What is wrong with my isPrime method?

问题

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?

回答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.

回答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;
}

回答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);
}

回答4:

Here are some hints:

The main bug is that you never check divisibility by sqrt(num) due to an off-by-one error in the loop.
The other bug is that you don't consider 2 to be prime (which it is).

回答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;
}

回答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;
  }

回答7:

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

回答8:

the loop is never executed so it returns directly true

回答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

回答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.

回答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;
}

回答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;
}

来源：https://stackoverflow.com/questions/20798391/what-is-wrong-with-my-isprime-method

标签

java

function

methods

primes