Java Program to Count Primes in Ranges

Given a range [L, R], we need to find the count of total numbers of prime numbers in the range [L, R] where 0 <= L <= R < 10000. Consider that there are a large number of queries for different ranges.
Examples:

Input : Query 1 : L = 1, R = 10
        Query 2 : L = 5, R = 10
Output : 4
         2
Explanation
Primes in the range L = 1 to R = 10 are 
{2, 3, 5, 7}. Therefore for query, answer 
is 4 {2, 3, 5, 7}.
For the second query, answer is 2 {5, 7}.

A simple solution is to do the following for every query [L, R]. Traverse from L to R, check if current number is prime. If yes, increment the count. Finally, return the count.
An efficient solution is to use Sieve of Eratosthenes to find all primes up to the given limit. Then we compute a prefix array to store counts till every value before limit. Once we have a prefix array, we can answer queries in O(1) time. We just need to return prefix[R] – prefix[L-1]. 

Java

// Java program to answer queries for
// count of primes in given range.
import java.util.*;

class GFG {

static final int MAX = 10000;

// prefix[i] is going to store count
// of primes till i (including i).
static int prefix[] = new int[MAX + 1];

static void buildPrefix() {

    // Create a boolean array "prime[0..n]". A
    // value in prime[i] will finally be false
    // if i is Not a prime, else true.
    boolean prime[] = new boolean[MAX + 1];
    Arrays.fill(prime, true);

    for (int p = 2; p * p <= MAX; p++) {

    // If prime[p] is not changed, then
    // it is a prime
    if (prime[p] == true) {

        // Update all multiples of p
        for (int i = p * 2; i <= MAX; i += p)
        prime[i] = false;
    }
    }

    // Build prefix array
    prefix[0] = prefix[1] = 0;
    for (int p = 2; p <= MAX; p++) {
    prefix[p] = prefix[p - 1];
    if (prime[p])
        prefix[p]++;
    }
}

// Returns count of primes in range
// from L to R (both inclusive).
static int query(int L, int R)
{
    return prefix[R] - prefix[L - 1];
}

// Driver code
public static void main(String[] args) {

    buildPrefix();
    int L = 5, R = 10;
    System.out.println(query(L, R));

    L = 1; R = 10;
    System.out.println(query(L, R));
}
}

// This code is contributed by Anant Agarwal.

Output:

2
4

Please refer complete article on Count Primes in Ranges for more details!