# Useful code snippet for Python list fold/reduce

Going through one of the MOOC courses, I came across a homework problem that I solved with one of Python’s list reduce functions, any(), throwing in an interesting functional programming twist. I save it here for my own reference. I won’t mention which course this is, lest I give the answer away to one of its many interesting assignments. But from what I learned so far, it is a great course from Udacity.

For those interested, this code snippet also uses various Python data structures like lists and dictionaries, recursion, not to mention the nice comments on FSM (Finite State Machine), regular expression implementation, and the meaning of determinism. All in all, a pretty good example to illustrate various important concepts in computer science.

Regarding recursion usage in this code, I *think* it is going to be pretty efficient, because the recursive calls won’t create additional frames that the program has to keep track of. So no need for tail recursion.

[code language=”python”]
# Title: Simulating Non-Determinism

# Each regular expression can be converted to an equivalent finite state
# machine. This is how regular expressions are implemented in practice.
# We saw how non-deterministic finite state machines can be converted to
# deterministic ones (often of a different size). It is also possible to
# simulate non-deterministic machines directly — and we’ll do that now!
#
# In a given state, a non-deterministic machine may have *multiple*
# outgoing edges labeled with the *same* character.
#
# To handle this ambiguity, we say that a non-deterministic finite state
# machine accepts a string if there exists *any* path through the finite
# state machine that consumes exactly that string as input and ends in an
# accepting state.
#
# Write a procedure nfsmsim that works just like the fsmsim we covered
# together, but handles also multiple outgoing edges and ambiguity. Do not
# consider epsilon transitions.
#
# Formally, your procedure takes four arguments: a string, a starting
# state, the edges (encoded as a dictionary mapping), and a list of
# accepting states.
#
# To encode this ambiguity, we will change “edges” so that each state-input
# pair maps to a *list* of destination states.
#
# For example, the regular expression r”a+|(?:ab+c)” might be encoded like
# this:
edges = { (1, ‘a’) : [2, 3],
(2, ‘a’) : [2],
(3, ‘b’) : [4, 3],
(4, ‘c’) : [5] }
accepting = [2, 5]
# It accepts both “aaa” (visiting states 1 2 2 and finally 2) and “abbc”
# (visting states 1 3 3 4 and finally 5).

def nfsmsim(string, current, edges, accepting):
# fill in your code here
if string == “”:
return current in accepting
else:
letter = string[0]
# Is there a valid edge?
if (current, letter) in edges:
return any(nfsmsim(string[1:], i, edges, accepting) for i in edges[(current, letter)])
else:
return False

# This problem includes some test cases to help you tell if you are on
# the right track. You may want to make your own additional tests as well.

print “Test case 1 passed: ” + str(nfsmsim(“abc”, 1, edges, accepting) == True)
print “Test case 2 passed: ” + str(nfsmsim(“aaa”, 1, edges, accepting) == True)
print “Test case 3 passed: ” + str(nfsmsim(“abbbc”, 1, edges, accepting) == True)
print “Test case 4 passed: ” + str(nfsmsim(“aabc”, 1, edges, accepting) == False)
print “Test case 5 passed: ” + str(nfsmsim(“”, 1, edges, accepting) == False)
[/code]

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