# 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:

### Pseudo code

1. int i, sum = 0, num
2. input positive number
3. i = 0
4. do
5. sum = sum + i
6. i = i + 1
7. iterate the value of i < = num
8. 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

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

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

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

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

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

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

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

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
```

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