Python MiniCourse

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Day 4 Lesson 14Lists

• Python Mini-Course
• University of Oklahoma
• Department of Psychology

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Lesson objectives

• Describe the characteristics of the list data
  structure in Python
• Perform basic operations with lists including
  creation, concatenation, repetition, slicing, and
  traversing
• Use string methods that require lists (join,
  split)
• Use lists in functions

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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, 33

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List indexing

• Elements within a list are indexed (see Lesson
  10)
• print cheeses0
• Lists are mutable
• cheeses0 'Feta'
• print cheeses

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Slicing a list

• Like strings and other sequences, lists can be
  sliced
• print cheeses14
• print cheeses2
• print cheeses2

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Changing a slice

• roster 'Meghan', 'Tricia', 'Juan',
• 'Alton', 'Darrel', 'Jen'
• print roster
• roster13 'Sam', 'Kerri'
• print roster
• roster35 'Tayla'
• print roster

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Inserting elements

• Slice notation
• roster22 'Dana', 'Ryan'
• print roster

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Deleting elements

• Set slice to empty list
• roster35
• print roster
• The del keyword
• del roster23
• 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
• a2 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 rosterindex
• 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 nested0
• print nested01

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Traversing nested lists

• for i in range(len(nested))
• for j in range(len(nestedi))
• print nestedij

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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 functionsonly_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