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Program to find the Nth Prime Number

  • Difficulty Level : Hard
  • Last Updated : 26 Apr, 2021

Given an integer N. The task is to find the Nth prime number.

Examples:  

Input :
Output : 11

Input : 16 
Output : 53

Input : 1049 
Output : 8377 

Approach:  

  • Find the prime numbers up to MAX_SIZE using Sieve of Eratosthenes.
  • Store all primes in a vector.
  • For a given number N, return the element at (N-1)th index in a vector.

Below is the implementation of the above approach: 

C++




// C++ program to the nth prime number
 
#include <bits/stdc++.h>
using namespace std;
 
// initializing the max value
#define MAX_SIZE 1000005
 
// Function to generate N prime numbers using
// Sieve of Eratosthenes
void SieveOfEratosthenes(vector<int>& primes)
{
    // Create a boolean array "IsPrime[0..MAX_SIZE]" and
    // initialize all entries it as true. A value in
    // IsPrime[i] will finally be false if i is
    // Not a IsPrime, else true.
    bool IsPrime[MAX_SIZE];
    memset(IsPrime, true, sizeof(IsPrime));
 
    for (int p = 2; p * p < MAX_SIZE; p++) {
        // If IsPrime[p] is not changed, then it is a prime
        if (IsPrime[p] == true) {
            // Update all multiples of p greater than or
            // equal to the square of it
            // numbers which are multiple of p and are
            // less than p^2 are already been marked.
            for (int i = p * p; i < MAX_SIZE; i += p)
                IsPrime[i] = false;
        }
    }
 
    // Store all prime numbers
    for (int p = 2; p < MAX_SIZE; p++)
        if (IsPrime[p])
            primes.push_back(p);
}
 
// Driver Code
int main()
{
    // To store all prime numbers
    vector<int> primes;
 
    // Function call
    SieveOfEratosthenes(primes);
 
    cout << "5th prime number is " << primes[4] << endl;
    cout << "16th prime number is " << primes[15] << endl;
    cout << "1049th prime number is " << primes[1048];
 
    return 0;
}

Java




// Java program to the nth prime number 
import java.util.ArrayList;
class GFG
{
     
    // initializing the max value
    static int MAX_SIZE = 1000005;
     
    // To store all prime numbers
    static ArrayList<Integer> primes =
       new ArrayList<Integer>();
     
    // Function to generate N prime numbers
    // using Sieve of Eratosthenes
    static void SieveOfEratosthenes()
    {
        // Create a boolean array "IsPrime[0..MAX_SIZE]"
        // and initialize all entries it as true.
        // A value in IsPrime[i] will finally be false
        // if i is Not a IsPrime, else true.
        boolean [] IsPrime = new boolean[MAX_SIZE];
         
        for(int i = 0; i < MAX_SIZE; i++)
            IsPrime[i] = true;
         
        for (int p = 2; p * p < MAX_SIZE; p++)
        {
            // If IsPrime[p] is not changed,
            // then it is a prime
            if (IsPrime[p] == true)
            {
                // Update all multiples of p greater than or
                // equal to the square of it
                // numbers which are multiple of p and are
                // less than p^2 are already been marked.
                for (int i = p * p; i < MAX_SIZE; i += p)
                    IsPrime[i] = false;
            }
        }
     
        // Store all prime numbers
        for (int p = 2; p < MAX_SIZE; p++)
        if (IsPrime[p] == true)
                primes.add(p);
    }
     
    // Driver Code
    public static void main (String[] args)
    {
         
        // Function call
        SieveOfEratosthenes();
     
        System.out.println("5th prime number is " +
                                    primes.get(4));
        System.out.println("16th prime number is " +
                                    primes.get(15));
        System.out.println("1049th prime number is " +
                                    primes.get(1048));
    }
}
 
// This code is contributed by ihritik

Python3




# Python3 program to the nth prime number 
primes = []
 
# Function to generate N prime numbers using 
# Sieve of Eratosthenes
def SieveOfEratosthenes():
     
    n = 1000005
     
    # Create a boolean array "prime[0..n]" and
    # initialize all entries it as true. A value
    # in prime[i] will finally be false if i is
    # Not a prime, else true.
    prime = [True for i in range(n + 1)]
     
    p = 2
    while (p * p <= n):
           
