(This article was originally published at The DO Loop, and syndicated at StatsBlogs.)

In my previous post, I described how to implement an iterated function system (IFS) in the SAS/IML language to draw fractals. I used the famous Barnsley fern example to illustrate the technique. At the end of the article I issued a challenge: can you construct an IFS whose fractal attractor looks like a Christmas tree?

I started my attempt by "straightening" the fern example, but it ended up looking like a "fishbone fern" instead of an evergreen tree. I decided to abandon the four-function IFS and adopt a seven-function system. In addition to "left" and "right" branches, I introduced transformations for "left diagonal" and "right diagonal" branches. I also played with the relative probabilities and the translation parameters to get the right "fill" for the tree.

Eventually I constructed the following program, which implements an IFS that creates the "SAS Christmas tree" that is shown at the left. I'm not 100% satisfied with it, but as Linus says in *A Charlie Brown Christmas*, it's not "such a bad little tree."

proc iml; /* For an explanation of how to construct an iterated function system in SAS, see http://blogs.sas.com/content/iml/2012/12/12/iterated-function-systems-and-barnsleys-fern-in-sas/ */ /* Each row is a 2x2 linear transformation */ /* Christmas tree */ L = {0.03 0 0 0.1, 0.85 0.00 0.00 0.85, 0.8 0.00 0.00 0.8, 0.2 -0.08 0.15 0.22, -0.2 0.08 0.15 0.22, 0.25 -0.1 0.12 0.25, -0.2 0.1 0.12 0.2}; /* ... and each row is a translation vector */ B = {0 0, 0 1.5, 0 1.5, 0 0.85, 0 0.85, 0 0.3, 0 0.4 }; prob = { 0.02 0.6 0.1 0.07 0.07 0.07 0.07}; L = L`; B = B`; /* For convenience, transpose the L and B matrices */ /* Iterate the discrete stochastic map */ N = 1e5; /* number of iterations */ x = j(2,N); k = j(N,1); x[,1] = {0, 2}; /* initial point */ call randgen(k, "Table", prob); /* values 1-7 */ do i = 2 to N; x[,i] = shape(L[,k[i]], 2)*x[,i-1] + B[,k[i]]; /* iterate */ end; /* Plot the iteration history */ y = x`; create IFS from y[c={"x" "y"}]; append from y; close IFS; quit; /* basic IFS Christmas Tree */ ods graphics / width=200px height=400px; proc sgplot data=IFS; title "SAS Christmas Tree"; scatter x=x y=y / markerattrs=(size=1 color=ForestGreen); yaxis display=none; xaxis display=none; run;

As I looked at the tree, however, I decided that it seemed rather bare. It seemed to "need a little love," so I decided to add some brightly colored, ornamental, decorations. I took a random sample of points on the attractor and drew balls at those points. I also added a star on top, all courtesy of PROC SGPLOT. You can download the complete program that creates the final image, which is shown at left. I used a few programming tricks, such as a data attribute map to assign customized, bright, colors for the ornaments.

So now it's your turn! In the comments, submit or link to a SAS program that creates a Christmas tree, a snowflake, or some other seasonal image. Be creative! My colleague, Robert Allison, has already posted a few whimsical SAS graphs for the holidays.

As for me, I'm finished blogging until the New Year. However, I'm not exactly taking a vacation from SAS programming because I'll be working to finish my forthcoming book, *Simulating Data with SAS*.

Thank you, readers, for an amazing year of blogging, interacting, and sharing ideas in 2012. Thanks to you, this blog has become the most widely-read blog at SAS!

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