[Matthew Wilson] > The random.jumpahead documentation says this: > > Changed in version 2.3: Instead of jumping to a specific state, n steps > ahead, jumpahead(n) jumps to another state likely to be separated by > many steps.. > > I really want a way to get to the Nth value in a random series started > with a particular seed. Is there any way to quickly do what jumpahead > apparently used to do?
No known way, and it seems unlikely that any quick way will be found in my lifetime ;-) for the Mersenne Twister. In Pythons <= 2.2, the underlying generator was the algebraically _very_ much simpler original Wichman-Hill generator, and jumping ahead by exactly n steps was just a matter of a few modular integer exponentiations. That took time proportional to log(n), so was extremely fast. It was also much more _necessary_ using W-H, since W-H's period was only around 10**13, while the Twister's period is around 10**6001: even if you're going to use a billion random numbers per sequence, the Twister's period has a way-way-beyond astronomical number of non-overlapping subsequences of that length. The chance of hitting an overlap is correspondingly miniscule. > I devised this function, but I suspect it runs really slowly: Well, it takes exactly as much time as it takes to call random() n times. > def trudgeforward(n): > '''Advance the random generator's state by n calls.''' > for _ in xrange(n): random.random() > > So any speed tips would be very appreciated. What are you trying to achieve in the end? Take it as a fact that there is no known way to advance the Twister by n states faster than what you've already found. -- http://mail.python.org/mailman/listinfo/python-list
