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

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 6

(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 7

i * i < num的循环条件应该是i * i <= num

Answer 8

循环永远不会执行，所以它直接返回真

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;
}

Answer 13

    package Prime;

import javax.swing.*;
import java.util.Scanner;

public class Prime {
    // main method
    public static void main(String[] args) {

        // declare lower bound and upper bound variables and get their values
        Scanner scanner = new Scanner(System.in);

        System.out.print("Enter a Lower Bound number: ");
        int lowerBound = scanner.nextInt();

        System.out.print("Enter an Upper Bound number: ");
        int upperBound = scanner.nextInt();

        int counter;

        if ((lowerBound <= 0) || (upperBound <= 0)) {
            System.out.println("Bounds must be greater than 0, Invalid Bounds");
            } else if (upperBound < lowerBound) {
            System.out.println("Upper bound cannot be less than lower bound, Invalid Bounds\nProgram terminating.");
        } else {
            // loop from lower bound to upper bound
            System.out.println("The Prime numbers are: ");
            for (counter = lowerBound; counter <= upperBound;
                 counter++) {
                // if prime number, display in primeNumbersJTextArea
                if (isPrime(counter)) {
                    System.out.print(counter + ", ");
                }
            } // end for loop
        } // end else

    } // end method calculatePrimesJButtonActionPerformed


    // method to determine if a number is a Prime number?
    private static boolean isPrime(int number) {
        // number 1 is not a Prime number
        if (number == 1)
            return false;

        int limit = (int) Math.sqrt(number);

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