Review

• Time for Computer Operations
• Reading a File into a List
• Advantages of Using Lists with Files
• Problems with Copying Objects
• Copying Lists
• Functions That Return Objects
• Functions That Work with Lists
• Functions That Change Lists

New Material

• List Elements
• Two-Dimensional Lists
• Tuples
• The tuple Conversion Function
• Why Use Tuples?
• Tic-Tac-Toe
• Tic-Tac-Toe - Step 1
• Tic-Tac-Toe - Step 2
• Tic-Tac-Toe - Step 3
• Tic-Tac-Toe - Step 4
• Tic-Tac-Toe - Step 5
• Tic-Tac-Toe - Step 6
• Tic-Tac-Toe - Step 7
• Privacy

Graded Quiz 10

You can connect to Gradescope to take weekly graded quiz today during the last 15 minutes of the class.

Once you start the quiz you have 15 minutes to finish it.

This will be the last quiz.

Final Exam

The final exam will be held on Thursday, May 16th from 11:30AM - 2:30PM.

The exam will be given in this room.

If for some reason you are not able to take the Final at the time it will be offered, you MUST send an email to me before the exam so we can make alternative arrangements.

The final will consist of questions like those on the quizzes, along with questions asking you to write short segments of Python code.

60% of the points on this exam will consist of questions from the Ungraded Class Quizzes.

You do not need to study a Class Quiz question if the topic is not mentioned in either the Midterm or Final review.

The remaining 40% will come from 4 questions that ask you to write some code.

To study for the code questions you should be able to

• Write a function to create a list from a file
• Write a function that changes a list
• Write a function that performs a calculation using the elements of a list
• Write a function performs a calculation on the characters in a string

A good way to study for the code questions is to review the Class Exercises homework solutions and Class Notes.

The last class on Thursday, May 2nd, will be a review session.

You will only be responsible for the material in that review session, which you will find here, and the review for the Midterm, which you will find here.

Although the time alloted for the exam is 3 hours, I would expect that most of you would not need that much time.

The final is a closed book exam.

To prevent cheating, certain rules will be enforced during the exam.

Remember, the Midterm and Final determine 50% of your grade.

Announcement

There will be no graded quiz next week.

Questions

Are there any questions before I begin?

Review

Time for Computer Operations

• Computers are very fast
• But different computer operations proceed at different speeds
• There are three speed regimes for accessing data
  • Getting data from a user
  • Getting data from a file
  • Getting data from memory

Reading a File into a List

• Reading data from a file on disk is relatively slow
• It is much faster to read data from memory
• For this reason whenever you read a file in a script ...
• you should probably create a list ...
• from the data in a file
• Here an the algorithm for creating a list of integers ...
• from a file with one number on each line

  create an empty list
  for each line in the file:
      convert the line into an integer
      append the integer to the list
  return the list

• Using this algorithm we can write the following function ...
• which takes a file object as its only parameter

• def read_integers_into_list(file)
      numbs = []
      for line in file:
          num = int(line)
          numbs.append(num)
      return numbs

Advantages of Using Lists with Files

• Reading a file into a list has a number of advantages
• We do not have to count the number of entries in the file
• We can get this value using len on the list object
• We can also use min and max
• We can sort the file values in ascending order using the list sort method
• We can sort the values in descending order using sort and reverse

Problems with Copying Objects

• When a variable holds a number
• You can create a new variable with an assignment statement

  >>> n1 = 5
  >>> n2 = n1
  >>> n2
  5

• If you then change n1
• The value of n2 remains unchanged

  >>> n1 = 6
  >>> n1
  6
  >>> n2
  5

• This not true for objects ...
• such as lists
• Let's say we create a list object ...
• and store its location in the variable l1

  >>> l1 = [1,2,3,4,5,6,7]
  >>> l1
  [1, 2, 3, 4, 5, 6, 7]

• You can create a new variable l2 ...
• by assigning it the value of l1

  >>> l2 = l1
  >>> l2
  [1, 2, 3, 4, 5, 6, 7]

• But when you change something in l1 ...
• the same change appears in l2

  >>> l1[6] = 8
  >>> l1
  [1, 2, 3, 4, 5, 6, 8]
  >>> l2
  [1, 2, 3, 4, 5, 6, 8]

• Let's look at what is happening in memory
• We first created l1

• Then we copied l1 to l2

  l2 = l1

• After this the picture in RAM looks like this

• l1 and l2 point to the same object
• When we make a change using l1
• It's the same as using l2 to make the change

