Sum of first N natural numbers in C

As we know natural numbers contain all the positive numbers starting from 1, 2, 3, to n numbers or infinity. For example, suppose when we calculate the sum of the first 25 numbers. That means we start adding the numbers from 1 to the given number 25, and the process is called the sum of the first N natural number. In this topic, we will learn how to find the sum of first n numbers using a C program.

Mathematical Formula

Following is the representation to find the sum of n natural numbers using the mathematical formula:

Where n defines the natural number.

Suppose, we want to calculate the sum of the first 20 natural number, we need to put in a mathematical formula to get the sum:

Sum = 20 * (20 + 1) / 2 = 20 * 10.50 = 210
Or 
20 * (20 + 1) /2 = 10 * 21 = 210

Pseudo code

int i, sum = 0, num
input positive number
i = 0
do
sum = sum + i
i = i + 1
iterate the value of i < = num
display the sum of the first natural number.

Using for Loop

Let's create a C program that determines the sum of n natural numbers using for loop.

SumOfNaturalNumber1.c

#include <stdio.h>
#include <conio.h>
void main()
{
	int num, i, sum = 0; // declare local variables
	printf(" Enter a positive number: ");
	scanf("%d", &num); // take any positive number
	// executes until the condition remains true.
	for (i = 0; i <= num; i++)
	{
		sum = sum + i; // at each iteration the value of i is added to the sum variable
	}
	// display the sum of natural number
	printf("\n Sum of the first %d number is: %d", num, sum); 
	getch();
}

Output:

Enter a positive number: 25
Sum of the first 25 number is: 325

Using while Loop

Let's create a C program that determines the sum of n natural numbers using while loop.

SumOfNaturalNumber2.c

#include <stdio.h>
#include <conio.h>
void main()
{
	int num, i, sum = 0; // initialize and declare the local variables
	printf("Enter a positive number : ");
	scanf("%d", &num); // take a value up to which find the sum of n natural number
	i = 0;
	while (i <= num) // define the while loop and i should be less than num
	{
		sum = sum + i; // store the sum of natural number
		i++; // increment by 1
	}
	// print the sum of natural number
	printf(" \n Sum of first %d natural number is : %d", num, sum);
	getch();
}

Output:

Enter a positive number: 20
Sum of the first 20 natural number is: 210

In the above example, when we enter a positive number 20, the while loop continuously iterates over the counter value between i = 0 to 20. At each iteration, the value of i is added to the variable sum, and i is incremented by 1. When the while condition becomes false, it exits the loop and prints the sum of the first 20 natural numbers.

Using do-while Loop

Let us consider the following example to calculate the sum of natural number using Do while loop.

SumOfNaturalNumber3.c

#include <stdio.h>
#include <conio.h>
void main()
{
	int num, i, sum = 0; // initialize and declare the local variables
	printf("Enter a positive number: ");
	scanf("%d", &num); // take a value up to which find the sum of natural number
	i = 0;
	do
	{
	    sum = sum + i; // store the sum of natural number
		i++; // increment by 1	
	} while (i <= num); // define the while loop and i should be less than num
	
	// print the sum of natural number
	printf(" \n Sum of first %d natural number is : %d", num, sum);
	getch();
}

Output:

Enter a positive number: 30
Sum of the first 30 natural number is: 465

In the above example, when we enter a positive number 30, the do loop continuously iterates the counter value between i = 0 to 30. At each iteration, the value of i is added to the variable sum, and i is incremented by 1. When the while condition becomes false, it exits from the while loop and prints the sum of the first 30 natural numbers.

Using the Mathematical Formula

Let's write a program to print the sum of n natural number using the mathematical formula.

SumOfNaturalNumber4.c

#include<stdio.h>
int main()
{
	int n = 40; // declare & initialize local variable n.
	int sum = (n * (n + 1) ) / 2; /* define the mathematical formula to calculate the sum of given number. */
	printf("Sum of %d natural number is = %d", n, sum); // print the sum of natural number
	return 0;
}

Output:

Sum of 40 natural number is = 840

Using Function

Let's consider the following example to calculate the sum of natural number using function in C.

SumOfNaturalNumber5.c

#include <stdio.h>
#include <conio.h>
void main()
{
	int num, total; // local variable
	printf("Enter a natural number: ");
	scanf("%d", &num); // take a natural number from the user
	total = natural_no(num);  // call the function
	printf(" Sum of the %d natural number is: %d", num, total);
}
int natural_no(num)
{
	int i, sum = 0;
	// use for loop until the condition becomes false
	for (i = 0; i <= num; i++)
	{
         // adding the counter variable i to the sum value
		sum = sum + i;
	}
	return sum;
}

Output:

Enter a natural number: 100
Sum of the 100 natural number is: 5050

Sum of n natural numbers Between a Given Range

Calculate the sum of n natural number from any starting number to the specify last number.

SumOfNaturalNumber6.c

#include <stdio.h>
#include <conio.h>
void main()
{
	int num, i, sum = 0; // define the local variables
	printf("Enter the first number: ");
	scanf("%d", &i); // accept the starting number
	
	printf(" Up to the last natural number: ");
	scanf("%d", &num); // accept the last number
	
	// As long as the loop condition is true, it continuously iterates the statement	while(i <= num)
	{
		// adding the counter variable i to the sum variable
		sum = sum + i;
		i++; // increment by 1
	}
	printf("Sum of natural number is = %d", sum);
	getch();
}

Output:

Enter the first number: 1
Up to the last number natural number: 25
Sum of natural number is = 325

Using Recursion

Let's consider the following example to calculate the sum of natural number using recursion.

SumOfNaturalNumber7.c

#include <stdio.h>
#include <conio.h>
int sum_natural_no(int num); // declare function outside the main function
int
 main()
{
	int num, sum = 0; // declare local variable
	printf("Enter any positive number to calculate the sum of natural no. ");
	scanf("%d", &num); // take an input from the user
	
	sum = sum_natural_no(num); // call the function
	
	printf("Sum of the first %d natural number is: %d", num, sum); // print the sum of natural number
	return 0;
}
int sum_natural_no(int num)
{
	if( num == 0) // define if condition
	{
		return num;	
	}
	else
	{   // return the else condition
		return( num + sum_natural_no( num - 1));
	}
}

Output:

Enter any positive number to calculate the sum of natural no. 50
Sum of the first 50 natural number is: 1275

Using an Array

SumOfNaturalNumber8.c

#include <stdio.h>
int main()
{
	// declare & initialize local variable
   int num, sum = 0, i, array[50];
	printf(" Enter a positive number as we want to sum the natural number: ");
   scanf("%d", &num); // take a positive number 
   printf("\n Enter the number one by one: \n");
   for (i = 0; i < num; i++)
   {
   	
      scanf("%d", &array[i]); // read value one by one 
      sum = sum + array[i]; // store number into the sum variable
   }

   printf("Sum of the given number is = %d\n", sum);

   return 0;
}

Output:

Enter a positive number as we want to sum the natural number: 5
 Enter the number one by one: 
2
4
5
6
7
Sum of the given number is = 24

Next TopicWhat is getch() in C

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Sum of first N natural numbers in C

Mathematical Formula

Pseudo code

Using for Loop

Using while Loop

Using do-while Loop

Using the Mathematical Formula

Using Function

Sum of n natural numbers Between a Given Range

Using Recursion

Using an Array

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