CSCI/CMPE 4341 Topic: Programming in Python�Chapter 6: Lists, and Dictionaries

Xiang Lian

The University of Texas – Pan American

Edinburg, TX 78539

lianx@utpa.edu

1

Objectives

• In this chapter, you will:
• Understand Python sequences
• Learn the list AND dictionary data types
• Know how to create, initialize, and refer to individual elements of lists, and dictionaries
• Explore how to use lists to sort and search sequences of values
• Learn how to pass lists to functions
• Become familiar with list and dictionary methods

2

Introduction

• Data structures
• Structures that hold and organize information
• Sequences
• Also known as arrays in other languages (e.g., C++)
• Store related data types
• 3 types
• Strings
• Lists
• Tuples
• Mappings
• In most languages called hashes or associative arrays
• Dictionaries are the only python mapping container
• Key-value pairs

3

Sequences

• Sequences
• Series of items that are often related
• Strings are sequences in Python
• A sequence is returned by the range function

​

4

Example of Sequences

Name sequence (C)

Position number of the element within sequence C

5

Sequences (cont'd)

• Elements
• The individual items in each sequence
• Referred to by subscripts
• Obtain one by using sequenceName[ subscript ]
• The subscript of the first element is 0
• The subscript of the last element can be -1

​

6

Creating Sequences – String

• Strings
• Use quotes
• string1 = "hello"
• Empty string
• string2 = ""

​

7

Creating Sequences – List

• Lists
• Use brackets
• Separate multiple items with a comma
• list1 = [1, 2, 3, 4]
• Empty list
• list2 = []
• Length of the list
• len (list1)

8

Lists

• Not restricted to values of the same type
• Programmers use lists to hold data that is of the same type
• The length is usually not predetermined and can vary throughout the program
• Accessing elements out of range
• Python exits and an out of range error is displayed

​

9

Fig05_03.py

# Fig. 5.3: fig05_03.py

# Creating, accessing and changing a list.

aList = [] # create empty list

# add values to list

for number in range( 1, 11 ):

aList += [ number ]

print ("The value of aList is:", aList)

# access list values by iteration

print ("\nAccessing values by iteration:")

for item in aList:

print (item, end=" ")

​

print ()

​

# access list values by index

print ("\nAccessing values by index:")

print ("Subscript Value")

​

for i in range( len( aList ) ):

print ("%9d %7d" % ( i, aList[ i ] ))

​

# modify list

print ("\nModifying a list value...")

print ("Value of aList before modification:", aList)

aList[ 0 ] = -100

aList[ -3 ] = 19

print ("Value of aList after modification:", aList)

​

10

© 2002 Prentice Hall.�All rights reserved.

Outline

Syntax Error When Using Lists

Python 3.4 (#26, Nov 16 2001, 11:44:11) [MSC 32 bit (Intel)] on win32

Type "help", "copyright", "credits" or "license" for more information.

>>> aList = [ 1 ]

>>> print (aList[ 13 ])

Traceback (most recent call last):

File "<stdin>", line 1, in ?

IndexError: list index out of range

Fig. 5.4 Out-of-range error.

11

Fig05_05.py

# Fig. 5.5: fig05_05.py

# Creating a histogram from a list of values.

values = [] # a list of values

​

# input 10 values from user

print ("Enter 10 integers:")

​

for i in range( 10 ):

newValue = int( input( "Enter integer %d: " % ( i + 1 ) ) )

values += [ newValue ]

# create histogram

print ("\nCreating a histogram from values:" )

print ("%s %10s %10s" % ( "Element", "Value", "Histogram" ) )

for i in range( len( values ) ):

print ("%7d %10d %s" % ( i, values[ i ], "*" * values[ i ] ))

​

12

© 2002 Prentice Hall.�All rights reserved.

Outline

Dictionaries

• Key-value pairs
• Unordered collection of references
• Each value is referenced through key in the pair
• Curley braces ({}) are used to create a dictionary
• When entering values
• Use { key1:value1, … }
• Keys must be immutable values
• strings, numbers and tuples
• Values can be of any Python data type

13

Operations in Dictionaries

• Access dictionary element
• dictionaryName [key]
• dictionaryName [key] = value

# update the value of an existing key

• Add key-value pair
• dictionaryName [newKey] = newValue
• Delete key-value pair
• del dictionaryName [key]

14

Fig05_09.py

# Fig. 5.09: fig05_09.py

# Creating, accessing and modifying a dictionary.

# create and print an empty dictionary

emptyDictionary = {}

print ("The value of emptyDictionary is:", emptyDictionary )

# create and print a dictionary with initial values

grades = { "John": 87, "Steve": 76, "Laura": 92, "Edwin": 89 }

print ("\nAll grades:", grades )

# access and modify an existing dictionary

print ("\nSteve's current grade:", grades[ "Steve" ] )

grades[ "Steve" ] = 90

print ("Steve's new grade:", grades[ "Steve" ] )

# add to an existing dictionary

grades[ "Michael" ] = 93

print ("\nDictionary grades after modification:" )

print (grades )

# delete entry from dictionary

del grades[ "John" ]

print ("\nDictionary grades after deletion:" )

print (grades)

15

© 2002 Prentice Hall.�All rights reserved.

