C, 208 206 201 200 199 196 194 193 194 193 188 185 183 180 176 Bytes
(if newlines are removed):
main(int u,char**b){
for(int v,x,y,S=v=**++b-48;--v>-S;putchar(10))
for(u=-S;++u-u^v
Compiled with
> gcc -std=c99 -o ulam ulam.c
Warning. This program is slow, because is does a trial division up to 2^31. But is does produce the required output:
* *
* *
* * *
* * *
* ** *
* *
* *
* *
* *
In nicely formatted C and with redundant #includes:
#include
#include
int main(int argc, char** argv) {
int u,v,x,y,d,S = atoi(argv[1]);
/* v is the y coordinate of grid */
for (v=S; v>=-S; --v)
/* u is the x coordinate. The second operand (!putchar...) of the boolean or
* is only ececuted a a end of a x line and it prints a newline (10) */
for (u=-S; u<=S || !putchar(10); ++u) {
/* x,y are u,v after "normalizing" the coordintes to quadrant 0
normalizing is done with the two comparisions, swapping and and
an additional term later */
d = v
It works by transforming the coordinates of the grid to the appropriate number and then performing the primality test, intead of drawing in a snake-like manner. The different equations for the four "quadrants" can be collapsed into one with swapping x and y and an additional term for "backward counting".
