Question

I started working on Project Euler problems today to keep myself busy over break. One of the problems asks for the sum of all prime numbers below 2 million, so I threw together a Sieve of Eratosthenes to find all those numbers.

unsigned long i, j, sum = 0, limit = 2000000;

// Allocate an array to store the state of numbers (1 is prime, 0 is not).
int* primes = malloc(limit * sizeof(int));

// Initialize every number as prime.
for (i = 0; i < limit; i++)
    primes[i] = 1;

// Use the sieve to check off values that are not prime.
for (i = 2; i*i < limit; i++)
    if (primes[i] != 0)
        for (j = 2; i*j < limit; j++)
            primes[i*j] = 0;

// Get the sum of all numbers still marked prime.
for (i = 2; i < limit; i++)
    if (primes[i] != 0)
        sum += i;

printf("%d", sum);

This works perfectly up to limit around half a million. After this, it returns random values (for example, 1000000 returns -1104303641). I’ve tried declaring all the unsigned long variables as unsigned long long to no avail. The error seems to be happening in the last 4 lines, because primes[] contains nothing but 1’s and 0’s at that point. I figure this has something to do with the size of the values being worked with, can anyone offer any guidance?

Answer 1

Change the %d to %ld and you should get:

142913828922

which seems like it should be (close to or) the right answer.

..assuming your longs aren’t 32-bit.

If you’re on a 32-bit platform, you’re going to need some sort of third-party big int library.

BigInteger in C?

recommends: http://gmplib.org/