Day 4 Lesson 14 Lists Python MiniCourse University

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Day 4 – Lesson 14 Lists Python Mini-Course University of Oklahoma Department of Psychology

Day 4 – Lesson 14 Lists Python Mini-Course University of Oklahoma Department of Psychology 1 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Lesson objectives 1. Describe the characteristics of the list data structure in Python 2.

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 2 Python Mini-Course: Day 4 – Lesson 14 5/02/09

The list data structure In Python, a list is a mutable sequence of values

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 3 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Creating lists numbers = [1, 2, 3, 4] print numbers cheeses = ['swiss', 'cheddar',

Creating lists numbers = [1, 2, 3, 4] print numbers cheeses = ['swiss', 'cheddar', 'ricotta', 'gouda'] print cheeses 4 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Creating lists mixed = [1, 'a', 3. 45] print mixed single = ['z'] print

Creating lists mixed = [1, 'a', 3. 45] print mixed single = ['z'] print single, type(single) empty = [] print empty 5 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Repeating a list Use the * operator: meat = ['spam']*4 print meat print [1,

Repeating a list Use the * operator: meat = ['spam']*4 print meat print [1, 2, 3]*3 6 Python Mini-Course: Day 4 – Lesson 14 5/02/09

List indexing Elements within a list are indexed (see Lesson 10) print cheeses[0] Lists

List indexing Elements within a list are indexed (see Lesson 10) print cheeses[0] Lists are mutable cheeses[0] = 'Feta' print cheeses 7 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Slicing a list Like strings and other sequences, lists can be sliced print cheeses[1:

Slicing a list Like strings and other sequences, lists can be sliced print cheeses[1: 4] print cheeses[: 2] print cheeses[2: ] 8 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Changing a slice roster = ['Meghan', 'Tricia', 'Juan', 'Alton', 'Darrel', 'Jen'] print roster[1: 3]

Changing a slice roster = ['Meghan', 'Tricia', 'Juan', 'Alton', 'Darrel', 'Jen'] print roster[1: 3] = ['Sam', 'Kerri'] print roster[3: 5] = ['Tayla'] print roster 9 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Inserting elements Slice notation roster[2: 2] = ['Dana', 'Ryan'] print roster 10 Python Mini-Course:

Inserting elements Slice notation roster[2: 2] = ['Dana', 'Ryan'] print roster 10 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Deleting elements Set slice to empty list roster[3: 5] = [] print roster The

Deleting elements Set slice to empty list roster[3: 5] = [] print roster The del keyword del roster[2: 3] print roster 11 Python Mini-Course: Day 4 – Lesson 14 5/02/09

The insert and append methods The insert method roster. insert(2, 'Jakob') print roster The

The insert and append methods The insert method roster. insert(2, 'Jakob') print roster The append method roster. append('Tonya') print roster 12 Python Mini-Course: Day 4 – Lesson 14 5/02/09

The extend method Adds a list to the end of an existing list adds

The extend method Adds a list to the end of an existing list adds = ['Ian', 'Stacie'] roster. extend(adds) print roster 13 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Extending a list Can also use += operator roster += ['Anya'] print roster 14

Extending a list Can also use += operator roster += ['Anya'] print roster 14 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Using the + operator a = [1, 2, 3] b = [4, 5, 6]

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 15 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Note on list operations Be careful when using the + operator and append method

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 16 Python Mini-Course: Day 4 – Lesson 14 5/02/09

List assignment and aliasing a = [1, 2, 3, 4] b = a c

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 17 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Other list methods roster. sort() print roster. reverse() print roster 18 Python Mini-Course: Day

Other list methods roster. sort() print roster. reverse() print roster 18 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Other list methods print roster. index('Tonya') print roster. index('Tonya', 2, 5) print roster. count('Sam')

Other list methods print roster. index('Tonya') print roster. index('Tonya', 2, 5) print roster. count('Sam') roster. remove('Sam') print roster 19 Python Mini-Course: Day 4 – Lesson 14 5/02/09

The join string method Concatenates a sequence of strings into a single string with

The join string method Concatenates a sequence of strings into a single string with sep inserted between each item. Syntax: sep. join(list) 20 Python Mini-Course: Day 4 – Lesson 14 5/02/09

The split string method Returns a list of words from a string using sep

The split string method Returns a list of words from a string using sep as the delimiter string Syntax: sep. split(list) 21 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Example: join_split. py t = ['pining', 'for', 'the', 'fjords'] delimiter = '_' s =

Example: join_split. py t = ['pining', 'for', 'the', 'fjords'] delimiter = '_' s = delimiter. join(t) print s u = s. split(delimiter) print u 22 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Example print ''. join(t) print 't'. join(t) 23 Python Mini-Course: Day 4 – Lesson

Example print ''. join(t) print 't'. join(t) 23 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Traversing a list for index in range(len(roster)): print roster[index] for student in roster: print

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 24 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Traversing a list What does this do? empty = [] for x in empty:

Traversing a list What does this do? empty = [] for x in empty: print x 25 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Nested lists nested = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] print

Nested lists nested = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] print nested[0][1] 26 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Traversing nested lists for i in range(len(nested)): for j in range(len(nested[i])): print nested[i][j] 27

Traversing nested lists for i in range(len(nested)): for j in range(len(nested[i])): print nested[i][j] 27 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Traversing nested lists for nest in nested: for item in nest: print item 28

Traversing nested lists for nest in nested: for item in nest: print item 28 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Using lists: cumulate. py def cumulate(seq): c_sum = 0 for item in seq: c_sum

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 29 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Returning lists from functions: only_upper. py def only_upper(t): res = [] for s in

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 c. Ol. Ors Make for Easy Reading' secret = only_upper(text) print secret 30 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Modifying lists in functions In Python, arguments are passed by reference The parameter in

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 31 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Example: byref. py def change(seq): print 'Passed in: ' + str(seq) seq. append('new item')

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 32 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Example: byref 2. py def change(seq): print 'Passed in: ' + str(seq) seq. append('new

Example: byref 2. 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 33 Python Mini-Course: Day 4 – Lesson 14 5/02/09

Suggested exercises Exercise 10. 5 – Solving the "Birthday Paradox" by a Monte Carlo

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 34 Python Mini-Course: Day 4 – Lesson 14 5/02/09

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