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 numbers = 1, 2, 3, 4, 5 ...

This C program reads the value of number N which is natural number and then sum all other natural numbers up to number N start from 1. Suppose **N = 10** 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 following statement. The value of **sum** also increments with each iteration of variable** i** in the for loop.

sum = sum + i;

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

## Output

**Related Articles:**