# 第六章第二十六题（回文素数）(Palindromic prime) – 编程练习题答案

**6.26（回文素数）回文素数是指一个数同时为素数和回文数。例如：131是一个素数，同时也是一个回文素数。数学313和757也是如此。编写程序，显示前100个回文素数。每行显示10个数，数字中间用一个空格隔开。如下所示：

2 3 5 7 11 101 131 151 181 191

313 353 373 383 727 757 787 797 919 929

**6.26(Palindromic prime) A palindromic prime is a prime number and also palindromic. For example, 131 is a prime and also a palindromic prime, as are 313 and 757. Write a program that displays the first 120 palindromic prime numbers. Display 10 numbers per line, separated by exactly one space, as follows:

2 3 5 7 11 101 131 151 181 191

313 353 373 383 727 757 787 797 919 929

``````public class Ans6_26_page202 {
public static void main(String[] args) {
isPrime(100,10);
}
public static void isPrime(int n, int line) {
int count = 0; // Count the number of prime numbers
int number = 2; // A number to be tested for primeness

System.out.println("The first "+n+" numbers are \n");

// Repeatedly find prime numbers
while (count < n) {
// Assume the number is prime
boolean isPrime = true; // Is the current number prime?

// Test if number is prime
for (int divisor = 2; divisor <= number / 2; divisor++) {
if (number % divisor == 0) { // If true, number is not prime
isPrime = false; // Set isPrime to false
break; // Exit the for loop
}
}

// Test if number is palindrome
boolean isPalindrome = true;
String strNumber = number+"";
int low = 0;
int high = strNumber.length() - 1;
while (low < high) {
if (strNumber.charAt(low) != strNumber.charAt(high)) {
isPalindrome = false;
break;
}
low++;
high--;
}

// Print the prime number and increase the count
if (isPrime && isPalindrome) {
count++; // Increase the count

if (count % line == 0) {
// Print the number and advance to the new line
System.out.println(number);
}
else
System.out.print(number + " ");
}

// Check if the next number is prime
number++;
}
}
}``````