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?

Solution 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.

Solution 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;
}

Solution 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);
}

Solution 4

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;
  }  

Solution 5

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).

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Comments

[Tareq Salah]

@ADG thanks for the hint .. but when the question was asked .. he want to fix specific bug in the code .. I didn't notice case when num=2

[Stella Lie]

1 is not a prime number, shall add || num == 1 to the second line on the function

[friederbluemle]

@StellaLie Even more generically, numbers smaller than 2 are not prime.

[ssukienn]

I would downvote for putting code with exactly same mistake OP had but I can't. There should be i * i <= num

[Popeye]

1 is not a prime number, adjust your sample.

[Alex Riabov]

While this code may answer the question, providing additional context regarding why and/or how this code answers the question improves its long-term value.