What is wrong with my isPrime method?

Question

This is my isPrime method:

private static boolean isPrime(int num) {
    if (num % 2 == 0) return false;
    for (int i = 3; i * i < num; i += 2)
        if (num % i == 0) return false;
    return true;
}

I put isPrime(9) and it returns true. What is wrong with the method?

Answer 1

Your condition should be i * i <= num

private static boolean isPrime(int num) 
{
        if (num == 2) 
            return true;
        if (num < 2 || num % 2 == 0) 
            return false;
        for (int i = 3; i * i <= num; i += 2)
            if (num % i == 0) 
                return false;
        return true;
}

You didn't take number 9 in your consideration so 9<9 will result false. But you need to check 9.

Answer 2

my sample:

public boolean isPrime(int x) {
    if (x==1) {
        return true;
    } else {
        for(int i=2;i<=Math.sqrt(x);i++) {
            if (x%i==0) return false;          
    }
    return true;
}

Answer 3

Java 8: (Example with lambda expression and streams)

public static boolean isPrimeFunctionalStyle(int number) {
    return number > 1 && 
            IntStream.rangeClosed(2, (int) Math.sqrt(number))
                .noneMatch(i -> number % i == 0);
}

Answer 4

Here are some hints:

1. The main bug is that you never check divisibility by sqrt(num) due to an off-by-one error in the loop.

2. The other bug is that you don't consider 2 to be prime (which it is).

Answer 5

(Late) Sidenode:

private static boolean isPrime(int num) {
    if (num % 2 == 0) return false;
    for (int i = 3; i * i < num; i += 2)
        if (num % i == 0) return false;
    return true;
}

This code is missing 2; 2 is a primenumber. Everything dividable by 2 isn't, except the 2 - so, use:

private static boolean isPrime(int num) {
    if (num == 2) return true;
    if (num % 2 == 0) return false;
    for (int i = 3; i * i < num; i += 2)
        if (num % i == 0) return false;
    return true;
}

Answer 6

Change your code like this ( check condition) :

 private static boolean isPrime(int num) {
        if (num == 2) return true;
        if (num % 2 == 0)
            return false;
        for (int i = 3; i * i <= num; i += 2)
            if (num % i == 0) return false;
        return true;
  }  

Answer 7

the loop condition with i * i < num should be i * i <= num

Answer 8

the loop is never executed so it returns directly true

Answer 9

The loop does not run. It gets terminated in the very first value of i because 3 x 3 = 9 it does not meet the condition i * i < n

Answer 10

 for (int i = 3; i * i < num; i += 2)
        if (num % i == 0) return false;

i * i is 9, and 9 in not less than 9, thus for loop is not run.

Answer 11

you can simply use if and else statement to check to see if the number is prime. There is a pattern, all the numbers are either multiples of either 2 or 3 once your number reach certain limits.

public static boolean isPrime2 (int n)
{
    if (n == 1) {
        return false;
    } else if (n == 2 || n==3) {
        return true;
    } else if (n>2) {
        if(n % 2 ==0 || n % 3 == 0) {
            return false;
        }
    }
    return true;
}

Answer 12

public static boolean isPrime (int number) {
    if(number < 2) {
        return false;
    }
    int check = (int) Math.sqrt(number);


    for(int i = 2; i <= check; i++) {
        if(number % i == 0) {
            return false;
        }
    }
    return true;
}