A prime number is a number which is divisible by only two numbers: 1 and itself. So, if any number is divisible by any other number, it is not a prime number.
Example 1: Using a for loop
public class Prime { public static void main(String[] args) { int num = 31; if (!flag) |
Output
31 is a prime number.
In the above program, for loop is used to determine if the given number num is prime or not. We only have to loop through 2 to half of num , because no number is divisible by more than its half.
Inside the for loop, we check if the number is divisible by any number in the given range (2..num/2)
. If it is, flag is set to true
and we break out of the loop. This determines num is not a prime number.
If num isn't divisible by any number, flag is false and num is a prime number.
Example 2: Using a while loop
public class Prime { public static void main(String[] args) { int num = 37, i = 2; ++i; if (!flag) |
Output
37 is not a prime number.
In the above program, while loop is used instead of a for loop. The loop runs until i <= num/2
. On each iteration, whether num is divisble by i is checked and the value of i is incremented by 1.