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Print characters having prime frequencies in order of occurrence
  • Last Updated : 13 Apr, 2021

Given a string str containing only lowercase characters. The task is to print the characters having prime frequency in the order of their occurrence. 

Note that repeated elements with prime frequencies are printed as many times as they occur in order of their occurrence.

Examples: 

Input: str = “geeksforgeeks” 
Output: gksgks 
 

CharacterFrequency
‘g’2
‘e’4
‘k’2
‘s’2
‘f’1
‘o’1
‘r’1

‘g’, ‘k’ and ‘s’ are the only characters with prime frequencies.
Input: str = “aeroplane” 
Output: aeae 



Approach: Create a frequency array to store the frequency of each of the character of the given string str. Traverse the string str again and check whether the frequency of that character is prime using Sieve Of Eratosthenes.

Below is the implementation of the above approach:  

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
 
#define SIZE 26
 
// Function to create Sieve to check primes
void SieveOfEratosthenes(bool prime[], int p_size)
{
    // false here indicates
    // that it is not prime
    prime[0] = false;
    prime[1] = false;
 
    for (int p = 2; p * p <= p_size; p++) {
 
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p]) {
 
            // Update all multiples of p,
            // set them to non-prime
            for (int i = p * 2; i <= p_size; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to print the prime frequency characters
// in the order of their occurrence
void printChar(string str, int n)
{
 
    bool prime[n + 1];
    memset(prime, true, sizeof(prime));
 
    // Function to create Sieve to check primes
    SieveOfEratosthenes(prime, str.length() + 1);
 
    // To store the frequency of each of
    // the character of the string
    int freq[SIZE];
 
    // Initialize all elements of freq[] to 0
    memset(freq, 0, sizeof(freq));
 
    // Update the frequency of each character
    for (int i = 0; i < n; i++)
        freq[str[i] - 'a']++;
 
    // Traverse str character by character
    for (int i = 0; i < n; i++) {
 
        // If frequency of current character is prime
        if (prime[freq[str[i] - 'a']]) {
            cout << str[i];
        }
    }
}
 
// Driver code
int main()
{
    string str = "geeksforgeeks";
    int n = str.length();
 
    printChar(str, n);
 
    return 0;
}

Java




// Java implementation of the approach
class GFG
{
     
static int SIZE = 26;
 
// Function to create Sieve to check primes
static void SieveOfEratosthenes(boolean []prime,
                                int p_size)
{
    // false here indicates
    // that it is not prime
    prime[0] = false;
    prime[1] = false;
 
    for (int p = 2; p * p <= p_size; p++)
    {
 
        // If prime[p] is not changed,
        // then it is a prime
        if (prime[p])
        {
 
            // Update all multiples of p,
            // set them to non-prime
            for (int i = p * 2; i < p_size; i += p)
                prime[i] = false;
        }
    }
}
 
// Function to print the prime frequency characters
// in the order of their occurrence
static void printChar(String str, int n)
{
    boolean []prime = new boolean[n + 1];
    for(int i = 0; i < n + 1; i++)
        prime[i] = true;
 
    // Function to create Sieve to check primes
    SieveOfEratosthenes(prime, str.length() + 1);
 
    // To store the frequency of each of
    // the character of the string
    int []freq = new int[SIZE];
 
    // Initialize all elements of freq[] to 0
    for(int i =0; i< SIZE; i++)
        freq[i]=0;
 
    // Update the frequency of each character
    for (int i = 0; i < n; i++)
        freq[str.charAt(i) - 'a']++;
 
    // Traverse str character by character
    for (int i = 0; i < n; i++)
    {
 
        // If frequency of current character is prime
        if (prime[freq[str.charAt(i) - 'a']])
        {
            System.out.print(str.charAt(i));
        }
    }
}
 
// Driver code
public static void main(String[] args)
{
    String str = "geeksforgeeks";
    int n = str.length();
 
    printChar(str, n);
}
}
 
// This code is contributed by PrinciRaj1992

Python 3




# Python 3 implementation of the approach
SIZE = 26
 
from math import sqrt
 
# Function to create Sieve to check primes
def SieveOfEratosthenes(prime, p_size):
     
    # false here indicates
    # that it is not prime
    prime[0] = False
    prime[1] = False
 
    for p in range(2, int(sqrt(p_size)), 1):
         
        # If prime[p] is not changed,
        # then it is a prime
        if (prime[p]):
             
            # Update all multiples of p,
            # set them to non-prime
            for i in range(p * 2, p_size, p):
                prime[i] = False
 
