Economical Numbers | C ++ implementation to find Economical Numbers till n ; Array to store all prime less than and equal to MAX . ; Utility function for sieve of sundaram ; In general Sieve of Sundaram , produces primes smaller than ( 2 * x + 2 ) for a number given number x . Since we want primes smaller than MAX , we reduce MAX to half ; Main logic of Sundaram . Mark all numbers which do not generate prime number by doing 2 * i + 1 ; Since 2 is a prime number ; Print other primes . Remaining primes are of the form 2 * i + 1 such that marked [ i ] is false . ; Function to check if a number is a Economical number ; Count digits in original number ; Count all digits in prime factors of n pDigit is going to hold this value . ; Count powers of p in n ; If primes [ i ] is a prime factor , ; Count the power of prime factors ; Add its digits to pDigit . ; Add digits of power of prime factors to pDigit . ; If n != 1 then one prime factor still to be summed up ; ; If digits in prime factors is less than digits in original number then return true . Else return false . ; Driver code ; Finding all prime numbers before limit . These numbers are used to find prime factors .
#include <bits/stdc++.h> NEW_LINE using namespace std ; const int MAX = 10000 ; vector < int > primes ; void sieveSundaram ( ) { bool marked [ MAX / 2 + 1 ] = { 0 } ; for ( int i = 1 ; i <= ( sqrt ( MAX ) - 1 ) / 2 ; i ++ ) for ( int j = ( i * ( i + 1 ) ) << 1 ; j <= MAX / 2 ; j = j + 2 * i + 1 ) marked [ j ] = true ; primes . push_back ( 2 ) ; for ( int i = 1 ; i <= MAX / 2 ; i ++ ) if ( marked [ i ] == false ) primes . push_back ( 2 * i + 1 ) ; } bool isEconomical ( int n ) { if ( n == 1 ) return false ; int original_no = n ; int sumDigits = 0 ; while ( original_no > 0 ) { sumDigits ++ ; original_no = original_no / 10 ; } int pDigit = 0 , count_exp = 0 , p ; for ( int i = 0 ; primes [ i ] <= n / 2 ; i ++ ) { while ( n % primes [ i ] == 0 ) { p = primes [ i ] ; n = n / p ; count_exp ++ ; } while ( p > 0 ) { pDigit ++ ; p = p / 10 ; } while ( count_exp > 1 ) { pDigit ++ ; count_exp = count_exp / 10 ; } } if ( n != 1 ) { while ( n > 0 ) { pDigit ++ ; n = n / 10 ; } } return ( pDigit < sumDigits ) ; } int main ( ) { sieveSundaram ( ) ; for ( int i = 1 ; i < 200 ; i ++ ) if ( isEconomical ( i ) ) cout << i << " ▁ " ; return 0 ; }