Re: Re: simplifying ulam spiral code

*To*: mathgroup at smc.vnet.net*Subject*: [mg56464] Re: [mg56435] Re: simplifying ulam spiral code*From*: DrBob <drbob at bigfoot.com>*Date*: Tue, 26 Apr 2005 01:33:31 -0400 (EDT)*References*: <d4cmlr$39e$1@smc.vnet.net> <200504250530.BAA01491@smc.vnet.net>*Reply-to*: drbob at bigfoot.com*Sender*: owner-wri-mathgroup at wolfram.com

Brilliant! Bobby On Mon, 25 Apr 2005 01:30:51 -0400 (EDT), Carl K. Woll <carlw at u.washington.edu> wrote: > "zak" <chocolatez at gmail.com> wrote in message > news:d4cmlr$39e$1 at smc.vnet.net... >> hi >> in the site: >> http://teachers.crescentschool.org/weissworld/m3/spiral/spiral.html >> there is a mathematica code for drawing ULAM'S SPIRAL >> the code is: >> > [snip] > > I have to confess that I don't understand how zak's code relates to the > above link. The text at the link says that 1 is placed at the origin, 2 is > placed to the right of 1, and succeeding integers are placed in a > counterclockwise spiral. Hence, 3 ought to have the coordinates {1,1}, 4 > ought to have the coordinates {0,1}, etc. > > At any rate, it is not too difficult to program a function to determine the > coordinates of an integer in the above spiral. If one notices that the > bottom right and top left corners have the integer values n^2+1, then one > eventually gets > > coords[k_Integer] := Module[{n, a, b}, > n = Floor[Sqrt[k - .9]]; > a = k - n^2 - n - 1; > b = Quotient[2n + 1 - (-1)^n, 4]; > (-1)^n {Quotient[Abs[a] + a, 2] - b, Quotient[Abs[a] - a, 2] - b}] > > coords[k_List] := Module[{n, a, b, c}, > n = Floor[Sqrt[k - .9]]; > a = k - n^2 - n - 1; > c = (-1)^n; > b = Quotient[2n + 1 - c, 4]; > c Transpose[{Quotient[Abs[a] + a, 2] - b, Quotient[Abs[a] - a, 2] - b}]] > > A few comments may be in order. Concentrating on the second function > definition, I used a number of ideas to speed up it's execution. I used > Sqrt[k-.9] so that Mathematica is taking square roots of real numbers > instead of integers, which perhaps surprisingly is much faster. I used > Sqrt[k-.9] instead of Sqrt[k-1.] to avoid spurious cancellation errors when > the Floor of the result is evaluated. I wanted to make sure that all my > arrays were packed, so I used Quotient instead of dividing two integers. > Even though 2n+1-(-1)^n is always divisible by 4, (2n+1-(-1)^n)/4 is not > packed even when 2n+1-(-1)^n is packed. Finally, I used (Abs[a]+a)/2 (with > Quotient instead of using division) to change all negative values in the > list a to 0 and (Abs[a]-a)/2 to change all positive values in the list a to > 0. > > At any rate, using coords on a list of the first million integers takes a > bit less than 2 seconds on my machine. > > Now, we are ready to used coords to find the coordinates of the primes. For > example, if we are interested in the first million primes: > > data = Prime[Range[10^6]]; // Timing > {9.547 Second, Null} > > Now, we use coords to get the coordinates of the primes. > > pts = coords[data]; // Timing > {2.375 Second, Null} > > Applying Developer`ToPackedArray to the data would speed up the pts > computation by a bit. Looking at the first few points of pts reveals > > Take[pts, 10] > {{1, 0}, {1, 1}, {-1, 1}, {-1, -1}, {2, 0}, {2, 2}, {-2, 2}, {-2, 0}, > {0, -2}, {3, 1}} > > which looks correct to me. > > Carl Woll > > > > > -- DrBob at bigfoot.com

**References**:**Re: simplifying ulam spiral code***From:*"Carl K. Woll" <carlw@u.washington.edu>