        # If prime[p] is not changed,
        # then it is a prime
        if (prime[p] == True):
               
            # Update all multiples of p
            for i in range(p * p, n + 1, p):
                prime[i] = False
                 
        p += 1
       
    # Print all prime numbers
    for p in range(2, n + 1):
        if prime[p]:
            primes.append(p)
   
# Driver code
if __name__=='__main__':
     
    # Function call
    SieveOfEratosthenes()
     
    print("5th prime number is", primes[4]);
    print("16th prime number is", primes[15]);
    print("1049th prime number is", primes[1048]);
     
# This code is contributed by grand_master

C#




// C# program to the nth prime number
using System;
using System.Collections;
 
class GFG
{
     
// initializing the max value
static int MAX_SIZE = 1000005;
 
// To store all prime numbers
static ArrayList primes = new ArrayList();
 
// Function to generate N prime numbers using
// Sieve of Eratosthenes
static void SieveOfEratosthenes()
{
    // Create a boolean array "IsPrime[0..MAX_SIZE]"
    // and initialize all entries it as true.
    // A value in IsPrime[i] will finally be false
    // if i is Not a IsPrime, else true.
    bool [] IsPrime = new bool[MAX_SIZE];
     
    for(int i = 0; i < MAX_SIZE; i++)
        IsPrime[i] = true;
     
    for (int p = 2; p * p < MAX_SIZE; p++)
    {
        // If IsPrime[p] is not changed,
        // then it is a prime
        if (IsPrime[p] == true)
        {
            // Update all multiples of p greater than or
            // equal to the square of it
            // numbers which are multiple of p and are
            // less than p^2 are already been marked.
            for (int i = p * p; i < MAX_SIZE; i += p)
                IsPrime[i] = false;
        }
    }
 
    // Store all prime numbers
    for (int p = 2; p < MAX_SIZE; p++)
    if (IsPrime[p] == true)
            primes.Add(p);
}
 
// Driver Code
public static void Main ()
{
     
    // Function call
    SieveOfEratosthenes();
 
    Console.WriteLine("5th prime number is " +
                                   primes[4]);
    Console.WriteLine("16th prime number is " +
                                   primes[15]);
    Console.WriteLine("1049th prime number is " +
                                   primes[1048]);
}
}
 
// This code is contributed by ihritik

Javascript




<script>
 
// Javascript program to the nth prime number
 
// initializing the max value
var MAX_SIZE = 1000005;
 
// Function to generate N prime numbers using
// Sieve of Eratosthenes
function SieveOfEratosthenes(primes)
{
    // Create a boolean array
    // "IsPrime[0..MAX_SIZE]" and
    // initialize all entries it as true.
    // A value in
    // IsPrime[i] will finally be false if i is
    // Not a IsPrime, else true.
    var IsPrime = Array(MAX_SIZE).fill(true);
     
    var p,i;
    for (p = 2; p * p < MAX_SIZE;p++)
    {
        // If IsPrime[p] is not changed,
        // then it is a prime
        if (IsPrime[p] == true)
        {
            // Update all multiples of p
            // greater than or
            // equal to the square of it
            // numbers which are multiple
            // of p and are
            // less than p^2 are already
            // been marked.
            for(i = p * p; i < MAX_SIZE; i += p)
                IsPrime[i] = false;
        }
    }
 
    // Store all prime numbers
    for (p = 2; p < MAX_SIZE; p++)
        if (IsPrime[p])
            primes.push(p);
}
 
// Driver Code
    // To store all prime numbers
    var primes = [];
 
    // Function call
    SieveOfEratosthenes(primes);
 
    document.write(
    "5th prime number is "+primes[4]+"<br>"
    );
    document.write(
    "16th prime number is "+primes[15]+"<br>"
    );
    document.write(
    "1049th prime number is "+primes[1048]+"<br>"
    );
 
</script>
Output: 
5th prime number is 11
16th prime number is 53
1049th prime number is 8377

 


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