PRIME1 Segmentation fault

[saintjulien_sof]

Hi guys can you look over my code for PRIME1 (spoj.com/problems/PRIME1/) and see what the issue is?

#include <iostream>
#include <cstring>
void SieveOfEratosthenes(bool arr[])
{	
	int n = 2; // smallest prime is 2
        int num = 0;
	for (int i = 1; i < 1000000000; ++i) { //1,000,000,000 is the constraint in the question
		while ((num = n * (i + 1)) <= 1000000000){
			arr[num - 1] = 1; //if a number is not a prime, its position is set to 1
			++n;
		}
	}
}
int main()
{
	bool arr[1000000000]; //create boolean array of size 1000000000
	memset(arr, 0, sizeof(arr));
	SieveOfEratosthenes(arr);
	int t = 0;
	scanf("%d", &t);
	typedef unsigned long ul;
	for (int i = 0; i < t; ++t){
		unsigned long n=0, m=0;
		scanf("%lu%lu", &m, &n);
		for (int i = m-1; m<=(n-1); ++m){
			if (arr[i] == 0){
                        //if it has not been changed to 1, it is a prime.
				printf("%d\n", arr[i]);
				}
		}
		printf("\n");
	}
	return 0;
}

On the ideone.com C++ 4.3.2 compiler, it gives the following error:

prog.cpp:4: error: variable or field 'SieveOfEratosthenes' declared void
prog.cpp:4: error: 'arr' was not declared in this scope

In my code, I want to create an array that is the size of the constraint in the problem. Then set all non-prime positions in the array to 1.
Then, print all numbers whose position in the array is still 0 (if left as 0, it means it is a prime).

Additionally, does my algorithm work?

[leppyr64]

To fix your compiler error you should declare arr as a pointer in your function declaration, then you not have to worry about giving it a size.

You will not be able to declare an array of size 10^9. Think about how much memory that would actually use.

You are off to a good start but you need to think some more.

[saintjulien_sof]

Should I then use a vector? That would still use a lot of memory right? So then I should only calculate primes between m and n. But then how do I not recalculate it every loop, i.e. in your other post you said "think about what happens if the inputs are
1000 2000
1000 2000
1000 2000
etc.
Can you give me a hint?

[leppyr64]

Vector? No. Primes between m and n? Yes.

You will need to recalculate all of the primes between m and n every test case. The difficulty is in determining how to do that efficiently. You need to determine the simplest definition of a prime number to do that.