Uram Spiral from wikipeda http://en.wikipedia.org/wiki/Ulam_spiral.

ulam_spiral.cpp

#include <iostream>
#include <cstring>

using namespace std;

const int N = 30;
const int LENGTH = 2*N+1;
const int dx[4] = {1, 0, -1, 0};
const int dy[4] = {0, -1, 0, 1};
bool feald[LENGTH][LENGTH]; 
bool is_prime[LENGTH*LENGTH];

void sieve_of_eratosthenes() {
  memset(is_prime, true, sizeof(feald));
  is_prime[0] = false, is_prime[1] = false;
  for (int i = 2; i < LENGTH*LENGTH; ++i) {
    if (is_prime[i]) {
      for (int j = 2; i * j < LENGTH*LENGTH; ++j) {
	is_prime[i*j] = false;
      }
    }
  }
}

void make_ulam_spiral() {
  int step = 1, cnt = 1, dir = 0;
  int x = N, y = N;
  while (cnt <= LENGTH*LENGTH) {
    for (int k = 0; k < 2; ++k) {
      for (int i = 0; i < step; ++i) {
	feald[y][x] = is_prime[cnt++];
	x += dx[dir];
	y += dy[dir];
      }
      dir = (dir + 1) % 4;
    }
    ++step;
  }
  return;
}


void output() {
  for (int i = 0; i < LENGTH; ++i) {
    for (int j = 0; j < LENGTH; j++) {
      cout << (feald[i][j] ? "*" : ".");
    }
    cout << endl;
  }
}
int main() {

  sieve_of_eratosthenes();
  make_ulam_spiral();
  output();

  return 0;
}