Outline

List and Dictionary Methods

• Method
• A function that performs the behaviors of an object
• List methods
• append - add values on to the end of a list
• Dictionary methods
• Allows manipulation of the items just as in a list

​

16

List Methods

17

Fig05_13.py

# Fig. 5.13: fig05_13.py

# Dictionary methods.

monthsDictionary = { 1 : "January", 2 : "February", 3 : "March",

4 : "April", 5 : "May", 6 : "June", 7 : "July",

8 : "August", 9 : "September", 10 : "October",

11 : "November", 12 : "December" }

print ("The dictionary items are:")

print (monthsDictionary.items() )

print ( "\nThe dictionary keys are:")

print ( monthsDictionary.keys() )

print ("\nThe dictionary values are:")

print (monthsDictionary.values() )

print ("\nUsing a for loop to get dictionary items:")

for key in monthsDictionary.keys():

print ("monthsDictionary[", key, "] =", monthsDictionary[key])

18

© 2002 Prentice Hall.�All rights reserved.

Outline

Dictionary Methods

19

Dictionary Methods (cont'd)

20

References and Reference Parameters

• Pass-by-value
• Copies the value and passes the copy
• Pass-by-reference
• Allows a function to access the caller data and to modify it
• Can increase performance
• Prevents the copying of large data
• Can weaken security
• Allows direct access to the data
• Pass by object reference
• Only thing allowed in Python
• Combination of pass-by-value and pass-by-reference
• Can modify mutable objects and not immutable objects

21

Passing Lists to Functions

• Passing a list
• The same applies for other mutable objects in Python
• To pass a list, pass it without its brackets
• This allows the entire list to be changed
• Items in the list that are immutable (numbers or strings) cannot be changed by the function when passed individually

22

Fig05_16.py

# Fig. 5.16: fig05_16.py

# Passing lists and individual list elements to functions.

def modifyList( aList ):

for i in range( len( aList ) ):

aList[ i ] *= 2

def modifyElement( element ):

element *= 2

aList = [ 1, 2, 3, 4, 5 ]

print ("Effects of passing entire list:")

print ("The values of the original list are:")

for item in aList:

print (item,end=" ")

modifyList( aList )

print ("\n\nThe values of the modified list are:")

for item in aList:

print (item,end=" ")

print ("\n\nEffects of passing list element:")

print ("aList[ 3 ] before modifyElement:", aList[ 3 ] )

modifyElement( aList[ 3 ] )

print ("aList[ 3 ] after modifyElement:", aList[ 3 ] )

print ("\nEffects of passing slices of list:")

print ("aList[ 2:4 ] before modifyList:", aList[ 2:4 ] )

modifyList( aList[ 2:4 ] )

print ("aList[ 2:4 ] after modifyList:", aList[ 2:4 ] )

​

23

© 2002 Prentice Hall.�All rights reserved.

Outline

Sorting and Searching Lists

• Sorting a list
• Use the sort method
• Searching a list
• Use the index method

​

24

Fig05_17.py������������Program Output

# Fig. 5.17: fig05_17.py

# Sorting a list.

aList = [ 2, 6, 4, 8, 10, 12, 89, 68, 45, 37 ]

print ("Data items in original order")

for item in aList:

print (item, end=" ")

aList.sort()

print ("\n\nData items after sorting")

for item in aList:

print (item, end=" ")

print()

25

Data items in original order

2 6 4 8 10 12 89 68 45 37

Data items after sorting

2 4 6 8 10 12 37 45 68 89

© 2002 Prentice Hall.�All rights reserved.

Outline

Fig05_18.py���������Program Output

# Fig. 5.18: fig05_18.py

# Searching a list for an integer.

# Create a list of even integers 0 to 198

aList = range( 0, 199, 2 )

searchKey = int(input( "Enter integer search key: " ) )

if searchKey in aList:

print ("Found at index:", aList.index( searchKey ) )

else:

print ("Value not found")

​

26

Enter integer search key: 36

Found at index: 18

Enter integer search key: 37

Value not found

© 2002 Prentice Hall.�All rights reserved.

Outline

27