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#!/usr/bin/env python
letters = 'abcdefghijklmnopqrstuvwxyz '
def file2prob(filename):
"""
Read a file and return a dictionary of letters and
their probabilities
"""
letter_dict = { c: 0 for c in letters }
letter_total = 0
with open(filename) as fp:
for c in fp.read():
if c.lower() not in letter_dict:
continue
letter_dict[c.lower()] += 1
letter_total += 1
probs = { c: letter_dict[c]/letter_total for c in letter_dict }
return probs
def file2pairs(filename):
"""
Read a file and return a dictionary of letters and
the probabilities of following letters. That is, the
conditional probability of a letter given its
predecessor.
"""
letter_dict = { c: { a: 0 for a in letters }
for c in letters }
previous = None
with open(filename) as fp:
for c in fp.read():
if c not in letter_dict:
continue
c = c.lower()
if previous is None:
previous = c
continue
letter_dict[previous][c] += 1
previous = c
probs = { c: { d: letter_dict[c][d]/sum(letter_dict[c].values())
for d in letters } for c in letters }
return probs
if __name__ == '__main__':
import sys
if len(sys.argv) != 3:
print("Usage: %s filename letter" % sys.argv[0])
sys.exit(-1)
filename = sys.argv[1]
letter = sys.argv[2]
probs = file2prob(filename)
print(probs[letter])