Palindrome Prime program

[Zora]

Hey guys,

I have to write a program using methods that displays all palindrome prime numbers when a user inputs a number, N, up to 9626. I really need some guidelines and advice because I honestly do not know where to start. Hope you can help, thanks.

[mwr1976]

Start with writing out your algorithm. Define your input, output,variables and data types that you will need.

[pbrockway2]

Try to break the problem down into more tractable pieces that, together, will give you what you want. For example write a method that checks whether a given number is a prime. And another to check whether a number is a palindrome.

Come up with a *plan*, as this may guide how you break the problem into pieces. For instance, it strikes me that it might be more efficient to examine just palindromes for primeness rather than checking every number to see if it is a palindrome. So a method that can be used to create all the palindromic numbers might be useful (A method that takes a number and returns a palindrome would be useful here: 41 -> 141, etc. We needn't worry about the even length palindromes 41 -> 1441: why?)

[Zora]

This might be a bit of a bump, but I don't want to create another thread. Thanks for the suggestions pbrockway2.

I've written out my code for this program and I have a couple of errors that I can't seem to fix.

import java.util.*;
public class Lab5
{
   public static void main(String[] args)
   {
       int i=0;
       int j=0;
       int userInput;
       Scanner scn = new Scanner(System.in);
       System.out.println("Enter a positive number less than 9626");
       userInput=scn.nextInt();
	if (userInput>9626) {
		System.out.println("Number cannot be greater than 9626.");
		System.exit(0);
	}
	   
	for (i=1; i<userInput; i++) {
		i=isPrime(i);
		i=isPalindrome(i);
	}
	System.out.println(i);
}
     

//checks whether an int is prime or not.
public static boolean isPrime(int n) {
    //check if n is a multiple of 2
    if (n%2==0) return false;
    //if not, then just check the odds
    for(int i=3;i*i<=n;i+=2) {
        if(n%i==0)
            return false;
    }
    return true;
}
    
public static boolean isPalindrome(int n) {
	int dig;
        int rev=0;
        while (n>0) {
            dig=n%10;
            rev=rev*10+dig;
            n=n/10;
        }
        if (n==rev) {
            return false;
	}
	return true;
	}
    }

The errors are in the two methods that I've called. I need to print out numbers less than the user's input, so I'm sure it doesn't make sense right now. Could I get some help with this?

Lab5.java:32: error: incompatible types
i=isPrime(i);
^
required: int
found: boolean
Lab5.java:33: error: incompatible types
i=isPalindrome(i);
^
required: int
found: boolean

[pbrockway2]

required: int
found: boolean

isPrime() and is Palindrome() both return a boolean value (true/false, yes/no) but you are assigning them (for some reason) to an int variable i.

What boolean expressions are good for is putting inside if statements: "if(isPalindrome(i))". Or joining together with and or or.

-----

Do check that your methods work as advertised.

[Zora]

isPrime() and is Palindrome() both return a boolean value (true/false, yes/no) but you are assigning them (for some reason) to an int variable i.

What boolean expressions are good for is putting inside if statements: "if(isPalindrome(i))". Or joining together with and or or.

-----

Do check that your methods work as advertised.

Well I fixed the errors and it prints out prime numbers without any problems, but it seems like the equation for the palindrome is wrong.

[DiamondSoul]

Two problems with the palindrome method:

Firstly, you did it backwards. You want it to return true if n and rev are equal, and false if they are not. So instead of:

    if (n==rev) {
        return false;
    }
    return true;

say:

    if (n==rev) {
        return true;
    }
    return false;

Better yet, since the expression "n==rev" is itself a boolean value, you can just return it without using an if statement, like so:

return n==rev;

The second problem is that you lose the original value of n. By the time you get out of your while loop, n will always be zero. To fix this, back up the value of n at the start of the method:

int backup=n;

Then, at the end, rather than comparing n to rev, compare backup to rev:

return backup==rev;

[Zora]

Oh awesome, it works! Thanks DiamondSoul.