The program adds first N natural numbers (starts from 1) and prints the sum in the console. We wrote this program using Dev C++ version 4.9.9.2 on a Windows 7 64-bit system. You may try other standard C compilers to run the program with modifications.

Learn following c programming concepts before you attempt this example program.

- C Assignment Operators
- C Arithmetic Operators
- C Reading Input Values
- C Printing Output Values
- C For Loop

## Problem Definition

Natural numbers are all positive numbers greater than number 0 and it does not include the number zero.

For example

Natural\hspace{2mm} numbers = 1, 2, 3, 4, 5, \cdots

This C program reads the value of number N which is a natural number and then sum all other natural numbers up to number N start from 1. Suppose then

Sum = 1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 + 10 = 55

## Flowchart – Sum of First N Natural Numbers

## Program Code – Sum of First N Natural Numbers

```
/* Program for sum of First N Natural Numbers */
#include <stdio.h>
#include <stdlib.h>
main() {
int num,i,sum;
sum = 0; /* read the number */
printf("Enter number to sum:");
scanf("%d",&num);
for(i=1;i<=num;i++) {
sum = sum + i;
}
printf("sum of %d number is %d\n",num,sum);
getch();
return 0;
}
```

The most important piece of code in the above example is the* for loop*.

```
for ( i = 1; i <= num; i++)
{
sum = sum + 1;
}
```

The *for* loop starts from *1* and goes through each digit between *1* and *num *because variable `i` is incremented with each iteration (` i++`

). The value of *num* is user input and it can be a large number.

The value of **i** is added to variable *sum* using the following statement. The value of `sum` also increments with each iteration of the variable` i` in the for loop.

`sum = sum + i;`

The final value of **the sum** is printed as output.

## Output

```
How many numbers to add?:10
------------------------
sum of 10 number is 55
------------------------
```