  >>>  l1[6] = 8

• Using an assignment statement to create a new list variable ...
• does not create a new object
• It merely creates a new variable referring to the same list

Copying Lists

• To copy a list you need to create a new list object
• We can do this using concatenation and the empty list

  >>> l1
  [1, 2, 3, 4, 5, 6, 7]
  >>> l2 = [] + l1
  >>> l2
  [1, 2, 3, 4, 5, 6, 7]

• Here is the picture in memory

• Now when you change one list
• The other remains unchanged

  >>> l1[6] = 8
  >>> l1
  [1, 2, 3, 4, 5, 6, 8]
  >>> l2
  [1, 2, 3, 4, 5, 6, 7]

• In memory, it looks like this

• You can also copy a list using a slice

  >>> l1 = [1,2,3,4,5,6,7]
  >>> l2 = l1[:]
  >>> l1[6] = 10
  >>> l1
  [1, 2, 3, 4, 5, 6, 10]
  >>> l2
  [1, 2, 3, 4, 5, 6, 7]

Functions That Return Objects

• The function above returns a pointer to a list it creates
• But where is the list?
• When a function runs it gets its own chunk of memory
• All the function's variables live inside this memory
• The variables disappear when the function ends
• This brings up two questions
  • Why doesn't the list disappear when the function ends?
  • What's the point of returning the list variable?
• The list remains after the function ends ...
• because it was not created in the function's memory space
• Objects are created in the memory space for the script
• Not the memory space for the function
• Script memory space does not change when the function ends
• When a function returns a variable it returns the value of that variable
• The value of new_list is the memory address of the list
• And that memory address is outside the function's memory space
• The situation in RAM looks like this

Functions That Work with Lists

• Functions can work on the values contained in a list
• When we use such a function we give it a list variable as an argument
• The function uses the memory address of the list to access the list
• Here is an algorithm for a function that returns the average of a list of numbers

  set an accumulator to zero
  loop through the list using the list address:
      add each number to the accumulator
  return the accumulator divided by the length of the list

• Here is the function

  def average_list(list):
      total = 0
      for num in list:
          total += num
      return total/len(list)

Functions That Change Lists

• Lists are mutable
• Here is a function that doubles all the elements in a list of numbers

  >>> def double_list(list):
  ...    for index in range(len(list)):
  ...       list[index] =  2 * list[index]
  ... 
  >>> numbers = [1,2,3,4,5]
  >>> double_list(numbers)
  >>> numbers
  [2, 4, 6, 8, 10]

• Notice that the function did not return a value
• It doesn't have to
• The calling function already has a variable pointing to the list

Attendance

New Material

List Elements

• The elements of a list can be anything

  >>> l1 = [1 , 2.5, True, "foo"]
  >>> for index in range(len(l1)):
  ...    print(l1[index], type(l1[index]))
  ... 
  1 <class 'int'>
  2.5 <class 'float'>
  True <class 'bool'>
  foo <class 'str'>

• The elements of a list can even be another list

  >>> l2 = [ 1, 2, 3, 4, [5, 6, 7,]]
  >>> for index in range(len(l2)):
  ...    print(l2[index], type(l2[index]))
  ... 
  1 <class 'int'>
  2 <class 'int'>
  3 <class 'int'>
  4 <class 'int'>
  [5, 6, 7] <class 'list'>

Two-Dimensional Lists

• When all the elements of a list are lists
• We have a two-dimensional list

  >>> 2dl = [[1, 2, 3], [4, 5, 6], [7, 8, 9]] 
  >>> 2dl
  [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

• Each element of 2dl is also a list

  >>> for element in 2dl:
  ...     print(element)
  ... 
  [1, 2, 3]
  [4, 5, 6]
  [7, 8, 9]

• We can use indexing to get each sublist

  >>> 2dl[0]
  [1, 2, 3]
  >>> 2dl[1]
  [4, 5, 6]
  >>> 2dl[2]
  [7, 8, 9]

• How do we get each element within the sublist?
• With another [ ]

  >>> 2dl[0][0]
  1
  >>> 2dl[0][1]
  2
  >>> 2dl[0][2]
  3
  

• We can think of this two-dimensional list as a rectangle

  1 2 3
  4 5 6
  7 8 9

• Here is a script that prints each number individually

  >>> for row in range(len(2dl)):
  ...     for column in range(len(2dl[row])):
  ...         print(2dl[row][column], end=" ")
  ...     print()
  ... 
  1 2 3 
  4 5 6 
  7 8 9 

