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Program to print characters present at prime indexes in a given string
• Last Updated : 01 May, 2021

Given a string, our task is to print the characters present at prime index.

Examples :

```Input : I love programming
Output : lv gan
Explanation :
prime index characters in a string are : lv gan

Input : Happy coding everyone
Output : apycn ro```

Approach :

1. Use two loops to divide the numbers upto the value of length of string.
2. Increment variable result when the remainder is 0.
3. If the variable result = 1, then print the corresponding character.

Below is the implementation of above approach :

## C++

 `// C++ Program to print Characters at``// Prime index in a given String``#include ``using` `namespace` `std;` `bool` `isPrime(``int` `n)``{``    ``// Corner case``    ``if` `(n <= 1) ``return` `false``;` `    ``// Check from 2 to n-1``    ``for` `(``int` `i = 2; i < n; i++)``        ``if` `(n % i == 0)``            ``return` `false``;` `    ``return` `true``;``}` `// Function to print``// character at prime index``void` `prime_index(string input)``{``    ``int` `n = input.length();` `    ``// Loop to check if``    ``// index prime or not``    ``for` `(``int` `i = 2; i <= n; i++)``        ``if` `(isPrime(i))``            ``cout << input[i - 1];        ``}` `// Driver Code``int` `main()``{``    ``string input = ``"GeeksforGeeks"``;``    ``prime_index(input);``    ``return` `0;``}`

## Java

 `// Java Program to print``// Characters at Prime index``// in a given String``class` `GFG``{``    ``static` `boolean` `isPrime(``int` `n)``    ``{``        ``// Corner case``        ``if` `(n <= ``1``) ``return` `false``;``    ` `        ``// Check from 2 to n-1``        ``for` `(``int` `i = ``2``; i < n; i++)``            ``if` `(n % i == ``0``)``                ``return` `false``;``    ` `        ``return` `true``;``    ``}``    ` `    ``// Function to print``    ``// character at prime index``    ``static` `void` `prime_index(String input)``    ``{``        ``int` `n = input.length();``    ` `        ``// Loop to check if``        ``// index prime or not``        ``for` `(``int` `i = ``2``; i <= n; i++)``            ``if` `(isPrime(i))` `                ``System.out.print``                ``(input.charAt(i - ``1``));        ``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `main (String[] args)``    ``{``        ``String input = ``"GeeksforGeeks"``;``        ` `        ``prime_index(input);``    ``}``}` `// This code is contributed by Anant Agarwal.`

## Python3

 `# Python3 program to print``# Characters at Prime index``# in a given String` `def` `isPrime(n):` `    ``# Corner case``    ``if` `n <``=` `1``:``        ``return` `False` `    ``# Check from 2 to n-1``    ``for` `i ``in` `range``(``2``, n):``        ``if` `n ``%` `i ``=``=` `0``:``            ``return` `False``;` `    ``return` `True` `# Function to print``# character at prime index``def` `prime_index (``input``):``    ``p ``=` `list``(``input``)``    ``s ``=` `""``    ` `    ``# Loop to check if``    ``# index prime or not``    ``for` `i ``in` `range` `(``2``, ``len``(p) ``+` `1``):``        ``if` `isPrime(i):``            ``s ``=` `s ``+` `input``[i``-``1``]``    ``print` `(s)``        ` `# Driver Code``input` `=` `"GeeksforGeeks"``prime_index(``input``)`

## C#

 `// C# Program to print Characters``// at Prime index in a given String``using` `System;` `class` `GFG``{``    ``static` `bool` `isPrime(``int` `n)``    ``{``        ``// Corner case``        ``if` `(n <= 1) ``return` `false``;``    ` `        ``// Check from 2 to n-1``        ``for` `(``int` `i = 2; i < n; i++)``            ``if` `(n % i == 0)``                ``return` `false``;``    ` `        ``return` `true``;``    ``}``    ` `    ``// Function to print character``    ``// at prime index``    ``static` `void` `prime_index(``string` `input)``    ``{``        ``int` `n = input.Length;``    ` `        ``// Loop to check if``        ``// index prime or not``        ``for` `(``int` `i = 2; i <= n; i++)``            ``if` `(isPrime(i))` `                ` `                ``Console.Write(input[i - 1]);        ``    ``}``    ` `    ``// Driver code``    ``public` `static` `void` `Main ()``    ``{``        ``string` `input = ``"GeeksforGeeks"``;``        ` `        ``prime_index(input);``    ``}``}` `// This code is contributed by Vt_m.`

## PHP

 ``

## Javascript

 ``

Output :

`eesoes`

Optimizations :
For large strings, we can use Sieve of Eratosthenes to efficiently find all prime numbers smaller than or equal to length of string.

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