prime.c

#include<stdio.h>
#include<time.h>

#define N (10*1000*1000)
int table[N+1];
int uncheck_min = 0;


int calc(int x){
  int i;
  for (i = 2; i*i  <= x; i++){
    if(isprime(i)){
      if (0 == (x% i )){
        return 0;
      }
    }
  }
  return 1;
}

int isprime(int n){
  while (!(n < uncheck_min)){
    table[uncheck_min] = calc(uncheck_min);
    uncheck_min += 1;
  }
  return table[n];
}


int main(){
  int i;
  clock_t start, end;
  FILE* fp;
  table[2] = 1;
  uncheck_min = 3;


  start = clock();
  for (i = 0; i <=N; i++){
    isprime(i);
  }
  end = clock();
  printf("%f s\n", (double)(end - start)/CLOCKS_PER_SEC);
  printf("%d\n", CLOCKS_PER_SEC);


  fp = fopen("result.txt", "w");
  for (i = 0; i <=N; i++){
      if(table[i]){
          fprintf(fp, "%d\n", i);
      }
  }
  fclose(fp);


  return 0;
}