Prime points (Points that split a number into two primes) - GeeksforGeeks


Prime points (Points that split a number into two primes) - GeeksforGeeks
Given a n-digit number. Prime point is the index of the digit whose left and right side numbers
are prime. Print all the prime points of the number. If no prime point exists print -1.

// Function to count number of digits
int countDigits(int n)
{
    int count = 0;
    while (n > 0)
    {
        count++;
        n = n/10;
    }
    return count;
}
 
// Function to check whether a number is
// prime or not. Returns 0 if prime else -1
int checkPrime(int n)
{
    // Corner cases
    if (n <= 1)
        return -1;
    if (n <= 3)
        return 0;
 
    // This is checked so that we can skip
    // middle five numbers in below loop
    if (n%2 == 0 || n%3 == 0)
        return -1;
 
    for (int i=5; i*i<=n; i=i+6)
        if (n%i == 0 || n%(i+2) == 0)
            return -1;
 
    return 0;
}
 
// Function to print prime points
void printPrimePoints(int n)
{
    // counting digits
    int count = countDigits(n);
 
    // As single and double digit numbers do not
    // have left and right number pairs
    if (count==1 || count==2)
    {
        cout << "-1";
        return;
    }
 
    // Finding all left and right pairs. Printing
    // the prime points accordingly. Discarding
    // first and last index point
    bool found = false;
    for (int i=1; i<(count-1); i++)
    {
        // Calculating left number
        int left = n / ((int)pow(10,count-i));
 
        // Calculating right number
        int right = n % ((int)pow(10,count-i-1));
 
        // Prime point condition
        if (checkPrime(left) == 0 &&
            checkPrime(right) == 0)
        {
            cout << i << " ";
            found = true;
        }
    }
 
    // No prime point found
    if (found == false)
        cout << "-1";
}
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