# C Program to Calculate Sum of Natural Numbers

To understand this example, you should have knowledge of following C programming topics:

Positive integers 1, 2, 3, 4... are known as natural numbers. This program takes a positive integer from user( suppose user entered `n` ) then, this program displays the value of 1+2+3+....n.

## Source Code to Calculate Sum of Natural Numbers

```
/* This program is solved using while loop. */
#include <stdio.h>
int main()
{
int n, count, sum=0;
printf("Enter an integer: ");
scanf("%d",&n);
count=1;
while(count<=n) /* while loop terminates if count>n */
{
sum+=count; /* sum=sum+count */
++count;
}
printf("Sum = %d",sum);
return 0;
}
```

### Source Code to Calculate Sum Using for Loop

```
/* This program is solve using for loop. */
#include <stdio.h>
int main()
{
int n, count, sum=0;
printf("Enter an integer: ");
scanf("%d",&n);
for(count=1;count<=n;++count) /* for loop terminates if count>n */
{
sum+=count; /* sum=sum+count */
}
printf("Sum = %d",sum);
return 0;
}
```

**Output**

Enter an integer: 100 Sum = 5050

Both program above does exactly the same thing. Initially, the value of `count` is set to 1. Both program have test condition to perform looping iteration until condition `count<=n`

becomes false and in each iteration `++count`

is executed.

This program works perfectly for positive number but, if user enters negative number or 0, `Sum = 0`

is displayed but, it is better is display the error message in this case. The above program can be made user-friendly by adding if else statement as:

```
/* This program displays error message when user enters negative number or 0 and displays the sum of natural numbers if user enters positive number. */
#include <stdio.h>
int main()
{
int n, count, sum=0;
printf("Enter an integer: ");
scanf("%d",&n);
if ( n<= 0)
printf("Error!!!!");
else
{
for(count=1;count<=n;++count) /* for loop terminates if count>n */
{
sum+=count; /* sum=sum+count */
}
printf("Sum = %d",sum);
}
return 0;
}
```

This program also can be solved using recursive function.