# Print characters having prime frequencies in order of occurrence

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

Character Frequency
‘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

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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++ implementation of the approach ` `#include ` `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 = ``false``; ` `    ``prime = ``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 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 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# 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 = ``false``; ` `        ``prime = ``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
```

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