Code Golf: Ulam Spiral

Answer 1

MATLAB: 182 167 156 characters

Script ulam.m:

A=1;b=ones(1,4);for i=0:(input('')-2),c=b(4);b=b+i*8+(2:2:8);A=[b(2):-1:b(1);(b(2)+1:b(3)-1)' A (b(1)-1:-1:c+1)';b(3):b(4)];end;disp(char(isprime(A)*10+32))

And formatted a little nicer:

A = 1;
b = ones(1,4);
for i = 0:(input('')-2),
  c = b(4);
  b = b+i*8+(2:2:8);
  A = [b(2):-1:b(1); (b(2)+1:b(3)-1)' A (b(1)-1:-1:c+1)'; b(3):b(4)];
end;
disp(char(isprime(A)*10+32))

Test cases:

>> ulam
2
* *
  *
*  
>> ulam
3
*   *
 * * 
*  **
 *   
  *  
>> ulam
5
    * *  
 *     * 
* *   *  
   * * * 
  *  ** *
 * *     
*   *    
 *   *   
*     *