C# 0 The Complete Reference

count

, is greater than 5 when the 

loop is first entered. This makes the conditional expression, 

count < 5

, false from the outset; 

thus, not even one iteration of the loop will occur.

The

for

 loop is most useful when you will be iterating a known number of times. For 

example, the following program uses two 

for

 loops to find the prime numbers between 2 

and 20. If the number is not prime, then its largest factor is displayed.

// Determine if a number is prime. If it is not, then

// display its largest factor.

using System;

class FindPrimes {

  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) {

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          // num is evenly divisible. Thus, it is not prime.

          isprime = false;

          factor = i;

        }

      }

      if(isprime)

        Console.WriteLine(num + " is prime.");

      else

        Console.WriteLine("Largest factor of " + num +

                          " is " + factor);

    }

  }

}

The output from the program is shown here:

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.