Transcript LING 681 Intro to Comp Ling

Q9
Structured programming
Day 31
LING 681.02
Computational Linguistics
Harry Howard
Tulane University
Course organization
 http://www.tulane.edu/~ling/NLP/
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Structured programming
NLPP §4
Assignment 1
>>> foo = 'Monty'
>>> bar = foo
>>> foo = 'Python'
>>> bar
'Monty'
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Assignment 2
>>> foo = ['Monty', 'Python']
>>> bar = foo
>>> foo[1] = 'Bodkin'
>>> bar
['Monty', 'Bodkin']
 Why?
 The second line copies a reference to the object 'bar',
not its content.
 Use id() to find the numerical identifier of each
variable.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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List assignment and
computer memory
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Assignment 3
>>> empty = []
>>> nested = [empty, empty, empty]
>>> nested
[[], [], []]
>>> nested[1].append('Python')
>>> nested
[['Python'], ['Python'], ['Python']]
 The third line creates a nested list,
 in which each element is a reference to the same list.
 use id() to find the identifier of each one.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Assignment 4
>>> nested = [[]] * 3
>>> nested[1].append('Python')
>>> nested[1] = ['Monty']
>>> nested
[['Python'], ['Monty'], ['Python']]
 Shows the difference between modifying an via an object
reference and overwriting an object reference.
 Use id() to find the identifier of each one.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Equality
>>> size = 5
>>> python = ['Python']
>>> snake_nest = [python] * size
>>> snake_nest[0] == snake_nest[1] ==
snake_nest[2] == snake_nest[3] == snake_nest[4]
True
>>> snake_nest[0] is snake_nest[1] is
snake_nest[2] is snake_nest[3] is snake_nest[4]
True
 is tests for object identity.
 Check by:
 [id(snake) for snake in snake_nest]
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Equality 2
>>> import random
>>> position = random.choice(range(size))
>>> snake_nest[position] = ['Python']
>>> snake_nest
[['Python'], ['Python'], ['Python'], ['Python'], ['Python']]
>>> snake_nest[0] == snake_nest[1] == snake_nest[2] ==
snake_nest[3] == snake_nest[4]
True
>>> snake_nest[0] is snake_nest[1] is snake_nest[2] is
snake_nest[3] is snake_nest[4]
False
 Find the interloper:
 [id(snake) for snake in snake_nest]
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Conditionals
>>> mixed = ['cat', '', ['dog'], []]
>>> for element in mixed:
...
if element:
...
print element
...
cat
['dog']
 In the condition part of an if statement, a
nonempty string or list is evaluated as true, while
an empty string or list evaluates as false.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Conditionals 2
 What's the difference between using if...elif as opposed to using a couple of if statements
in a row?
>>> animals = ['cat', 'dog']
>>> if 'cat' in animals:
...
print 1
... elif 'dog' in animals:
...
print 2
...
1
 Since the if clause of the statement is satisfied, Python never tries to evaluate the elif
clause, so we never get to print out 2.
 By contrast, if we replaced the elif by an if, then we would print out both 1 and 2.
 So an elif clause potentially gives us more information than a bare if clause; when it
evaluates to true, it tells us not only that the condition is satisfied, but also that the
condition of the main if clause was not satisfied.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Universal quantification
>>> sent = ['No', 'good', 'fish', 'goes',
'anywhere', 'without', 'a', 'porpoise',
'.']
>>> all(len(w) > 4 for w in sent)
False
>>> any(len(w) > 4 for w in sent)
True
 The functions all() and any() can be applied to a
list (or other sequence) to check whether all or any
items meet some condition.
09-Nov-2009
LING 681.02, Prof. Howard, Tulane University
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Next time
Continue §4