• I could rewrite this function without using indexes

  >>> for row in 2dl:
  ...     for column in row:
  ...         print(column, end=" ")
  ...     print()

• Notice that when I print a number I don't move down to the next line
• But when I finish a row I have to call print to start a new line

Tuples

• A tuple is a sequence of values that cannot be changed
• You create a tuple literal using parentheses, ( )
• They enclose a comma separated list of values

  >>> t1 = (1, 2, 3, 4, 5)
  >>> t1
  (1, 2, 3, 4, 5)

• Tuples can contain elements of any type

  >>> t2 = (1, 2.5, False, "Sam")
  >>> t2
  (1, 2.5, False, 'Sam')

• You can access the elements of a tuple with an index

  >>> t1[0]
  1

• You can use a for loop to print all the elements

  >>> for number in t1:
  ...     print(number)
  ... 
  1
  2
  3
  4
  5

• You can use the concatenation operator

  >>> t3 = t1 + t2
  >>> t3
  (1, 2, 3, 4, 5, 1, 2.5, False, 'Sam')

• The repetition operator

  >>> t4 = t1 * 3
  >>> t4
  (1, 2, 3, 4, 5, 1, 2, 3, 4, 5, 1, 2, 3, 4, 5)

• And the in operator

  >>> "Sam" in t2
  True

• The len function works with tuples

  >>> t1
  (1, 2, 3, 4, 5)
  >>> len(t1)
  5

• As do min and max

  >>> min(t1)
  1
  >>> max(t1)
  5

• And the index method

  >>> t1.index(3)
  2

• But you cannot use the following methods
  • append
  • pop
  • insert
  • sort
  • reverse
• Because they change the tuple ...
• and tuples cannot be changed
• Slices also work with tuples

  >>> t1
  (1, 2, 3, 4, 5)
  >>> t1[1:3]
  (2, 3)
  >>> t1[:3]
  (1, 2, 3)
  >>> t1[1:]
  (2, 3, 4, 5)
  >>> t1[:]
  (1, 2, 3, 4, 5)

• But there is one unusual feature of tuples
• That feature is how you create a tuple literal with only one entry
• If you try the obvious

  >>> t5 = (1)

• Something strange happens

  >>> t5
  1

• If you run type on the variable you get

  type(t5)
  <class 'int'>

• The interpreter did not see (1) as a tuple
• It saw it as the integer 1 ...
• inside parentheses
• Parentheses in Python have two meanings
• They are used to group part of an expression to raise it's priority

  >>> 4 + 3 * 5
  19
  >>> (4 + 3) * 5
  35
  

• As well as being used in a tuple literal
• When in doubt, Python assumes parentheses are for grouping
• Which is why (1) is an integer
• To make it clear that you are defining a tuple with one element ...
• you must use as comma, , , after the single element

  >>> t5 = (1,)
  >>> t5
  (1,)
  >>> type(t5)
  <class 'tuple'>
  >>> t5[0]
  1

• Python lets you create an empty tuple like this

  >>> empty_tuple = ()
  >>> empty_tuple
  ()

• It is a perfectly good tuple

  >>> type(empty_tuple)
  <class 'tuple'>

The tuple Conversion Function

• Every one of the basic data types in Python has a conversion function
  • int
  • float
  • str
  • bool
  • list
• The conversion function for tuples is tuple
• It will work on strings

  >>> name = "Glenn"
  >>> tuple(name)
  ('G', 'l', 'e', 'n', 'n')

• And lists

  >>> digits = [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
  >>>  tuple(digits)
  (1, 2, 3, 4, 5, 6, 7, 8, 9, 0)

• But not on integers

  >>> tuple(1)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: 'int' object is not iterable

• Or booleans

  >>> tuple(True)
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: 'bool' object is not iterable

• tuple only works on iterables
• An iterable is an object that can be used in a for loop

Why Use Tuples?

