嗨，我想打印用户输入的 2 个数字之间的所有回文素数

Question

So, I am done with first part and most of 2nd one but can not figure out how to do Palindromic primes, so please help. The code i am attaching is p1b and I have no idea what to do in p1c so please help.

I. Introduction: The purpose of this project is to help you learn the basic JAVA programming language elements based upon your C/C++ background that you gained from CSC114.

II. Assignment: Develop a JAVA program according to the following steps: Part A: Name this step p1a.java and you don't need to write a method other than main for this. 1) The user is asked to enter an integer and your program is to produce the reverse of that number. Example follows: Please enter an integer number: 2341 The reverse of 2341 is 1432

Part B: Name this step p1b.java. 1) Modify the last step using a method called reverse that returns the reverse value of a given integer. Then, modify the program so that it will runs as the following: 2) The user is asked to enter two integers, say x and y; 3) So long as x is less than y, your program will do the following: a) Find and print all prime numbers between x and y. (Need a method called isPrime that checks whether a given number is a prime or not.) b) Find and print all prime numbers that are also palindromes; c) Ask for another pair of x and y. Example follows: Please enter two integer numbers: 5 400 Prime: 5 7 11 13 17 19 23 29 31 37 41 43 47 53 59 61 67 71 73 79 83 89 97 101 103 107 109 113 127 131 137 139 149 151 157 163 167 173 179 181 191 193 197 199 211 223 227 229 233 239 241 251 257 263 269 271 277 281 283 293 307 311 313 317 331 337 347 349 353 359 367 373 379 383 389 397 Palindromic Prime: 5 7 11 101 131 151 181 191 313 353 373 383 Please enter two integer numbers: 9 1 Done

Part C: Name this step p1c.java. 1) Traditionally, in order to produce the above output, the approach that one normally has is to go through the numbers twice: the first time is to find and print all prime numbers and then go through the numbers again to find and print all palindromic primes. However, if we are able to keep the result and then print it when the process is completed, then it will only need 1 pass. 2) Additionally, please display your output in a formatted way (such as aligned output in the previous page). 3) Modify p1b.java so that it will only go through one pass to produce the same result as produced in part B.

import java.util.Scanner;

public class p1b {

    public static void main(String[] args) {    
        System.out.println("Program for Finding Primem Numbers: ");
        System.out.println("==================================");   
        Scanner input = new Scanner(System.in);

        int num1, num2, i;
        int choice;
        System.out.print("Please enter the first number: ");
        num1 = input.nextInt();
        System.out.print("Please enter the Second number: ");
        num2 = input.nextInt();

        if (num1 > num2) {
            System.out.println("        Thank you.");
            System.exit(1); 
        }
        System.out.println( isPrime(num1, num2));
        System.out.println( reverse(num1));

        System.out.print("\nWould you like to enter two more numbers, "
                + "Enter 1 for Yes or 0 for No: ");
        choice = input.nextInt();
        if(choice == 1) {
            System.out.print("Please enter the first number: ");
            num1 = input.nextInt();
            System.out.print("Please enter the Second number: ");
            num2 = input.nextInt();
            System.out.println( isPrime(num1, num2));
        }

        if(choice == 0) {
            System.out.println("Bye!!");
            System.exit(0);
        }
    }

    public static int isPrime(int num1, int num2) {
        final int displayPerLine = 50;
        System.out.print("Prime: ");
        for ( int i = num1; i <= num2; i++ ) {
            int j;
            for ( j = 2; j < i; j++) {
                int number = i % j;
                if (number == 0) {
                    break;
                    //return n;
                }   
            }
            if(i == j) {
                System.out.printf("%-5d " + i);
            }
            if (i % displayPerLine == 0) {
                System.out.println();
            }
        }
        System.out.println();
            return num1;
    }

    //public static int reverse(int num1, int num2) {
    public static int reverse(int num1, int num2) {
        System.out.print("Palindrome: ");
        int palindrome = num1; // copied number into variable
        int reverse = palindrome;

        while (palindrome != 0) {
            int remainder = palindrome % 10;
            reverse = reverse * 10 + remainder;
            palindrome = palindrome / 10;
        }
        // if original and reverse of number is equal means
        // number is palindrome in Java
        if (num1 == reverse) {
            return reverse;
        }
        return palindrome;        
    }

    public static int reverse(int num) {
        int test = 0;           
        while (num != 0) {
            int lastdigit = num % 10;           
            test = test * 10 + lastdigit;
            num = num / 10;
        }
        return test;
    }       

    public static boolean isPalindrome(int num) {
        return num == reverse(num);
    }
}

Answer 1

Let's start by fixing isPrime , first test if the number is even; if so, it isn't prime. Next, test odd numbers from 3 to the square root of the number. If none of those are factors, then the number is prime. Something like,

public static boolean isPrime(int i) {
    if (i % 2 == 0) {
        return false;
    }
    double sqrt = Math.sqrt(i);
    for (int t = 3; t <= sqrt; t += 2) {
        if (i % t == 0) {
            return false;
        }
    }
    return true;
}

Then we can implement isPalindrome with a StringBuilder like

public static boolean isPalindrome(int i) {
    String str = String.valueOf(i);
    return new StringBuilder(str).reverse().toString().equals(str);
}

Finally, we can prompt for input in an infinite (here a while ) loop and use an inner for loop for printing primes that are also palindromes . And, you can use Math.max and Math.min for getting the max and min respectively. Like,

public static void main(String[] args) {
    System.out.println("\t\tProgram for Finding Prime Numbers: ");
    System.out.println("\t\t===================================");
    Scanner input = new Scanner(System.in);
    while (true) {
        System.out.print("Please enter the first number: ");
        int num1 = input.nextInt();
        System.out.print("Please enter the second number: ");
        int num2 = input.nextInt();
        int lo = Math.min(num1, num2);
        int hi = Math.max(num1, num2);
        for (int i = lo; i <= hi; i++) {
            if (isPalindrome(i) && isPrime(i)) {
                System.out.println(i);
            }
        }
        System.out.print("Would you like to enter two more numbers, "
                + "Enter 1 for Yes or 0 for No: ");
        int choice = input.nextInt();
        if (choice == 0) {
            break;
        }
    }
}