LOOPS - Introduction to Java Programming: Comprehensive Version - page 189

while (count < NUMBER_OF_PRIMES) {
Test whether number is prime;

if number is prime {
Display the prime number and increase the count;
}

Increment number by 1;
}

To test whether a number is prime, check whether it is divisible by 2 , 3 , 4 , and so on up to
number/2 . If a divisor is found, the number is not a prime. The algorithm can be described
as follows:

Use a boolean variable isPrime to denote whether
the number is prime; Set isPrime to true initially;

for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) {
Set isPrime to false
Exit the loop;
}
}

The complete program is given in Listing 5.15.

L ISTING 5.15

PrimeNumber.java
1 public class PrimeNumber {
2 public static void main(String[] args) {

3 final int NUMBER_OF_PRIMES = 50 ; // Number of primes to display

4 final int NUMBER_OF_PRIMES_PER_LINE = 10 ; // Display 10 per line

5 int count = 0 ; // Count the number of prime numbers

6 int number = 2 ; // A number to be tested for primeness

7
8 System.out.println( "The first 50 prime numbers are \n" );
9
10 // Repeatedly find prime numbers

11 while (count < NUMBER_OF_PRIMES) {

count prime numbers

12 // Assume the number is prime

13 boolean isPrime = true ; // Is the current number prime?

14
15 // Test whether number is prime
16 for ( int divisor = 2 ; divisor <= number / 2 ; divisor++) {
17 if (number % divisor == 0 ) { // If true, number is not prime
18 isPrime = false ; // Set isPrime to false
19

check primeness

break; // Exit the for loop

exit loop

20 }
21 }

22
23 // Display the prime number and increase the count
24 if (isPrime) {
25 count++; // Increase the count
26
27 if (count % NUMBER_OF_PRIMES_PER_LINE == 0 ) {
28 // Display the number and advance to the new line
29 System.out.println(number);
30 }
31 else
32 System.out.print(number + " " );

display if prime