how do i make a array with lots of places in c? - c

Question

I'm making the sieve of erasthostenes in c. I have programmed it in other languages before but I never encountered this problem. Here's my algorithm:
#include <stdio.h>
#include <stdbool.h>
#include <math.h>
#include <time.h>
#include <limits.h>
int main(){
clock_t t;
t = clock();
int n = 1299710;
bool pPrimes[n];
for(int i = 0; i<n; i++){
pPrimes[i] = true;
}
pPrimes[0] = false;
pPrimes[1] = false;
for(int i = 2; i<sqrt(n); i++){
if(pPrimes[i]){
for(int x = i*i; x<n; x+=i){
pPrimes[x] = false;
}
}
}
for(int i = 2; i<n; i++){
if (pPrimes[i]){
printf("%d\n", i);
}
}
t = clock() - t;
double time_taken = ((double)t)/CLOCKS_PER_SEC;
printf("%f", time_taken);
return 0;
}
It's when I declare pPrimes that n can't be large enough. A million or so works but not more. Is there anyway of fixing this?
I tried debugging but I only get this error message:
line 1: 4320 Segmentation fault: 11

Answer 1

Some issues:
Out of local space
OP reports with large n, there is trouble. Better to allocate with malloc().
bool is not certainly the narrowest type - use unsigned char. Better to allocate with an unsigned size such as unsigned or size_t.
//int n = 1299710;
//bool pPrimes[n];
unsigned n = 1299710;
if (n < 2) { // Avoid edge cases
fprintf(stderr, "Use larger value, not %u\n", n);
return EXIT_FAILURE;
}
unsigned char *pPrimes = malloc(sizeof *nPrimes * n);
if (pPrimes == NULL) {
fprintf(stderr, "Out-of-memory %u\n", n);
return EXIT_FAILURE;
}
Off by one
I would expect int n = 1299710; to imply finding all the primes up to and including n.
unsigned char *pPrimes = malloc(sizeof *nPrimes * (n+1));
// for(int i = 2; i<n; i++){
for(unsigned i = 2; i <= n; i++){ // <=
Referencing this pseudocode, edge tests are off by one.
Do not trust a raw sqrt() for an integer problem. When the expected result is x.00000..., that function may return x_minus_1.99999....
unsigned qsrt_n = lround(sqrt(n));
// for(int i = 2; i<sqrt(n); i++){
for(unsigned i = 2; i <= sqrt_n; i++){ // <=
if(pPrimes[i]){
// for(int x = i*i; x<n; x+=i){
for(unsigned x = i*i; x <= n; x+=i){ // <=
pPrimes[x] = false;
}
}
}

Answer 2

You are allocating too much memory in the stack. This is known as a stack overflow.
Either (as #Gerhardh said in a comment):
Use a static array
Use a global array
Use malloc()
1. With a static array:
int main(void)
{
#define n 1299710
static bool primes[n] = {false, false};
for (size_t i = 2; i < n; ++i) {
primes[i] = true;
}
long srn = sqrt(n) + 1;
for (size_t i = 0; i < srn; ++i) {
if (!primes[i]) continue;
for (size_t ii = i*i; ii < n; ii += i)
primes[ii] = false;
}
// print...
}
2. With a global array:
#define n 1299710
/*static?*/ bool primes[n] = {false, false};
int main(void)
{
for (size_t i = 2; i < n; ++i) {
primes[i] = true;
}
long srn = sqrt(n) + 1;
for (size_t i = 0; i < srn; ++i) {
if (!primes[i]) continue;
for (size_t ii = i*i; ii < n; ii += i)
primes[ii] = false;
}
// print...
}
3. With a dynamic array:
int main(void)
{
const size_t n = 1299710;
bool *primes = malloc(n * sizeof bool);
if (!primes) {
printf("Memory issues. Goodbye!\n");
return EXIT_FAILURE;
}
for (size_t i = 2; i < n; ++i) {
primes[i] = true;
}
primes[0] = false;
primes[1] = false;
long srn = n == 1 ? 1 : sqrt(n) + 1;
for (size_t i = 0; i < srn; ++i) {
if (!primes[i]) continue;
for (size_t ii = i*i; ii < n; ii += i)
primes[ii] = false;
}
// print...
}