• Lists and tuples are similar
• But tuples have many limitations
• So why should we use tuples?
• There are two reasons
  • Performance
  • Security
• Tuples are simpler that lists
• So a list takes more memory than a tuple of the same size
• The smaller size makes them somewhat faster to work with
• The savings can be significant when working with big data sets
• Consider the data collected by the European Gaia satellite
• It measured the position and movement of 1.7 billion stars
• You would not want to use lists for that data set ...
• Because the memory cost would be prohibitive
• Tuples also provide security
• They cannot be changed
• So any data you put in a tuple cannot be modified or corrupted ...
• by other functions
• This can be important in team projects ...
• where many people work with the same data

Tic-Tac-Toe

• Two-dimensional lists can be used for many things
• For example, we can use it to represent a game of tic-tak-toe
• Tic-Tac-Toe is a game for two players on a 3 by 3 square
• One player marks a cell in the square with an "X"
• The other with an "O"
• The goal is to get three in a row
• Either across, down or diagonally
• Like this

• We can represent the board above with a two-dimensional array

  board = [["O", "X", "X"], ["X", "0", "O"], ["X", "0", "O"]]

• As you can see by printing the elements

  >>> for row in range(len(game)):
  ...     for column in range(len(game[row])):
  ...         print(game[row][column], end=" ")
  ...     print()
  ... 
  O X X 
  X 0 O 
  X 0 O 

• Let's write a Python script for the game
• A player will make moves against the computer
• Once again we will use incremental development
• When writing this program

Tic-Tac-Toe - Step 1

• We need to define the board
• The board is a 3 by 3 square represented as a two-dimensional list
• In the beginning each square is empty

  board = [[" ", " ", " "],[" ", " ", " "],[" ", " ", " "]]

• We need to be able to print the board ...
• and have to be careful when we do this
• A simple for loop won't work

  >>> board = [[" ", " ", " "],[" ", " ", " "],[" ", " ", " "]]
  >>> for row in board:
  ...     print(row)
  ... 
  [' ', ' ', ' ']
  [' ', ' ', ' ']
  [' ', ' ', ' ']

• We need to see the lines between each entry ...
• and each row
• So an empty board should look like this

   | | 
  -----
   | | 
  -----
   | | 

• Printing a row is a discrete task
• So we should create, the helper function print_row, to do this
• A row will have the contents of each square
• With a vertical bar, | , between them
• Here is the code

  def print_row(row):
      print(row[0] + "|" + row[1] + "|" + row[2])

• When I print the first row I get

   | | 

Tic-Tac-Toe - Step 2

• Now we need to print the entire board
• We need to call print_row for each row
• And print a line of dashes after each row
• Here is print_board

  def print_board():
      print_row(board[0])
      print("-----")
      print_row(board[1])
      print("-----")
      print_row(board[2])

• When I run this I get

   | | 
  -----
   | | 
  -----
   | | 

Tic-Tac-Toe - Step 3

• Now we need a function that asks the user for a move
• It should prompt the user for two integers
• One for row and one for column
• The rows and columns will have numbers from 1 to 3
• But the function has to check that the entry is valid
• It has to check that
  • There are two values
  • Both values are integers
  • Both values are between 1 and 3
• To validate the data we need a while loop
• We will use input to ask the user for two values ...
• and call split on their answer to create variables ...
• for row and col
• The loop will keep running until the user enters good values
• To check we have two values we use len ...
• on the list returned by split
• We need to convert these values into integers ...
• inside a try/except statement
• If the except clause fires ...
• we print and error message ...
• and try again
• To check that the values are between 1 and 3
• We create the following list constant

  GOOD_VALUES = [1,2,3]

• Then we check that each number is contained in GOOD_VALUES
• The loop will keep running until the user entry is correct
• Then it will return the values
• Here is the function user_move

  def user_move():
      while True:
          reply = input("Next move (row col): ")
          fields = reply.split()
          if len(fields) < 2:
              print("Need a row number and a column number")
              continue
          row, col = fields
          try:
              row = int(row)
              col = int(col)
          except:
              print(row, "or", col, "is not a number")
              continue
          if row not in GOOD_VALUES or col not in GOOD_VALUES:
              print("Row and column values must be 1, 2, or 3")
              continue
          return row, col

• Notice the use of continue in the except clause
• If converting a string into an integer causes a runtime error ...
• we want to print an error message ...
• and get a new pair of values from the user ...
• so there is no reason to continue with the rest of the loop code
• continue let's us do that

Tic-Tac-Toe - Step 4

• There is one thing that user_move does not test for
• It needs to be sure that the square is empty
• To do this we need a global variable

  prev_moves = []

