Course Work

• Importance of Exams
• Preparing for Exams

Tips and Examples

• Read the Homework Assignments Carefully

New Material

• Sequences
• Dictionaries
• Dictionary Literals
• Getting Values from a Dictionary
• Empty Dictionaries
• Adding Entries to a Dictionary
• Changing a Dictionary Value
• Dictionaries Are Not Sequences
• Looping Through a Dictionary
• Tuples As Dictionary Values
• When To Use a Dictionary

Questions

Are there any questions before I begin?

Readings

If you have the textbook read Chapter 9, Dictionaries and Sets section 9.1, Dictionaries.

Homework 3

I have posted homework 3 here.

It is due this coming Sunday at 11:59 PM.

Late Penalty

You will lose 2 points for every day that your assignment is late.

Every Unix file has a time stamp that changes every time the file is modified

My scripts look at this time stamp to determine whether a submission is late.

If your assignment is not working by the due date, you can continue to work on it but you will lose a few points.

If your assignment is working, do not go back and change it, because that will change the time stamp and result in a late penalty.

Do Not Email Me about Missing Assignments

If you get an email from me saying an assignment is missing do not email me about this.

I get far too many emails.

Instead of sending me an email, fix the problem.

If you do not know how to fix it, post a question on Piazza or contact the Class Assistant.

I collect homework assignments and and check Class Exercises several times during the week.

I will check or collect your assignment later in the week.

Class Exercises

Scripts for each Class Exercise must have the correct name and be in the right directory or my scripts will not see them.

If my scripts do not see them, your score with be zero.

If you receive no email from me about your Class Exercise script, that means your script will get full credit, less any late penalty that applies.

Course Work

Importance of Exams

• Many people find exams difficult
• But the best way for me to know if you have really learned the material is by giving exams
• It is too easy to cheat on Class Exercises and homework assignments
• This is why the two exams account for 50% of your grade
• Too many students do well on the graded quizzes, Class Exercises and homework assignment ...
• only to do poorly on the exams
• This lowers their grade considerably
• The purpose of the Class Exercises and homework assignments is to give you hands on experience with the material
• And to better prepare you for the exams

Preparing for Exams

• Both the midterm and final exams follow the same format
• 60% of the points on the exams come from questions taken from the ungraded class quizzes
• The remaining 40% of the points come from 4 code questions
• These questions ask you to write short segments of Python code
• Two things will help you prepare for exams
  • Flash cards
  • Class Exercises and homework assignments
• Flash cards should be made for all questions from Ungraded Class Quizzes
• You should review them frequently throughout the semester
• Pay attention when you are working on the Class Exercises and homeworks
• Work on these assignments is teaching you how to write Python code
• This will come in handy for answering the code questions on the midterm and final

Tips and Examples

Read the Homework Assignments Carefully

• Homework assignments may ask you to do many things
• So you may need to read the assignment more than once
• A single reading might not be enough to understand what I want you to do
• Take the time to read these documents slowly
• This will help you understand all the details
• If you don't, you will make mistakes and lose points
• Technical work is detailed work
• So you must learn to read documents with a lot of detail
• If you do not understand something post a question on Piazza
• NEVER GUESS at the meaning of something I have written

Attendance

New Material

Sequences

• Sequences are Python objects that hold more than one value
• The values are stored one right after the other
• So the elements in a sequence have a definite order
• In IT 116 you encountered three types of sequences
  • Lists
  • Tuples
  • Strings
• A list contains a series of values that can be changed at any time
• A tuple is also a series of values but a tuple is immutable
• That means it cannot be changed
• A string is a sequence of characters which cannot be changed
• But you can make a new string from a previously created string
• Strings also have special methods that only make sense when applied to characters

Dictionaries

• Dictionaries can also contain multiple entries
• But each entry has two parts
• The first part is the key
• The second is a value associated with that key
• You use the key to get the value
• An entry in a Python dictionary is a key-value pair

Dictionary Literals

• When you create a list literal you enclose the individual values in square brackets, [   ] ...
• and put commas between the values

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

• With tuples the values are enclosed in parentheses, (   )
• Again, commas separate the values

  numbers = (1, 2, 3, 4, 5)

• When we create strings we enclose them in single or double quotes

  numbers = "12345"
  name    = 'Bob'

• In a dictionary literal we enclose the entries inside curly braces, {   }, with commas between the entries
• In each entry, there is a colon, :, between the key and the value

  email_addresses = {"Chris":"chris57@gmail.com" , "Alan":"ahurley175@yahoo.com", "Frank":"frank27@hotmail.com"}

