The positive numbers such as 1,2,3,4,5…. etc are known as natural numbers.

In this program, you will see how to calculate the sum up to n natural numbers. We will ask a user to enter a number and then calculate the sum up to that number.

For example, if the user enters 5, then we will calculate 1+2+3+4+5.

To understand this program better you should have the knowledge of –

- Loops.
- Assignment Operators.

## Program using for loop

```
#include <stdio.h>
int main()
{
int num, i, sum = 0;
printf("Enter a positive number: ");
scanf("%d", &num);
if(num > 0)
{
for (i = 1; i <= num; i++)
{
sum += i; //sum = sum + i
}
printf("The sum is %d ", sum);
}
else
{
printf("You have entered an invalid number");
}
return 0;
}
```

**Output**

`Enter a positive number:10`

`The sum is 55`

## Program using while loop

```
#include <stdio.h>
int main()
{
int num, i = 1, sum = 0;
printf("Enter a positive number: ");
scanf("%d", &num);
if(num > 0)
{
while (i <= num)
{
sum += i; //sum = sum + i
i++;
}
printf("The sum is %d ", sum);
}
else
{
printf("You have entered an invalid number");
}
return 0;
}
```

**Output**

`Enter a positive number:100`

`The sum is 5050`

### Explanation

- In both the programs, we have declared three variables num, i, and sum. The sum is also initialized with 0.
- The variable num will store the number entered by the user. The variable i is used as a counter for the loop.
- The loop would execute from 1 to the number stored inside ‘num’. So, for instance, if the user enters 10, the loop will run 10 times from 1 to 10.
- We have put an if-else block to check if the user has entered a positive number. If the number is positive, the control will enter the loop block and the sum would be generated, else a message would be shown “You have entered an invalid number”.
- Every time the loop runs, the value of sum is incremented by the current value of i.

In the first iteration sum += i// sum += 1 // Sum is storing 1

In the first iteration sum += i// sum += 2 // Sum is storing 3

In the first iteration sum += i// sum += 1 // Sum is storing 6

and so on.

You can be more specific by placing an if-else if-else block.

### Program

```
#include <stdio.h>
int main()
{
int num, i = 1, sum = 0;
printf("Enter a positive number: ");
scanf("%d", &num);
if(num > 0)
{
while (i <= num)
{
sum += i; //sum = sum + i
i++;
}
printf("Sum = %d", sum);
}
else if(num == 0)
{
printf("You have entered 0");
}
else
{
printf("You have entered a negative number");
}
return 0;
}
```