--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/ch5ex6-2.py	Wed Jan 09 20:51:16 2013 -0600
@@ -0,0 +1,50 @@
+"""Chapter 5.5, exercise 6 in Allen Downey's Think Complexity book.
+
+2. Use the WS model to generate the largest graph you can in a reasonable amount
+   of time. Plot P(k) versus k and see if you can characterize the tail
+   behavior.
+
+"""
+import sys
+
+from matplotlib import pyplot
+
+from Graph import Vertex
+from SmallWorldGraph import SmallWorldGraph
+
+
+
+def main(script, n, k, p):
+
+    # create a SmallWorldGraph with n vertices, k regular edges between
+    # vertices, and rewiring probability p.
+
+    n, k, p = int(n), int(k), float(p)
+    vs = [Vertex() for i in xrange(n)]
+
+    g = SmallWorldGraph(vs, k, p)
+
+    # retrieve probabilities
+    p = g.get_p()
+
+    # plot P(k) versus k on a log-log scale
+
+    vals = p.items()
+    vals.sort(key=lambda t: t[0])
+    x, y = zip(*vals)
+
+    assert abs(sum(y) - 1.0) < 1e-6
+
+    pyplot.clf()
+    pyplot.xscale('log')
+    pyplot.yscale('log')
+    pyplot.title('P(k) versus k')
+    pyplot.xlabel('k')
+    pyplot.ylabel('P(k)')
+    pyplot.plot(x, y, label='P(k) vs. k', color='green', linewidth=3)
+    pyplot.legend(loc='upper right')
+    pyplot.show()
+
+
+if __name__ == '__main__':
+    main(*sys.argv)