Writing Donald E. Knuth based code in Python, cont'd
Juhani Ylikoski
antti.ylikoski at elisanet.fi
Mon Nov 12 22:02:36 CET 2012
Following comes a working, debugged Python program which computes the
permutations of the integers 1, 2, 3 - n after Donald E. Knuth. I
present it as an example of writing straightforward, easy Knuth-based
code in a language with no GOTO statement.
The Python program has been written after the DFA construct I
previously discussed in this newsgroup, and after Knuth's discussion
of the solution of the problem; and according the (very good)
discussions in this newsgroup. To my opinion, it no more is a "crow's
nest" as they say in Finnish.
This program was needed for a real problem, namely computing optimal
tournament tables for a Bughouse (Tandem) chess tournament. See
http://en.wikipedia.org/wiki/Bughouse_chess
Knuth became criticized in the newsgroup; but to my opinion his books
are still useful and nontrivially needed.
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class DFA(object):
# Iteratively generate all permutations of n integers 1-n.
# After Donald Knuth, The Art of Computer Programming, Vol4, Fascicle 2,
# ISBN 0-201-85393-0, on Pages 39-40.
def __init__(self, n):
self.n = n
self.listofPerm = [] # list of lists to collect permutations
self.nextStat = self.E1 # next phase in Knuth's text
self.a = list(range(0, n+1)) # [0, 1, 2, 3, 4, ..., n] -- see Knuth
def E1(self): # Phase 1 in Knuth's text
self.app = self.listofPerm.append(self.a[1:self.n+1])
return self.E2 # next state: E2
def E2(self): # Phase 2 in Knuth's text
self.j = self.n - 1
while self.a[self.j] >= self.a[self.j+1]:
self.j -= 1
if self.j == 0:
return None # algorithm finishes
else:
return self.E3 # next state: E3
def E3(self): # Phase 3 in Knuth
self.l = self.n
while self.a[self.j] >= self.a[self.l]:
self.l -= 1
self.temp = self.a[self.j]
self.a[self.j] = self.a[self.l]
self.a[self.l] = self.temp
return self.E4 # next state: E4
def E4(self): # Phase 4
self.k = self.j + 1
self.l = self.n
while self.k < self.l:
self.temp = self.a[self.k]
self.a[self.k] = self.a[self.l]
self.a[self.l] = self.temp
self.k += 1
self.l -= 1
return self.E1 # following phase: Phase 1
def runDFA(self):
self.nextState = self.E1
while self.nextState is not None:
self.nextState = self.nextState()
return(self.listofPerm)
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yours sincerely, Antti J Ylikoski
Helsinki, Finland
PhD student in the Aalto University
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