Prime with Prime

The 4 one - digit prime no's are 2 , 3 , 5 and 7 . Arranging them to form 2 digit no's , the required 2 digit prime no's are 23 , 37 , 53 , 73 . Therefore , required answer is 4

// Another Java Solution Code for the problem

public static boolean[] getPrimes(int range) {
    boolean[] bs = new boolean[range + 1];
    Arrays.fill(bs, true);
    bs[0] = bs[1] = false;
    for (int i = 2; i * i < bs.length; i++) {
        if (bs[i]) {
            for (int j = i * 2; j < bs.length; j += i) {
                bs[j] = false;
            }
        }
    }
    return bs;
}


// No throws clause here
public static void main(String[] args) throws java.io.IOException {

    BufferedReader reader = new BufferedReader(new InputStreamReader(
            System.in));
    int t = Integer.valueOf(reader.readLine());
    boolean[] bs = getPrimes(10000);
    while (t-- > 0) {
        String[] strings = reader.readLine().split(" ");
        int start = Integer.valueOf(strings[0]), end = Integer
                .valueOf(strings[1]);
        int counter = 0;
        for (int i = start; i <= end; i++) {
            if (bs[i] && bs[i % 10] && bs[i / 10]) {
                counter++;
            }
        }
        System.out.println(counter);
    }

Java solution -

Actually this program could be much more shorter but I value efficiency more than shortness :)

public static void main(String[] args){
    for(int i = 10; i < 100; i++){
//loop from 10 to 100 (2 digits numbers)
        String t = String.valueOf(i); 
//convert the current number (in the loop) into a string
        if(prime(i)) 
//check if the number is prime
            if(prime(Integer.parseInt(t.substring(0,1))) && prime(Integer.parseInt(t.substring(1,2)))) 
//if the number is prime then split the number into two parts or digits and check if they are prime
                System.out.println(i); 
//if the two digits are prime, then print the number as the number itself is prime and it's digits are prime too.
    }
}

static boolean prime(int n){
 //check for prime numbers. Returns true if the number is prime and false if the number is composite or neither of the two
    if(n==2) return true; 
//if the number is 2, then return 2
    if(n==1) return false; 
//if it's 1, then return false
//this is because the loop below considers 1 and 2 prime

    if(n%2==0) return false; 
//if the number is divisible by 2 then return false (it's obviously not prime)
    for(int i=3;i*i<=n;i+=2) 
//then just loop through odd numbers
        if(n%i==0)
            return false;
    return true;
}

Otherwise, I could have done it with a external library for checking primes (which is not very efficient and not legible too) -

for(int i = 10; i < 100; i++)
if(prime(i) && prime(Integer.parseInt(String.valueOf(i).substring(0,1))) && prime(Integer.parseInt(String.valueOf(i).substring(1,2))))  System.out.println(i);