• Before returning the move, user_move needs to check prev_moves
• If the move has been taken, it should print an error message
• And keep looping
• Otherwise it should add the move to prev_moves ...
• before returning the move
• Here is the new version of user_move

  def user_move():
      while True:
          reply = input("Next move (row col): ")
          fields = reply.split()
          if len(fields) != 2:
              print("Need a row number and a column number")
              continue
          row, col = fields
          try:
              row = int(row)
              col = int(col)
          except:
              print(row, "or", col, "is not a number")
              continue
          if row not in GOOD_VALUES or col not in GOOD_VALUES:
              print("Row and column values must be 1, 2, or 3")
              continue
          if (row, col) in prev_moves:
              print(row, col, "is already taken")
              continue
          else:
              prev_moves.append((row, col))
              return row, col

• Notice that each entry in prev_moves ...
• is a tuple of two integers
• We never want entries in this list to change
• So we use tuples

Tic-Tac-Toe - Step 5

• Now we need to create the function mark_square ...
• to record a move
• It is a very simple function

  def mark_square(row, col, mark):
      board[row - 1][col - 1] = mark

• Notice that I have to subtrack 1 from both row and col
• That's because list uswe zero-based indexing ...
• but we want the user to use the values 1, 2 and 3
• To make things a little clearer, I define two constants

  USER_MARK    = "X"
  MACHINE_MARK = "O"

• We need a while loop to run the game
• Eventually this loop will stop when the game is over
• For now, I just need a loop that will call user_move
• Then call mark_square

  print_board()
  print()
  while True:
      row, col = user_move()
      mark_square(row, col, USER_MARK)
      print_board()
      print()

Tic-Tac-Toe - Step 6

• Now we need to write machine_move
• We can use randint to create the integers
• And we can specify that they must be between 1 and 3
• The only data validation we need is to check that the space is free
• The moves have to be created inside a while loop

  def machine_move():
      while True:
          row = random.randint(1,3)
          col = random.randint(1,3)
          if (row, col) not in prev_moves:
              prev_moves.append((row, col))
              return row, col

• Now we can call machine_move inside the game loop

  print_board()
  print()
  while True:
      row, col = user_move()
      mark_square(row, col, USER_MARK)
      print_board()
      print()
      row, col = machine_move()
      mark_square(row, col, MACHINE_MARK)
      print_board()
      print()

Tic-Tac-Toe - Step 7

• To jump out of the game loop and end play ...
• we need to know when someone has won
• We will create the function game_over to do this
• The code will be complicated
• We can make the problem simpler if we only check if a specific player has won
• Each time a player makes a move we call game_over for that mark
• There are three ways a player can win
  • The player's mark fills a row
  • The player's mark fills a column
  • The player's mark fills a diagonal
• But each possibility can be satisfied in more than one way
• We have to check each row for the correct set of marks
• There are three rows, so there are are three possible ways to win
• We have the same situation for each column
• But there are only two diagonals
• We are going to need a for loop to check the rows and columns
• And two separate if statements for the diagonals
• Each loop will have an inner loop
• Which checks each square in the row or column
• To make things cleaner, I will define the helper function cell_match
• It returns True if the mark is found in a specific square

  def cell_match(row, col, mark):
      return board[row][col] == mark

• The function returns True if a mark is in the cell
• Each for loop checks three squares
• If one square does not match it returns False
• If it gets to the end of the inner loop ...
• it prints a win message ...
• and returns true
• Here is game_over

  def game_over(mark):
      # check rows
  	for row in [0,1,2]:
  	    if cell_match(row, 0, mark) and cell_match(row, 1, mark) and cell_match(row, 2, mark):
  	        print(mark , "wins!")
              return True
      # check columns
      for col in [0,1,2]:
      	if cell_match(0, col, mark) and cell_match(1, col, mark) and cell_match(2, col, mark):
          	print(mark , "wins!")
              return True
      # check diagonals
      if cell_match(0, 0, mark) and cell_match(1, 1, mark) and cell_match(2, 2, mark):
          print(mark , "wins!")
          return True
      if cell_match(0, 2, mark) and cell_match(1, 1, mark) and cell_match(2, 0, mark):
          print(mark , "wins!")
          return True
      return False

• Now I need to modify the game loop to call this function
• The call needs to happen after each player makes a move

  print_board()
  print()
  while True:
      row, col = user_move()
      mark_square(row, col, USER_MARK)
      print_board()
      if game_over(USER_MARK):
          break
      print()
      row, col = machine_move()
      mark_square(row, col, MACHINE_MARK)
      print_board()
      if game_over(MACHINE_MARK):
          break
      print()

• You will find the complete script here

Class Exercise

Class Quiz