使用自定义IsPrime方法查找素数

Question

I started learning Java about one month ago and today I saw this question I couldn't solve.

The question was:

Write a method named isPrime , which takes an integer as an argument and returns true if the argument is a prime number, or false otherwise. Demonstrate the method in a complete program.

And the second part says:

Use the isPrime method that you wrote in previous program in a program that stores a list of all the prime numbers from 1 through 100 in a file.

Here's my code, which doesn't work:

import java.io.*;

public class PrimeNumbers {

    public static void main (String args[]) throws IOException {

        PrintWriter outputFile = new PrintWriter("PrimeNumber.txt");

        int j = 0;

        for (int i = 0; i < 100; i++) {
            isPrime(i); 
            outputFile.println("Prime nums are:" + i);
        }
    }
    public static boolean isPrime (int j) {

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

Answer 1

Your condition for returning true in isPrime - if (i == j) - can never be met, since it's inside a loop whose condition is j < i . Instead, just return true after the loop. If the loop ends without returning false , you know for sure that the input number is prime.

Your code that uses isPrime is not checking the value returned by this method. You must check it in order to decide whether to write the number to the output file.

Answer 2

import java.io.IOException;
import java.io.PrintWriter;

public class PrimeNumbers
{
    public static void main(String args[]) throws IOException
    {
        PrintWriter primeNumbersWriter = new PrintWriter("PrimeNumber.txt");

        for (int i = 0; i < 100; i++)
        {
            // You didn't do anything with the return value of isPrime()
            if (isPrime(i))
            {
                primeNumbersWriter.println("Prime numbers are: " + i);
            }
        }

        // Please close writers after using them
        primeNumbersWriter.close();
    }

    public static boolean isPrime(int prime)
    {
        // Do not use the number to check for prime as loop variable, also it's
        // sufficient to iterate till the square root of the number to check
        for (int number = 2; number < Math.sqrt(prime); number++)
        {
            if (prime % number == 0)
            {
                return false;
            }
        }

        // You didn't always return a value, it won't let you compile otherwise
        return true;
    }
}

Answer 3

Prime Number
A prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

---

What should be the logic?

• Pass a number to method.
• Use loop to start a check of modulo % to find at least one number which can divide the passed number.
• Check until we reached the value passedNumber .
• If the modulo gives 0 for atleast one, it's not prime thank god!
• If modulo is not 0 for any number ...oh man it's Prime.

What are the problems in your code?

• You are looping correctly but using the variable j which is the limit and you are incrementing it!
• If you want to loop through i < j how can the condition i == j be true?
• If method is returning boolean why are you using method as void ! Use that returned value.
• You can just return false at the end if divisor not found!

We did it...Just try now!

Answer 4

about isPrime: First of all, u should loop for( j = 2; j*j <= i; j++)

the reason for this loop is that if a number isn't prime, its factor must be less or equal to the squared root of i, so there is no need to loop after that point now, if loop didn't return false - return true`

about second function: use if before checking isPrime - if(isPrime(i)) {add i to list}

Answer 5

see here is your solution.

 import java.util.Scanner;
    public class Testing {

        public static void main(String args[]) {
            Scanner scnr = new Scanner(System.in);
            int number = Integer.MAX_VALUE;
            System.out.println("Enter number to check if prime or not ");
            while (number != 0) {
                number = scnr.nextInt();
                System.out.printf("Does %d is prime? %s %s  %s %n", number,
                        isPrime(number), isPrimeOrNot(number), isPrimeNumber(number));
            }
        }


        /*
         * Java method to check if an integer number is prime or not.
         * @return true if number is prime, else false
         */
        public static boolean isPrime(int number) {
            int sqrt = (int) Math.sqrt(number) + 1;
            for (int i = 2; i < sqrt; i++) {
                if (number % i == 0) {
                    // number is perfectly divisible - no prime
                    return false;
                }
            }
            return true;
        }


        /*
         * Second version of isPrimeNumber method, with improvement like not
         * checking for division by even number, if its not divisible by 2.
         */
        public static boolean isPrimeNumber(int number) {
            if (number == 2 || number == 3) {
                return true;
            }
            if (number % 2 == 0) {
                return false;
            }
            int sqrt = (int) Math.sqrt(number) + 1;
            for (int i = 3; i < sqrt; i += 2) {
                if (number % i == 0) {
                    return false;
                }
            }
            return true;
        }


        /*
         * Third way to check if a number is prime or not.
         */
        public static String isPrimeOrNot(int num) {
            if (num < 0) {
                return "not valid";
            }
            if (num == 0 || num == 1) {
                return "not prime";
            }
            if (num == 2 || num == 3) {
                return "prime number";
            }
            if ((num * num - 1) % 24 == 0) {
                return "prime";
            } else {
                return "not prime";
            }
        }
    }