mirror of
https://github.com/TheAlgorithms/C
synced 2024-11-25 23:09:36 +03:00
94 lines
2.2 KiB
C
94 lines
2.2 KiB
C
/**
|
|
* \file
|
|
* \brief [Problem 21](https://projecteuler.net/problem=21) solution
|
|
* \author [Krishna Vedala](https://github.com/kvedala)
|
|
*/
|
|
#include <stdio.h>
|
|
#include <stdlib.h>
|
|
#include <time.h>
|
|
|
|
/**
|
|
* function to return the sum of proper divisors of N
|
|
*/
|
|
unsigned long sum_of_divisors(unsigned int N)
|
|
{
|
|
unsigned long sum = 1 + N; /* 1 and itself are always a divisor */
|
|
/* divisors are symmertically distributed about the square-root */
|
|
for (unsigned int i = 2; i * i < N; i++)
|
|
{
|
|
if ((N % i) != 0)
|
|
/* i is not a divisor of N */
|
|
continue;
|
|
|
|
// #ifdef DEBUG
|
|
// printf("%4d, %4d,", i, N / i);
|
|
// #endif
|
|
|
|
sum += i + (N / i);
|
|
}
|
|
// #ifdef DEBUG
|
|
// printf("\nSum of divisors of %4d: %4d\n", N, sum);
|
|
// #endif
|
|
return sum;
|
|
}
|
|
|
|
/** Main function */
|
|
int main(int argc, char **argv)
|
|
{
|
|
unsigned long sum = 0;
|
|
unsigned int MAX_N = 500;
|
|
if (argc == 2)
|
|
MAX_N = atoi(argv[1]);
|
|
|
|
/*
|
|
* We use an array of flags to check if a number at the index was:
|
|
* not-processed = 0
|
|
* is amicable = 1
|
|
* not amicable = -1
|
|
*/
|
|
char *flags = (char *)calloc(MAX_N, sizeof(char));
|
|
|
|
clock_t start_time = clock();
|
|
int i;
|
|
/* there are no such numbers till 10. Lets search from there on */
|
|
for (i = 10; i < MAX_N; i++)
|
|
{
|
|
if (flags[i] != 0)
|
|
/* already processed, skip */
|
|
continue;
|
|
|
|
unsigned int b = sum_of_divisors(i);
|
|
if (b >= MAX_N)
|
|
flags[i] = -1;
|
|
else if (flags[b] == -1)
|
|
continue;
|
|
|
|
unsigned int c = sum_of_divisors(b);
|
|
if (c == i && b != i)
|
|
{
|
|
/* found amicable */
|
|
flags[b] = 1;
|
|
flags[i] = 1;
|
|
sum += b + i;
|
|
#ifdef DEBUG
|
|
printf("Amicable: %4d : %4d\n", i, b);
|
|
#endif
|
|
}
|
|
else
|
|
{
|
|
flags[i] = -1;
|
|
if (b < MAX_N)
|
|
flags[b] = -1;
|
|
}
|
|
}
|
|
|
|
clock_t end_time = clock();
|
|
|
|
printf("\nTime taken: %.4g millisecond\n",
|
|
1e3 * (end_time - start_time) / CLOCKS_PER_SEC);
|
|
printf("Sum of all numbers = %lu\n", sum);
|
|
|
|
free(flags);
|
|
return 0;
|
|
}
|