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Count pairs with sum as a prime number and less than n
  • Last Updated : 23 Jan, 2019

Given a positive integer n, count distinct number of pairs (x, y) that satisfy following conditions :

  • (x + y) is a prime number.
  • (x + y) < n
  • x != y
  • 1 <= x, y

Examples:

Input : n = 6
Output : 3
prime pairs whose sum is less than 6 are:
(1,2), (1,4), (2,3) 

Input : 12
Output : 11
prime pairs whose sum is less than 12 are:
(1,2),  (1,4), (2,3), (1,6), (2,5), (3,4), 
(1,10), (2,9), (3,8), (4,7), (5,6)

Approach:

1) Find all prime numbers less than n using
   Sieve of Sundaram

2) For each prime number p, count distinct
   pairs that sum up to p.
   
For any odd number n, number of distinct pairs
that add upto n are n/2
Since, a prime number is a odd number, the 
same applies for it too. 

Example,
For prime number p = 7
distinct pairs that add upto p: p/2 = 7/2 = 3
The three pairs are (1,6), (2,5), (3,4)



For prime number p = 23
distinct pairs that add upto p: p/2 = 23/2 = 11

C++




// C++ implementation of prime pairs
// whose sum is less than n
#include <bits/stdc++.h>
using namespace std;
  
// Sieve of Sundaram for generating
// prime numbers less than n
void SieveOfSundaram(bool marked[], int nNew)
{
    // Main logic of Sundaram.  Mark all numbers
    // of the form i + j + 2ij as true where
    // 1 <= i <= j
    for (int i=1; i<=nNew; i++)
        for (int j=i; (i + j + 2*i*j) <= nNew; j++)
            marked[i + j + 2*i*j] = true;
}
  
// Returns number of pairs with fiven conditions.
int countPrimePairs(int n)
{
    // In general Sieve of Sundaram, produces
    // primes smaller than (2*x + 2) for a number
    // given number x. Since we want primes smaller
    // than n, we reduce n to half
    int nNew = (n-2)/2;
  
    // This array is used to separate numbers of
    // the form i+j+2ij from others where
    // 1 <= i <= j
    bool marked[nNew + 1];
  
    // Initialize all elements as not marked
    memset(marked, false, sizeof(marked));
  
    SieveOfSundaram(marked, nNew);
  
    int count = 0, prime_num;
  
    // Find primes. Primes are of the form
    // 2*i + 1 such that marked[i] is false.
    for (int i=1; i<=nNew; i++)
    {
        if (marked[i] == false)
        {
            prime_num = 2*i + 1;
  
            // For a given prime number p
            // number of distinct pairs(i,j)
            // where (i+j) = p are p/2
            count = count + (prime_num / 2);
        }
    }
  
    return count;
}
  
// Driver program to test above
int main(void)
{
    int n = 12;
    cout << "Number of prime pairs: "
         << countPrimePairs(n);
    return 0;
}


Java




// Java implementation of prime pairs 
// whose sum is less than n 
  
class GFG
{
      
// Sieve of Sundaram for generating 
// prime numbers less than n 
static void SieveOfSundaram(boolean marked[], int nNew) 
      
    // Main logic of Sundaram. Mark all numbers 
    // of the form i + j + 2ij as true where 
    // 1 <= i <= j 
    for (int i = 1; i <= nNew; i++) 
        for (int j = i; (i + j + 2 * i * j) <= nNew; j++) 
            marked[i + j + 2 * i * j] = true
  
// Returns number of pairs with fiven conditions. 
static int countPrimePairs(int n) 
    // In general Sieve of Sundaram, produces 
    // primes smaller than (2*x + 2) for a number 
    // given number x. Since we want primes smaller 
    // than n, we reduce n to half 
    int nNew = (n - 2) / 2
  
    // This array is used to separate numbers of 
    // the form i+j+2ij from others where 
    // 1 <= i <= j 
    // Initialize all elements as not marked
    boolean marked[]=new boolean[nNew + 1]; 
  
    SieveOfSundaram(marked, nNew);
    int count = 0, prime_num; 
  
    // Find primes. Primes are of the form 
    // 2*i + 1 such that marked[i] is false. 
    for (int i = 1; i <= nNew; i++) 
    
        if (marked[i] == false
        
            prime_num = 2 * i + 1
  
            // For a given prime number p 
            // number of distinct pairs(i, j) 
            // where (i + j) = p are p/2 
            count = count + (prime_num / 2); 
        
    
    return count; 
  
// Driver code
public static void main (String[] args) 
{
    int n = 12
    System.out.println("Number of prime pairs: " +
    countPrimePairs(n)); 
}
  
// This code is contributed by mits


Python3




# Python3 implementation of prime pairs
# whose sum is less than n
  
# Sieve of Sundaram for generating
# prime numbers less than n
def SieveOfSundaram(marked, nNew):
      
    # Main logic of Sundaram. Mark all numbers
    # of the form i + j + 2ij as true where
    # 1 <= i <= j
    for i in range(1, nNew + 1):
        for j in range(i, nNew):
            if i + j + 2 * i * j > nNew:
                break
            marked[i + j + 2 * i * j] = True
  
