Writing a program that determines a prime number

Question

I'm trying to write a program that asks the user to enter a number, then I need to create a method called isPrime to create the calculation and print out the result in main. I'm sure it's something small that I'm missing, but I can't get it to produce an accurate result.

public static void main(String[] args) {
    System.out.print("Enter number: ");
    int num = s.nextInt();

    if (isPrime(num) == true) {
        System.out.println("Number is prime");
    } else if (isPrime(num) == false) {
        System.out.println("Number is not prime");
    }    
}  

public static boolean isPrime(int num){
    for(int i = 2; i <= num/2; i++) {
        if (num%i != 0) {
            return true;
        }
    }
    return false;
}

Answer 1

Use an if and else (don't retest your boolean condition in the first case). And don't test for == true in an if . This

if(isPrime(num) == true)
{
    System.out.println("Number is prime");
}
else if(isPrime(num) == false)
{
    System.out.println("Number is not prime");
}

Should just be

if (isPrime(num)) {
    System.out.println("Number is prime");
} else {
    System.out.pritnln("Number is not prime");
}

or even something like

System.out.print("Number is ");
if (!isPrime(num)) {
    System.out.print("not ");
}
System.out.println("prime");

If you want to put the braces on their own lines go ahead. As for your isPrime method; you have your return conditions reversed (and the test as well). Also we can optimize it a bit. Unroll the first even test, because then we can skip every other element. Also we only need to test to the square root of the input number. Like,

public static boolean isPrime(int num) {
    if (num == 2) {
        return true; // two is prime.
    }
    if (num < 1 || num % 2 == 0) {
        return false; // all other even numbers are not prime.
    }
    for(int i = 3; i <= Math.sqrt(num); i += 2) {
        if (num % i == 0) {
            return false;
        }
     }
     return true;
}

Answer 2

You should change your "if (num%i!=0)" condition to if(num % i == 0)

See the following code:

public class Prime {

public static void main(String[] args) {

    int num = 29;
    boolean flag = false;
    for(int i = 2; i <= num/2; ++i)
    {
        // condition for nonprime number
        if(num % i == 0)
        {
            flag = true;
            break;
        }
    }

    if (!flag)
        System.out.println(num + " is a prime number.");
    else
        System.out.println(num + " is not a prime number.");
}
}

Answer 3

The isPrime(int num) method may need to check if num % i == 0 instead of if num % i != 0 because many non-prime numbers will pass the condition and return true.

As an example, if isPrime(9) is called, the conditional will check if 9 % 2 != 0, which is true, and the method will say that 9 is prime when it's not.

As a result, you could try changing the method to something like this:

public static boolean isPrime(int num)
{
    for(int i = 2; i <= num/2; i++)
    {
        if (num%i==0)
        {
         return false;
        }

    }
    return true;
}