Objectives

1
Objectives

• More on Dictionaries
• Loose ends
• Using/improving documentation
• Default parameter values

2
Dictionaries in Python

• Map keys to values
• Keys are not necessarily alphabetized
• Mappings are from one key to one or more values
• Keys are unique, Values are not necessarily
  unique
• Example student ids --gt last names
• Keys must be immutable (numbers, strings)
• Similar to Hashtables/Hashmaps in other languages

How would we handle if there is more than one
value for a key?
3
Why Dictionaries?

• Useful for fast lookups
• Lookup by keyword
• Dont need to look through entire list or sort
  list first

4
Examples of Dictionaries

• What are the keys and values for
• Dictionary
• Textbooks index
• Cookbook
• URL (Uniform Resource Locator)
• Examples from class
• Function name --gt function definition
• Variable name --gt value
• ASCII value --gt character

5
Creating Dictionaries in Python

• Syntax
• ltkeygtltvaluegt, , ltkeygtltvaluegt

empty ascii a97, b98, c99, ,
z122
6
Accessing Values using Keys

• Syntax
• ltdictionarygtltkeygt
• Examples
• KeyError if key is not in dictionary

asciiz directoryregistrar
7
Adding/Modifying Key-Value Pairs

• Syntax
• ltdictionarygtltkeygt ltvaluegt
• asciia 97

ascii_dictionary.py
8
Dictionary Operations
Unlike strings and lists, doesnt make sense to
do slicing, concatenation, repetition for
dictionaries
9
Dictionary Methods
10
Special Value None

• Special value we can use
• E.g., Return value from function when there is an
  error
• Similar to null in Java
• If you execute
• list list.sort()
• print list
• Prints None because list.sort() does not return
  anything

11
Example Problem

• Given a file of the form
• ltlastnamegt ltmajorgt
• Create a mapping between the last names and majors

majors_dictionary.py
12
Example Problem

• Modify the previous program to keep track of the
  number of majors of each type
• Could we solve this using a list?

majors_dictionary2.py
13
Getting Documentation

• dir function that returns a list of methods and
  attributes in an object
• dir(lttypegt)
• help get documentation
• In the Python shell
• help(lttypegt)
• import ltmodulenamegt
• help(ltmodulenamegt)

14
Where is Documentation Coming From?

• Comes from the code itself in doc strings
• i.e., documentation strings
• Doc strings are simply strings after the function
  header
• Typically use triple-quoted strings because
  documentation goes across several lines

def verse(animal, sound) prints a verse of
Old MacDonald, filling in the strings for animal
and sound
15
Defaults for Parameters

• Saw with the xrange function
• Didnt have to specify start or increment when
  calling the function
• Default start to 0
• Default increment to 1
• Can assign a default value to a parameter
• In general, default parameter should come after
  all the parameters that need to be defined

16
Using Default Parameters

• By default the rollDie function could assume that
  a die has 6 sides

Assigns a value ONLY IF not passed a parameter
def rollDie(sides6) return random.randint(1,si
des)
Examples of calling the function
rollDie(6) rollDie() rollDie(12)
game.py
17
Problem Student Majors

• We want to keep track of the number of majors of
  each type
• Twist Not every student has a major (dont
  declare until sophomore year)

18
Problem Student Majors, revised

• Students can have more than one major
• Should count these separately
• How can we modify the previous program to do that?

19
Problem Music Files

• We have an album file that has the format
• ltArtist namegt
• ltAlbum namegt
• ltSong name 1gt
• ltSong length 1gt
• ltSong name ngt
• ltSong length ngt
• Given an album file, print out the number of
  songs and the total length of the album

Length has the format minseconds