armstrong.cpp

/**
 * Coding challenge : Armstrong Numbers
 *
 * Write a program using any language or technique that can accept two positive
 * integers defining a range (such as 0 and 100 for the range 0 <= x <= 100) and
 * will print all Armstrong Numbers in that range.
 *
 * An Armstrong Number is fully defined at
 * http://en.wikipedia.org/wiki/Narcissistic_number
 *
 * In short it is any number where the value is the same as the sum of it's 
 * digits raised to the power of the number of it's digits.
 *
 * So 153 = 1^3 + 5^3 + 3^3
 *
 * Given the range 0 to 10,000 you should have the following output:
 * 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 153, 370, 371, 407, 1634, 8208, 9474
 *
 * We are assuming base 10.
 */

#include <iostream>

bool is_armstrong(int number, int * powers) {
    int sum = 0, temp = number;

    while ( temp != 0 ) {
        sum += powers[temp % 10];
        temp /= 10;
    }

    return sum == number;
}

void bump_powers(int * powers) {
    for(int i = 2; i < 10; ++i) {
        powers[i] *= i;
    }
}

int main (int argc, char * argv[]) {
    int digits = 0;
    int mod = 10;
    int powers[10] = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9};

    for(int i = 0; i <= 1000000; ++i) {
        if ( is_armstrong(i, powers) )
            std::cout << i << " ";

        if ( ++digits % mod == 0 ) {
            digits = 0;
            mod *= 10;
            bump_powers(powers);
        }
    }

    std::cout << std::endl;

    return 0;
}