In this example, we calculate the sum of natural numbers entered by the user.

The positive numbers like 1,2,3,4,5... are known as natural numbers. In this example we take one natural number as input from user and print sum like :

` ````
sum = 1 + 2 + 3 + ... + 10
```

Example :

` ````
#include <stdio.h>
int main() {
int n, i, sum = 0;
printf("Enter a positive integer: ");
scanf("%d", &n);
for (i = 1; i <= n; ++i) {
sum += i;
}
printf("Sum = %d", sum);
return 0;
}
```

Output :

` ````
Enter a positive integer: 10
Sum = 55
```

The above program we take input from the user and stores it in the variable n. Then, for loop is used to calculate the sum up to n. As per above output loop runs for 10 times.

` ````
#include <stdio.h>
int main() {
int n, i, sum = 0;
printf("Enter a positive integer: ");
scanf("%d", &n);
i = 1;
while (i <= n) {
sum += i;
++i;
}
printf("Sum = %d", sum);
return 0;
}
```

Output :

` ````
Enter a positive integer: 50
Sum = 1275
```

In both examples, the loop is iterated n number of times. And, in each iteration, the value of i is added to sum and i is incremented by 1. Though both programs are technically correct, it is better to use for loop in this case. It's because the number of iterations is known.

` ````
#include <stdio.h>
int main() {
int n, i, sum = 0;
printf("Enter a positive integer: ");
scanf("%d", &n);
i = 1;
do {
sum += i;
++i;
} while (i <= n);
printf("Sum = %d", sum);
return 0;
}
```

Output :

` ````
Enter a character: 5
Sum : 15
```

` ````
#include<stdio.h>
int sum(int num)
{
if(num)
return(num + sum(num-1));
else
return 0;
}
int main()
{
int count;
printf("Enter a positive no :: ");
scanf("%d", &count);
printf("Sum of 1st %d natural numbers is %d\n", count, sum(count));
return 0;
}
```

Output :

` ````
Enter a positive no :: 50
Sum of 1st 50 natural numbers is 1275
```

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