C Program to Find the Sum of Natural Numbers

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The positive numbers such as 1,2,3,4,5…. etc are known as natural numbers.

In this program, you will see how to calculate the sum up to n natural numbers. We will ask a user to enter a number and then calculate the sum up to that number.

For example, if the user enters 5, then we will calculate 1+2+3+4+5.

To understand this program better you should have the knowledge of –

  • Loops.
  • Assignment Operators.

Program using for loop

#include <stdio.h>
int main()
{
    int num, i, sum = 0;
    printf("Enter a positive number: ");
    scanf("%d", &num);

    if(num > 0)
    {
        for (i = 1; i <= num; i++)
        {
            sum += i; //sum = sum + i
        }
        printf("The sum is %d ", sum);
    }
    else
    {
        printf("You have entered an invalid number");
    }
    return 0;
}

Output

Enter a positive number:10

The sum is 55

Program using while loop

#include <stdio.h>
int main()
{
    int num, i = 1, sum = 0;
    printf("Enter a positive number: ");
    scanf("%d", &num);
    if(num > 0)
    {
        while (i <= num)
        {
            sum += i; //sum = sum + i
            i++;
        }
        printf("The sum is %d ", sum);
    }
    else
    {
        printf("You have entered an invalid number");
    }
    return 0;
}

Output

Enter a positive number:100

The sum is 5050

Explanation

  • In both the programs, we have declared three variables num, i, and sum. The sum is also initialized with 0.
  • The variable num will store the number entered by the user. The variable i is used as a counter for the loop.
  • The loop would execute from 1 to the number stored inside ‘num’. So, for instance, if the user enters 10, the loop will run 10 times from 1 to 10.
  • We have put an if-else block to check if the user has entered a positive number. If the number is positive, the control will enter the loop block and the sum would be generated, else a message would be shown “You have entered an invalid number”.
  • Every time the loop runs, the value of sum is incremented by the current value of i.

In the first iteration sum += i// sum += 1 // Sum is storing 1

In the first iteration sum += i// sum += 2 // Sum is storing 3

In the first iteration sum += i// sum += 1 // Sum is storing 6

and so on.

You can be more specific by placing an if-else if-else block.

Program

#include <stdio.h>
int main()
{
    int num, i = 1, sum = 0;
    printf("Enter a positive number: ");
    scanf("%d", &num);

    if(num > 0)
    {
        while (i <= num)
        {
            sum += i; //sum = sum + i
            i++;
        }
        printf("Sum = %d", sum);
    }
    else if(num == 0)
    {
        printf("You have entered 0");
    }
    else
    {
        printf("You have entered a negative number");
    }

    return 0;
}

 

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