# Returns number of pairs with fiven conditions.
def countPrimePairs(n):
      
    # In general Sieve of Sundaram, produces
    # primes smaller than (2*x + 2) for a number
    # given number x. Since we want primes smaller
    # than n, we reduce n to half
    nNew = (n - 2) // 2
  
    # This array is used to separate numbers 
    # of the form i+j+2ij from others where
    # 1 <= i <= j
    marked = [ False for i in range(nNew + 1)]
  
    SieveOfSundaram(marked, nNew)
  
    count, prime_num = 0, 0
  
    # Find primes. Primes are of the form
    # 2*i + 1 such that marked[i] is false.
    for i in range(1, nNew + 1):
        if (marked[i] == False):
  
            prime_num = 2 * i + 1
  
            # For a given prime number p
            # number of distinct pairs(i,j)
            # where (i+j) = p are p/2
            count = count + (prime_num // 2)
  
    return count
  
# Driver Code
n = 12
print("Number of prime pairs: "
             countPrimePairs(n))
  
# This code is contributed by Mohit kumar 29


C#




// C# implementation of prime pairs 
// whose sum is less than n 
using System;
  
class GFG
{
      
// Sieve of Sundaram for generating 
// prime numbers less than n 
static void SieveOfSundaram(bool[] marked, 
                            int nNew) 
      
    // Main logic of Sundaram. Mark all numbers 
    // of the form i + j + 2ij as true where 
    // 1 <= i <= j 
    for (int i = 1; i <= nNew; i++) 
        for (int j = i; 
            (i + j + 2 * i * j) <= nNew; j++) 
            marked[i + j + 2 * i * j] = true
  
// Returns number of pairs with fiven conditions. 
static int countPrimePairs(int n) 
    // In general Sieve of Sundaram, produces 
    // primes smaller than (2*x + 2) for a  
    // number given number x. Since we want 
    // primes smaller than n, we reduce n to half 
    int nNew = (n - 2) / 2; 
  
    // This array is used to separate numbers 
    // of the form i+j+2ij from others where 
    // 1 <= i <= j 
    // Initialize all elements as not marked
    bool[] marked = new bool[nNew + 1]; 
  
    SieveOfSundaram(marked, nNew);
    int count = 0, prime_num; 
  
    // Find primes. Primes are of the form 
    // 2*i + 1 such that marked[i] is false. 
    for (int i = 1; i <= nNew; i++) 
    
        if (marked[i] == false
        
            prime_num = 2 * i + 1; 
  
            // For a given prime number p 
            // number of distinct pairs(i, j) 
            // where (i + j) = p are p/2 
            count = count + (prime_num / 2); 
        
    
    return count; 
  
// Driver code
public static void Main () 
{
    int n = 12; 
    Console.WriteLine("Number of prime pairs: "
                             countPrimePairs(n)); 
}
  
// This Code is Contribute by Mukul Singh.


PHP




<?php
// PHP implementation of prime pairs
// whose sum is less than n
  
// Sieve of Sundaram for generating
// prime numbers less than n
function SieveOfSundaram(&$marked, $nNew)
{
    // Main logic of Sundaram. Mark all 
    // numbers of the form i + j + 2ij
    // as true where 1 <= i <= j
    for ($i = 1; $i <= $nNew; $i++)
        for ($j = $i
            ($i + $j + 2 * $i * $j) <= $nNew; $j++)
            $marked[$i + $j + 2 * $i * $j] = true;
}
  
// Returns number of pairs with
// given conditions.
function countPrimePairs($n)
{
    // In general Sieve of Sundaram, produces
    // primes smaller than (2*x + 2) for a 
    // number given number x. Since we want 
    // primes smaller than n, we reduce n to half
    $nNew = ($n - 2) / 2;
  
    // This array is used to separate numbers 
    // of the form i+j+2ij from others where
    // 1 <= i <= j
    $marked = array_fill(0, $nNew + 1, false);
  
    SieveOfSundaram($marked, $nNew);
  
    $count = 0;
  
    // Find primes. Primes are of the form
    // 2*i + 1 such that marked[i] is false.
    for ($i = 1; $i <= $nNew; $i++)
    {
        if ($marked[$i] == false)
        {
            $prime_num = 2 * $i + 1;
  
            // For a given prime number p
            // number of distinct pairs(i,j)
            // where (i+j) = p are p/2
            $count = $count + (int)($prime_num / 2);
        }
    }
  
    return $count;
}
  
// Driver Code
$n = 12;
echo "Number of prime pairs: "
            countPrimePairs($n);
  
// This code is contributed by 
// chandan_jnu
?>



Output:

Number of prime pairs: 11

This article is contributed by Ayush Jauhari. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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