# Thread: Palindromic Prime - Combining isPrime and isPalindrome

1. Member Join Date
Mar 2012
Posts
2
Rep Power
0

## Palindromic Prime - Combining isPrime and isPalindrome

Hi guys I am somewhat new to Java programming and am looking for some help. I have 2 programs, isPrime and isPalindrome, and I would like to combine them so that the new program displays the first 100 prime numbers that are palindromes. I am having a hard time doing this and was wondering if anyone could help me. Thanks

isPrime Code:
Java Code:
```public class ShowPrimes
{
static final int TOTAL_PRIMES_TO_FIND = 100;	// number of primes to be found in this application
static final int PRIMES_SHOWN_PER_LINE = 10;	// number of primes shown per display line

public static void main(String[] args)
{
int lastPrimeFound;				// last prime number found via test
int count;						// count of number of primes found

System.out.println("Here are the first 100 prime numbers:  ");
System.out.printf("%5d%5d", 2,3);	// clearly  2 and 3 are  prime numbers!
lastPrimeFound = 3;						// our first number to test

for (count = 2; count < TOTAL_PRIMES_TO_FIND; )
{
++count;											// count of the prime we are presently finding
lastPrimeFound = getNextPrime(lastPrimeFound);		// candidate for next prime from previous prime
System.out.printf("%5d", lastPrimeFound);			// prime found, so display it
if (count % PRIMES_SHOWN_PER_LINE == 0)
System.out.printf("\n");
}
}

// returns the next prime number using value of primeJustFound to generate a next candidate

private static int getNextPrime(int primeJustFound)
{
int candidate;		// candidate value tested for a prime

candidate = primeJustFound + 2;
while (!isPrime(candidate))
{
candidate += 2;
}

return candidate;
}
// returns true if testValue is prime and false if testValue is divisible by some number other than 1 or itself

private static boolean isPrime(int possiblePrime)
{
boolean factorFound = false;				// set to true if loop finds a factor
double limit = Math.sqrt(possiblePrime);	// upper limit on integers to test
int possibleFactor;							// int value that could be a factor of possiblePrime

possibleFactor = 3;								// even numbers are not part of the logic
while (possibleFactor <= limit && !factorFound)	// exits with factor found or no more factors to test
{
if (possiblePrime % possibleFactor == 0)	// factor found; no longer prime
factorFound = true;
else
possibleFactor+= 2;
}

return !factorFound;
}
}```

isPalindrome Code:
Java Code:
```import java.util.*;

// Main is a driver for the method isPalindrome

public class TestIsPalindrome
{
static Scanner keyboard = new Scanner(System.in);

public static void main(String[] args)
{
int testValue;

System.out.print("Enter a test value:  ");
testValue = keyboard.nextInt( );
keyboard.nextLine( );

if (isPalindrome(testValue))
System.out.println("The number " + testValue + " is a palindrome.");
else
System.out.println("The number " + testValue + " is not a palindrome.");
}

// returns true if testValue is a palindrome; otherwise returns false

private static boolean isPalindrome(int testValue)
{
int reversedNumber;				// least significant digit of testValue is most significant and vice-versa
int generatorForReversedNumber;	// gotten by "extracting" and weighting each digit from testValue
int nextDigit;					// to be extracted from testValue

reversedNumber = 0;				// sum accumulator.. initially no digits accumulated
generatorForReversedNumber = testValue;
while (generatorForReversedNumber > 0)
{
nextDigit = generatorForReversedNumber % 10;
reversedNumber = 10 * reversedNumber + nextDigit;
generatorForReversedNumber /= 10;
}

return testValue == reversedNumber;
}
}```
how would I go about combining them? I tried changing up the main but still doesn't work.  Reply With Quote

2. ## Re: Palindromic Prime - Combining isPrime and isPalindrome

Could you call one method and if it returns true, then call the other?  Reply With Quote

3. Member Join Date
Mar 2012
Posts
2
Rep Power
0

## Re: Palindromic Prime - Combining isPrime and isPalindrome

not sure how to do that. could you demonstrate?  Reply With Quote

4. ## Re: Palindromic Prime - Combining isPrime and isPalindrome

Method to check if a number is a palindrome
Java Code:
```public boolean isPalindrome(int number) {
//code to find out if it is a palindrome
... //if it is a palindrome, return true;
//otherwise:
return false;
}```
The part of your code where you are checking for both palindrome and prime
Java Code:
```List<Integer> listOfPrimes = getANumberOfPrimes(100);
for (int prime:listOfPrimes) {
if (isPalindrome(prime)) {
//it is both a palindrome and a prime
...
}
}```
Method to get a number of primes
Java Code:
```public List<Integer> getANumberOfPrimes(int number) {
...
}```
Last edited by ozzyman; 03-14-2012 at 01:52 AM. Reason: correction of my excessive misspelling of palindrome  Reply With Quote

5. ## Re: Palindromic Prime - Combining isPrime and isPalindrome

while count < 100
get next prime
is it a palindrome?
save it and incr the count
end loop  Reply With Quote

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•