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, \cdotsThis 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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