# Function to print the prime frequency characters
# in the order of their occurrence
def printChar(str, n):
    prime = [True for i in range(n + 1)]
 
    # Function to create Sieve to check primes
    SieveOfEratosthenes(prime, len(str) + 1)
 
    # To store the frequency of each of
    # the character of the string
    freq = [0 for i in range(SIZE)]
 
    # Update the frequency of each character
    for i in range(n):
        freq[ord(str[i]) - ord('a')] += 1
 
    # Traverse str character by character
    for i in range(n):
        # If frequency of current character is prime
        if (prime[freq[ord(str[i]) - ord('a')]]):
            print(str[i], end = "")
 
# Driver code
if __name__ == '__main__':
    str = "geeksforgeeks"
    n = len(str)
 
    printChar(str, n)
     
# This code is contributed by Surendra_Gangwar

C#




// C# implementation of the approach
using System;
 
class GFG
{
    static int SIZE = 26;
 
    // Function to create Sieve to check primes
    static void SieveOfEratosthenes(bool[] prime,
                                      int p_size)
    {
        // false here indicates
        // that it is not prime
        prime[0] = false;
        prime[1] = false;
 
        for (int p = 2; p * p <= p_size; p++)
        {
            // If prime[p] is not changed,
            // then it is a prime
            if (prime[p])
            {
                // Update all multiples of p,
                // set them to non-prime
                for (int i = p * 2;
                         i < p_size; i += p)
                    prime[i] = false;
            }
        }
    }
 
    // Function to print the prime frequency characters
    // in the order of their occurrence
    static void printChar(string str, int n)
    {
        bool[] prime = new bool[n + 1];
        for (int i = 0; i < n + 1; i++)
            prime[i] = true;
 
        // Function to create Sieve to check primes
        SieveOfEratosthenes(prime, str.Length + 1);
 
        // To store the frequency of each of
        // the character of the string
        int[] freq = new int[SIZE];
 
        // Initialize all elements of freq[] to 0
        for (int i = 0; i < SIZE; i++)
            freq[i] = 0;
 
        // Update the frequency of each character
        for (int i = 0; i < n; i++)
            freq[str[i] - 'a']++;
 
        // Traverse str character by character
        for (int i = 0; i < n; i++)
        {
 
            // If frequency of current character is prime
            if (prime[freq[str[i] - 'a']])
            {
                Console.Write(str[i]);
            }
        }
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        String str = "geeksforgeeks";
        int n = str.Length;
 
        printChar(str, n);
    }
}
 
// This code is contibuted by
// sanjeev2552
Output
gksgks

Method #2: Using built-in functions:

Approach:

We will scan the string and count the occurrence of all characters using built-in Counter() function after that we traverse the string and check if the occurrences are prime or not if there is any prime frequency then we print it.

Note: This method is applicable for all type of characters

Below is the implementation of the above approach:

Python3




# Python code for the above approach
 
# importing Counter function
from collections import Counter
import math
 
# Function to check primes
def prime(n):
    if n <= 1:
        return False
    max_div = math.floor(math.sqrt(n))
    for i in range(2, 1 + max_div):
        if n % i == 0:
            return False
    return True
 
 
def checkString(s):
 
    # Counting the frequency of all
    # character using Counter function
    freq = Counter(s)
 
    # Traversing string
    for i in range(len(s)):
        if prime(freq[s[i]]):
            print(s[i], end="")
 
 
# Driver code
s = "geeksforgeeks"
# passing string to checkString function
checkString(s)
 
# This code is contributed by vikkycirus

C#




// C# code for the above approach
using System;
using System.Collections.Generic;
 
class GFG{
     
// Function to check primes
static bool prime(int n)
{
    if (n <= 1)
        return false;
         
    int max_div = (int)Math.Floor(Math.Sqrt(n));
    for(int i = 2; i < 1 + max_div; i++)
    {
        if (n % i == 0)
            return false;
    }
    return true;
}
 
static void checkString(string s)
{
     
    // Counting the frequency of all
    // character using Counter function
    Dictionary<char,
               int> freq = new Dictionary<char,
                                          int>();
    for(int i = 0; i < s.Length; i++)
    {
        if (!freq.ContainsKey(s[i]))
            freq[s[i]] = 0;
             
        freq[s[i]] += 1;
    }
     
    // Traversing string
    for(int i = 0; i < s.Length; i++)
    {
        if (prime(freq[s[i]]))
            Console.Write(s[i]);
    }
}
 
// Driver code
public static void Main()
{
    string s = "geeksforgeeks";
     
    // Passing string to checkString function
    checkString(s);
}
}
 
// This code is contributed by ukasp
Output
gksgks

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