# Nested for loop to calculate prime number : For « Statement « C# / CSharp Tutorial

```using System;

class MainClass {
public static void Main() {
int num;
int i;
int factor;
bool isprime;

for(num = 2; num < 20; num++) {
isprime = true;
factor = 0;

// see if num is evenly divisible
for(i=2; i <= num/2; i++) {
if((num % i) == 0) {
// num is evenly divisible -- not prime
isprime = false;
factor = i;
}
}

if(isprime)
Console.WriteLine(num + " is prime.");
else
Console.WriteLine("Largest factor of " + num +
" is " + factor);
}

}
}
```
```2 is prime.
3 is prime.
Largest factor of 4 is 2
5 is prime.
Largest factor of 6 is 3
7 is prime.
Largest factor of 8 is 4
Largest factor of 9 is 3
Largest factor of 10 is 5
11 is prime.
Largest factor of 12 is 6
13 is prime.
Largest factor of 14 is 7
Largest factor of 15 is 5
Largest factor of 16 is 8
17 is prime.
Largest factor of 18 is 9
19 is prime.```
 4.3.For 4.3.1. for loop 4.3.2. Demonstrate a block of code inside if statement 4.3.3. Use block inside for statement: Compute the sum and product of the numbers from 1 to 10 4.3.4. Use byte to control for loop 4.3.5. A negatively running for loop 4.3.6. Nested for loop to calculate prime number 4.3.7. Use commas in a for statement. 4.3.8. Use commas in a for statement to find the largest and smallest factor of a number 4.3.9. Loop condition can be any bool expression. 4.3.10. Parts of the for can be empty. 4.3.11. Move 'update' out of the for loop 4.3.12. For loop with multiple expressions 4.3.13. The body of a loop can be empty 4.3.14. Declare loop control variable inside the for 4.3.15. Using break to exit a for loop