Transcript Day 4 – Lesson 14 Lists Python Mini-Course University of Oklahoma Department of Psychology Python Mini-Course: Day 4 – Lesson 14 5/02/09

Day 4 – Lesson 14
Lists
Python Mini-Course
University of Oklahoma
Department of Psychology
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Python Mini-Course: Day 4 – Lesson 14
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Lesson objectives
1. Describe the characteristics of the
list data structure in Python
2. Perform basic operations with lists
including creation, concatenation,
repetition, slicing, and traversing
3. Use string methods that require lists
(join, split)
4. Use lists in functions
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Python Mini-Course: Day 4 – Lesson 14
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The list data structure
 In Python, a list is a mutable
sequence of values
 Each value in the list is an element
or item
 Elements can be any Python data
type
 Lists can mix data types
 Elements can be nested lists
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Creating lists
numbers = [1, 2, 3, 4]
print numbers
cheeses = ['swiss', 'cheddar',
'ricotta', 'gouda']
print cheeses
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Creating lists
mixed = [1, 'a', 3.45]
print mixed
single = ['z']
print single, type(single)
empty = []
print empty
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Repeating a list
 Use the * operator:
meat = ['spam']*4
print meat
print [1, 2, 3]*3
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List indexing
 Elements within a list are indexed
(see Lesson 10)
print cheeses[0]
 Lists are mutable
cheeses[0] = 'Feta'
print cheeses
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Slicing a list
 Like strings and other sequences,
lists can be sliced
print cheeses[1:4]
print cheeses[:2]
print cheeses[2:]
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Changing a slice
roster = ['Meghan', 'Tricia', 'Juan',
'Alton', 'Darrel', 'Jen']
print roster
roster[1:3] = ['Sam', 'Kerri']
print roster
roster[3:5] = ['Tayla']
print roster
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Inserting elements
 Slice notation
roster[2:2] = ['Dana', 'Ryan']
print roster
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Deleting elements
 Set slice to empty list
roster[3:5] = []
print roster
 The del keyword
del roster[2:3]
print roster
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The insert and append methods
 The insert method
roster.insert(2,'Jakob')
print roster
 The append method
roster.append('Tonya')
print roster
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The extend method
 Adds a list to the end of an
existing list
adds = ['Ian', 'Stacie']
roster.extend(adds)
print roster
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Extending a list
 Can also use += operator
roster += ['Anya']
print roster
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Using the + operator
a = [1, 2, 3]
b = [4, 5, 6]
c = a + b
print a, b, c
*The + operator returns a new list that
is a concatenation of two lists
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Note on list operations
 Be careful when using the +
operator and append method
 Try this:
d = c + 7
 Or this
c.append(b)
print c
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List assignment and aliasing
a = [1, 2, 3, 4]
b = a
c = a[:]
a[2] = 9
print a, b, c
*The slice operator returns a copy of a list
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Other list methods
roster.sort()
print roster
roster.reverse()
print roster
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Other list methods
print roster.index('Tonya')
print roster.index('Tonya', 2, 5)
print roster.count('Sam')
roster.remove('Sam')
print roster
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The join string method
 Concatenates a sequence of
strings into a single string with
sep inserted between each item.
 Syntax: sep.join(list)
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The split string method
 Returns a list of words from a
string using sep as the delimiter
string
 Syntax: sep.split(list)
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Example: join_split.py
t = ['pining', 'for', 'the', 'fjords']
delimiter = '_'
s = delimiter.join(t)
print s
u = s.split(delimiter)
print u
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Example
print ''.join(t)
print ' '.join(t)
print '\t'.join(t)
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Traversing a list
for index in range(len(roster)):
print roster[index]
for student in roster:
print student
for index, student in enumerate(roster):
print index, student
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Traversing a list
 What does this do?
empty = []
for x in empty:
print x
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Nested lists
nested = [[1,2,3],[4,5,6],[7,8,9]]
print nested
print nested[0]
print nested[0][1]
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Traversing nested lists
for i in range(len(nested)):
for j in range(len(nested[i])):
print nested[i][j]
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Traversing nested lists
for nest in nested:
for item in nest:
print item
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Using lists: cumulate.py
def cumulate(seq):
c_sum = 0
for item in seq:
c_sum += item
return c_sum
a = [12, 78, 32, 82]
s = cumulate(a)
print s
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Returning lists from functions:
only_upper.py
def only_upper(t):
res = []
for s in t:
if s.isupper():
res.append(s)
return res
text = 'Bold cOlOrs Make for Easy Reading'
secret = only_upper(text)
print secret
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Modifying lists in functions
 In Python, arguments are passed
by reference
 The parameter in the function is an
alias for the argument that was
passed in
 If a mutable object is changed inside
the function, it is also changed
outside the function
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Example: byref.py
def change(seq):
print 'Passed in: ' + str(seq)
seq.append('new item')
print 'Changed to: ' + str(seq)
original = [1, 2, 3]
print original
change(original)
print original
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Example: byref2.py
def change(seq):
print 'Passed in: ' + str(seq)
seq.append('new item')
print 'Changed to: ' + str(seq)
new_seq = ['created','in','function']
print 'New seq: ' + str(new_seq)
original = [1, 2, 3]
new_seq = ['outside','the','function']
print original
change(original)
print original
print new_seq
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Suggested exercises
 Exercise 10.5 – Solving the
"Birthday Paradox" by a Monte
Carlo simulation
 Exercise 10.6 – Removing
duplicates from a list
 Exercise 10.8 – Bisection search
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