will prompt the user to enter any two numbers as the starting and ending numbers or points, after which it will check and print all perfect numbers that exist between the given two numbers:

#include<stdio.h>
#include<conio.h>
int main()
{
    int n1, n2, i, j, sum, temp;
    printf("Enter the value of n1 (starting point): ");
    scanf("%d", &n1);
    printf("Enter the value of n2 (ending point): ");
    scanf("%d", &n2);
    printf("\nPerfect numbers between %d to %d are:\n", n1, n2);
    for(i=n1; i<=n2; i++)
    {
        temp = i;
        sum = 0;
        for(j=1; j<i; j++)
        {
            if(i%j==0)
            {
                sum = sum + j;
            }
        }
        if(temp==sum)
        {
            printf("%d\n", sum);
        }
    }
    getch();
    return 0;
}

Here is the first snapshot of the sample run:

print all perfect number c

To see if the perfect number exists between any two numbers, enter 1 as the starting number and 1000 as the ending number. Here is the second snapshot of the sample run:

find all perfect number c

C Quiz


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