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    <h1>C, <strike>208</strike> <strike>206</strike> <strike>201</strike> <strike>200</strike> <strike>199</strike> <strike>196</strike> <strike>194</strike> <strike>193</strike> <strike>194</strike> <strike>193</strike> <strike>188</strike> <strike>185</strike> <strike>183</strike> <strike>180</strike> 176 Bytes</h1> <p>(if newlines are removed):</p> <pre><code>main(int u,char**b){ for(int v,x,y,S=v=**++b-48;--v&gt;-S;putchar(10)) for(u=-S;++u&lt;S;){ x=u;y=v;v&gt;-u^v&lt;u?:(x=v,y=u); x=4*y*y-x-y+1+2*(v&lt;u)*(x-y); for(y=1;x%++y;); putchar(y^x?32:42);}} </code></pre> <p>Compiled with</p> <pre><code>&gt; gcc -std=c99 -o ulam ulam.c </code></pre> <p>Warning. This program is slow, because is does a trial division up to 2^31. But is does produce the required output:</p> <pre><code> * * * * * * * * * * * ** * * * * * * * * * </code></pre> <p>In nicely formatted C and with redundant #includes:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; 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&gt;=-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&lt;=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&lt;u; x=u; y=v; if (v&lt;=-u ^ d) { x=v; y=u; } /* reuse x, x is now the number at grid (u,v) */ x = 4*y*y -x-y+1 +2*d*(x-y); /* primality test, y resused as loop variable, won't win a speed contest */ for (y=2; y&lt;x &amp;&amp; x%y; ++y) ; putchar(y!=x?' ':'*'); } } </code></pre> <p>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".</p>
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    1. POCode Golf: Ulam Spiral
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    1. USGunther Piez
    1. USGunther Piez
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    1. COThe primality test in the compacted version doesn't work with gcc 4.3.3. You can use y^x in the final ternary to get the char back.
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      1. USJohn La Rooy
    2. COI just tested it with 4.3.4 and 4.4.2, both do work. Can you specify what the problem is? And thanks for the xor-tip ;-)
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      1. USGunther Piez
    3. COFor 3-9, I think this is one off. ./a.out "2" shows the proper output for three. Two doesn't work, nor do double-digit numbers.
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      1. USACoolie
 

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