General Advice

• Learn to Read Carefully

Course Work

• Graded Quiz Today
• Late Penalty
• Do Not Email Me about Missing Assignments
• Importance of Exams
• Preparing for Exams

Tips and Examples

• Do Not Fall Behind in This Course
• 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.

Today's Class

Today I will talk about dictionaries, a new king of object that holds multiple values and can be very useful.

General Advice

Learn to Read Carefully

• Most of us spend a large part of each day reading
• Most of this reading is done quickly ...
• because the material is familiar ...
• and the details are not that important
• Most of the time when we read, we know what to expect
• For example, if you get a text from a friend ...
• you don't linger over each word
• You have some idea of what your friend is going to say ...
• so you can read it quickly
• The same thing happens if you read the about your favorite singer
• The details aren't important ...
• just the overall picture
• The reading you must do for this course ...
• is fundamentally different
• Technical work is detailed work
• If you read technical material too quickly ...
• you will miss important details
• You must learn to read technical material slowly and carefully
• You will sometime have to read something a second or third time ...
• until you fully understand it
• Learning how to read technical material ...
• may be the most important thing you take away from this class

Course Work

Graded Quiz Today

• After I finish my lecture today I will hand out papers for today's graded quiz
• Write your name clearly at the top
• When you finish the Quiz hand it to me
• Then you can work on the Class Exercise

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 date 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
• Any changes you make will change the date stamp ...
• which will cost you points

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
  • Come to Zoom Office Hours
  • Post a question of Piazza
  • Contact the Class Assistant
• I collect homework assignments and check Class Exercises several times during the week
• I will check or collect your assignment later in the week.

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

Do Not Fall Behind in This Course

• Technical courses require that you master a great deal of detail
• You will need to study for this course several times during the week
• Studying once a week is usually not enough
• The key to success in a technical course is steady, consistant, work
• It should be spread out over several days
• Some important concepts do not sink in immediately
• It may take several days for something new to become clear
• Material in this course often depends on what came before
• So it is critical that you not fall behind
• If you fall behind, it can be VERY hard to catch up

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 in memory 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"}

• In the first entry, "Chris" is the key ...
• and "chris57@gmail.com" is the value
• 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 variable pointing to a list 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 = {}

• To add value to a list we use the append method
• But dictionaries have no methods to add entries
• Instead 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
• I took me a long time to get use to this ...
• when I first started using dictionaries

Changing a Dictionary Value

• You change an element in a list using [ ] ...
• with an index number inside the brackets ...
• 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 differently in the latest versions of Python
• The order in which entries are made ..
• is now the order in which they are printed
• But that order is not the order in which the entries are stored ...
• in RAM
• 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
• You can use a for loop with tuples

  >>> nonsense
  ('foo', 'bar', 'bletch')
  >>> for word in nonsense:
  ...   print(word)
  ... 
  foo
  bar
  bletch

• With lists

  >>> for number in [1, 2, 3, 4, 5]:
  ...    print(number)
  ... 
  1
  2
  3
  4
  5

• And strings

  >>> 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

• When running 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