thanks, your code is much faster than mine, and now it produce the same figure as in the
http://en.wikipedia.org/wiki/Ulam_spiral
after we invert the colors
one small note is that your code will plot number 1 as a prime number, we can find this if we insert
print "number of primes = $NumOfPrimes \n"; before the mainloop; and try the program with number 10 , it will give us the number of primes is 5
as a variation i suggest for the invistigators for small spirals to replace the main plotting code:
$c->createText( $x*+$o, $y+$o, -fill => "$colr", -text => '.'); with
$c->createText( $x*30+$o, $y*30+$o, -fill => "$colr", -text => "$num"); so we can see numbers instead of dots in wich the primes will be in white color , the factor 30 above is for magnification of the plotting.