C Program for Sum of First N Natural Numbers

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
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sum of 10 number is 55
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