Count characters in a string whose ASCII values are prime | C ++ implementation of above approach ; Function to find prime characters in the string ; USE SIEVE TO FIND ALL PRIME NUMBERS LESS THAN OR EQUAL TO max_val Create a Boolean array " prime [ 0 . . n ] " . A value in prime [ i ] will finally be false if i is Not a prime , else true . ; 0 and 1 are not primes ; If prime [ p ] is not changed , then it is a prime ; Update all multiples of p ; Traverse all the characters ; Driver program ; print required answer
#include <bits/stdc++.h> NEW_LINE using namespace std ; #define max_val  257 NEW_LINE int PrimeCharacters ( string s ) { vector < bool > prime ( max_val + 1 , true ) ; prime [ 0 ] = false ; prime [ 1 ] = false ; for ( int p = 2 ; p * p <= max_val ; p ++ ) { if ( prime [ p ] == true ) { for ( int i = p * 2 ; i <= max_val ; i += p ) prime [ i ] = false ; } } int count = 0 ; for ( int i = 0 ; i < s . length ( ) ; ++ i ) { if ( prime [ int ( s [ i ] ) ] ) count ++ ; } return count ; } int main ( ) { string S = " geeksforgeeks " ; cout << PrimeCharacters ( S ) ; return 0 ; }