Three dumb questions (ordered by dumbness descending)

Thorsten Kampe thorsten at thorstenkampe.de
Mon Sep 23 23:03:21 CEST 2002


Okay, here they are:

1. Why is 'zip(zip(x)) != x' (should be the same because it's the 
transposed)


2. Peter Norvig mentions in "Python for Lisp Programmers" some 
"don'ts": "[x] + y" and "x[1:]". Are there more things to avoid 
(especially in a loop)?


3. random.shuffle: the documentation says: "Note that for even rather 
small len(x), the total number of permutations of x is larger than the 
period of most random number generators; this implies that most 
permutations of a long sequence can never be generated." I translate 
this to "don't shuffle lists with more than 15 elements if you want a 
randomly ordered list".

Alex Martelli says the same: "For example, Python users who are 
calling random.shuffle on a sequence of substantial length have a need 
for an underlying random generator with a *HUGE* period, as a 
necessary although not sufficient condition towards ensuring that all 
permutations of the shuffled sequence get generated with equal 
likelihood.

Wichman-Hill has a period of less than 7,000 billions (7e12),
insufficient to account even for the permutations of a sequence
whose length is just 16 (16! > 2e13)."

Otherwise in the "Python Cookbook" (Selecting Random Elements from a 
List Without Repetition) the shuffle version is called "the winner".

So: can I use random.shuffle or do I have to write my own - slower - 
version to get a random list?


Thorsten



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