Program to print characters present at prime indexes in a given string

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 <bits/stdc++.h>
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

<?php
// PHP Program to print
// Characters at Prime
// index in a given String

function isPrime($n)
{
    // Corner case
    if ($n <= 1) return false;

    // Check from 2 to n-1
    for ($i = 2; $i < $n; $i++)
        if ($n % $i == 0)
            return false;

    return true;
}

// Function to print
// character at prime index
function prime_index($input)
{
    $n = strlen($input);

    // Loop to check if
    // index prime or not
    for ($i = 2; $i <= $n; $i++)
        if (isPrime($i))
            echo $input[$i - 1];
}

// Driver Code
$input = "GeeksforGeeks";
prime_index($input);

// This code is contributed by ajit.
?>

Javascript

<script>

// Javascript program to print characters
// at Prime index in a given String
function isPrime(n)
{

    // Corner case
    if (n <= 1) return false;

    // Check from 2 to n-1
    for(let i = 2; i < n; i++)
        if (n % i == 0)
            return false;

    return true;
}

// Function to print character
// at prime index
function prime_index(input)
{
    let n = input.length;

    // Loop to check if
    // index prime or not
    for(let i = 2; i <= n; i++)
        if (isPrime(i))
            document.write(input[i - 1]);
}

// Driver code
let input = "GeeksforGeeks";

prime_index(input);

// This code is contributed by suresh07

</script>

Output

eesoes

Time Complexity: O(n²), where n is the length of the given string.
Auxiliary Space: O(1), no extra space is required, so it is a constant.

Optimizations : 

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