• While keys are often strings they can be values of many different data types
• So we could create a dictionary of integers ...
• and the words for the integer value

  integer_words = {1:"one", 2:"two", 3:"three", 4:"four", 5:"five"}

Getting Values from a Dictionary

• How do we get the value associated with a key?
• To get a value from a list we put an index inside square brackets, [ ]

  >>> numbers
  [1, 2, 3, 4, 5]
  >>> numbers[0]
  1
  >>> numbers[1]
  2
  >>> numbers[4]
  5

• The square brackets are actually an operator
• When used with lists the square bracket operator takes two operands
• A list variable and an index
• The operator gives us the value at that index position
• You can use the square bracket operator with a dictionary
• But we use the key, not an index number, inside the brackets
• So to get the email address for "Chris" I would write

  >>> email_addresses["Chris"]
  'chris57@gmail.com'

• And to get the email address for "Frank" I would write

  >>> email_addresses["Frank"]
  'frank27@hotmail.com'

• We don't use indexes to get the values from a dictionary ...
• as we do with lists, tuples, and strings
• Instead we use the key to get the value
• If you use the [ ] operator on a key that doesn't exist ...
• you will get a KeyError exception

  email_addresses["Waldo"]
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  KeyError: 'Waldo'

• In computer science, a dictionary is sometimes called an associative array ...
• or a map ...
• or a hash

Empty Dictionaries

• A special kind of dictionary literal is an empty dictionary
• We often create a list by starting with an empty list ...
• and then adding entries to it
• Similarly, we often create a dictionary by starting out with an empty dictionary
• Just as the empty list is represented by [ ]
• And an empty tuple is represented by ( )
• { } is an empty dictionary

Adding Entries to a Dictionary

• Just as with lists, there are two ways to create a directory
  • Use a dictionary literal
  • Add entries to an empty dictionary
• The second method is more common
• We define an empty dictionary like this

  email_addresses = {}

• Dictionaries have no methods to add entries to an existing dictionary
• Instead you use the [ ] operator to create an entry
• You create a new entry in a dictionary using [ ] with a new key
• Notice that I said a new key
• Here is an example

  >>> email_addresses = {}                           # create empty dictionary
  >>> email_addresses["Alan"]  = "alanh@gmail.com"   # add first entry
  >>> email_addresses["Chris"] = "chrisk@yahoo.com"  # add second entry
  >>> email_addresses                                # print the dictionary
  {"Chris": "christhek@gmail.com", "Alan": "alanh@gmail.com"}

• Here is another example

  >>> words_integers = {}
  >>> words_integers["one"]   = 1
  >>> words_integers["two"]   = 2
  >>> words_integers["three"] = 3
  >>> words_integers["four"]  = 4
  >>> words_integers["five"]  = 5
  >>> words_integers
  {'one': 1, 'two': 2, 'three': 3, 'four': 4, 'five': 5}

• You have to be careful when adding entries like this
• You have to make sure that the key you use for the new entry ...
• is not already in the dictionary
• If the key is already in use you will not add a new entry
• You will just change the value associated with the old key
• So adding entries to a dictionary is very different from adding to a list
• When adding to a list we use the list method append
• But there no dictionary method for creating a new entry

Changing a Dictionary Value

• You change an element in a list using [ ] and the assignment operator
• Like this

  >>> numbers
  [1, 2, 3, 4, 5]
  >>> numbers[0] = 83
  >>> numbers
  [83, 2, 3, 4, 5]

• We can use the square bracket operators to do the same thing with dictionaries

  >>> email_addresses
  {'Alan': 'ahurley175@yahoo.com', 'Frank': 'frank27@hotmail.com', 'Chris': 'chris57@gmail.com'}
  >>> email_addresses["Alan"] = "alan.hurley@gmail.com"
  >>> email_addresses
  {'Alan': 'alan.hurley@gmail.com', 'Frank': 'frank27@hotmail.com', 'Chris': 'chris57@gmail.com'}

• Note that the way you add an entry to a dictionary ...
• is the same way you change an entry in a dictionary
• They both have the same format

  DICTIONARY_VARIABLE[KEY] = EXPRESSSION

• If the KEY is already in the dictionary ...
• the value associated with it will be changed
• Otherwise the dictionary will get a new entry
• We use the same [ ] operator to add an entry ...
• as we used to change the value of an entry
• When I first started using dictionaries I found this confusing
• When you see a Python statement like this

  >>> words_integers["six"]  = 6

• It could either mean I have created a new entry ...
• or I've changed an existing entry ...
• whose key is "six"

Dictionaries Are Not Sequences

• I started out this discussion by talking about sequences
• But dictionaries are not sequences
• The values in a sequence are stored one right after the other
• So you can pick out a specific value using an index ...
• which specifies its position in the sequence
• Dictionaries use a data structure called a hash table
• Hash tables minimize the time it takes to find the value for a key
• The order of things in a hash table changes with time ...
• as entries are created and deleted
• In earlier version of Python 3 you could see this ...
• because the order in which dictionary entries were created ...
• was not the order in which they appeared when printed
• This has changed
• So dictionaries work different in the latest versions of Python
• The order in which entries are made is now the order in which they are now printed
• But that order is not the order in which the entries are stored
• So dictionaries are still not sequence
• I'll talk more about this in the next class

Looping Through a Dictionary

• The Python for loop has the following format

  for LOOP_VARIABLE in LIST_LIKE_THING:
      STATEMENT
      STATEMENT
      ...

• Where the LOOP_VARIABLE is assigned each value in the LIST_LIKE_THING ...
• for each pass through the loop

  >>> nonsense
  ('foo', 'bar', 'bletch')
  >>> for word in nonsense:
  ...   print(word)
  ... 
  foo
  bar
  bletch
  >>> for number in [1, 2, 3, 4, 5]:
  ...    print(number)
  ... 
  1
  2
  3
  4
  5
  >>> for char in "Hello":
  ...    print(char)
  ... 
  H
  e
  l
  l
  o

• But dictionaries are not sequences
• So you might think that dictionaries cannot be used with for loops
• But you would be wrong

  >>> for entry in email_addresses:
  ...    print(entry)
  ... 
  Chris
  Frank
  Alan

• In a for loop on a dictionary the loop variable does not get the value of the elements
• Instead, the loop variable gets the value of each key
• Of course we can use these keys to get their associated value

  >>>  for key in email_addresses:
  ...    print(key, "=", email_addresses[key])
  ... 
  Chris = chris57@gmail.com
  Frank = frank27@hotmail.com
  Alan = ahurley175@yahoo.com

Tuples As Dictionary Values

• What if I wanted to store more than one piece of information ...
• as the value for a given key?
• For example, say I wanted to create a dictionary with data for each student in a class
• The key would be the student ID
• But I would need to store the following data
  • First name
  • Last name
  • Unix username
  • Email
• Why first name and last name instead of just a student's name?
• Because I want to produce alphabetical lists
• To store this information do I need 4 dictionaries?
• No
• I can store all this data in one dictionary using the student ID as the key
• But the value will be a tuple containing all information about the student
• At the beginning of the semester I used to create a text file with data for each student
• The file looked something like this

  09329034 Jane        Adams      jadams   Jane.Adams001@umb.edu
  03687480 Alexander   Smith      bigboy   Alexander.Smith002@umb.ed
  05431692 Christopher Cannon     ccannon  Christopher.Cannon001@umb.edu
  07511379 Joseph      Malloney   jmal     Joseph.Malloney003@umb.edu
  07511379 Fatih       Jones      fjones   Fatih.Jones001@umb.edu
  04175276 James       Reynolds   jr       jim.reynolds002@umb.edu

• This is not real student data
• Confidentiality prevents me from doing that
• Here is a function that will read each line and return dictionary with the student information

  def student_dictionary_create(filename):
      try:
          file = open(filename,"r")
      except:
          print("Cannot open", filename)
      else:
          data = {}
          for line in file:
              data_list  = line.split()
              student_id = data_list[0]
              first_name = data_list[1]
              last_name  = data_list[2]
              username   = data_list[3]
              email      = data_list[4]
              student_tuple = (first_name, last_name, username, email)
              data[student_id] = student_tuple
          return data

• When I use this function and print the results

  student_data = student_dictionary_create("student_data.txt")
  for id in student_data:
      student_tuple = student_data[id]
      print(id, student_tuple[0], student_tuple[1], student_tuple[2], student_tuple[3])

• I get

  09329034 Jane Adams jadams Jane.Adams001@umb.edu
  03687480 Alexander Smith bigboy Alexander.Smith002@umb.ed
  05431692 Christopher Cannon ccannon Christopher.Cannon001@umb.edu
  07511379 Fatih Jones fjones Fatih.Jones001@umb.edu
  04175276 James Reynolds jr jim.reynolds002@umb.edu

When To Use a Dictionary

• When should we use a dictionary instead of a list or tuple?
• Lists and tuples are used when the elements are not particularly unique
• If we create a list of quiz scores there is nothing special about a particular score
• Generally you don't need to know the score for the 5th quiz
• But you do need a single object to hold all the scores ...
• so you can do certain things ...
• like finding the average score
• But every student is different
• Each one has a unique identity
• When you need to store information about unique things ...
• you should use a dictionary
• Or when you need to associate two things ...
• like a name and an email address

Class Exercise

Class Quiz