Review

• Sequences
• Lists
• The list Function
• Using a for Loop with Lists
• Empty Lists
• The len Function

New Material

• Operators That Work on Objects
• Concatenating Lists
• The Repetition Operator
• Indexing
• Changing Elements Using Indexes
• Slices
• Leaving Out Indexes in a Slice
• Skipping Elements in a Slice
• The in Operator
• List Methods
• The append Method
• The pop Method
• The index Method
• The insert Method
• The sort Method
• The reverse Method
• The del Statement
• The min and max Functions
• Fairness

Graded Quiz 9

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Solution to Homework 9

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Homework 11

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Questions

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Review

Sequences

• Sequences are Python data types that hold many values
• The values are stored one right after the other in RAM
• Like this

• There are three sequence data types
  • lists
  • tuples
  • strings
• Lists are sequences whose values can be changed
• Tuples are sequences that cannot be changed
• Strings are sequences whose elements are characters

Lists

• The values in a list are called elements
• You can change any value in the list ...
• and change their order
• You can also add and remove items from the list
• We can create a list literal by placing values inside square brackets
• There must be a comma between each value

  >>> even_numbers = [2, 4, 6, 8, 10]
  >>> even_numbers
  [2, 4, 6, 8, 10]

• Here is what this looks like in memory

• Lists can contain a value of any type
  • Integers
  • Floats
  • Strings
  • Booleans
• Lists can also contain other lists

The list Function

• list is a conversion function just like int and str
• list turns objects that are collections of things into a list
• Strings are sequences of characters in a specific order
• list will work on strings

  >>> name = "Glenn"
  >>> characters = list(name)
  >>> characters
  ['G', 'l', 'e', 'n', 'n']

Using a for Loop with Lists

• A for loop will work with a list

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

• When we loop through a list we are said to iterate over that list
• We can also use a for loop with strings

  >>> team = "Red Sox"
  >>> for char in team:
  ...     print(char)
  ... 
  R
  e
  d
   
  S
  o
  x
  >>>

Empty Lists

• We create an empty list using a list literal
• With nothing between the square brackets

  >>> new_list = []
  >>> new_list
  []

• The empty list serves the same purpose for lists ...
• that zero does for numbers

The len Function

• The built-in function len will return the length of any sequence

  >>> even_numbers = [2, 4, 6, 8, 10]
  >>> len(even_numbers)
  5
  >>> len("foo")

Attendance

New Material

Operators That Work on Objects

• Operators are special characters that perform some action
• Usually the action creates a new value
• Operators work on values that are called operands
• Every data type has its own set of operators
• For numbers, we have the arithmetic operators
  • +
  • -
  • *
  • /
  • //
  • %
  • **
• Sequences have only two operators
  • +
  • *

Concatenating Lists

• The concatenation operator that works on strings

  >>> name = "Glenn" + " " + "Hoffman"
  >>>  name
  'Glenn Hoffman'

• Also works on lists

  >>> n1 = [1,2,3]
  >>> n2 = [4,5,6]
  >>> n1 + n2
  [1, 2, 3, 4, 5, 6]

• That's because both strings and lists are sequence data types
• But it only works when both the operands are the same data type

  >>> n1 + name
  Traceback (most recent call last):
    File "<stdin>", line 1, in Monday
  TypeError: can only concatenate list (not "str") to list

The Repetition Operator

• You cannot subtract two list

  >>> n2 - n1
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: unsupported operand type(s) for -: 'list' and 'list'

• Nor can you divide them

  >>> n1 / n2
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  TypeError: unsupported operand type(s) for /: 'list' and 'list'

• But you can use the * on lists
• When you do the first operand is a list ...
• and the second is an integer
• The result is a new list ...
• containing multiple copies of the elements of the original list

  >>> zeros = [0] * 5
  >>> zeros
  [0, 0, 0, 0, 0]
  >>> numbers = [1, 2, 3] * 3
  >>> numbers
  [1, 2, 3, 1, 2, 3, 1, 2, 3]

• The repetition operator also works on strings

  >>> "Go " * 3
  'Go Go Go '

Indexing

• What if we wanted to get the 3rd element in a list?
• We can do this with a for loop
• But it is very cumbersome

  >>> even_numbers = [2, 4, 6, 8, 10]
  >>> count = 0
  >>> for n in even_numbers:
  ...   count += 1
  ...   if count == 3:
  ...      third_element = n
  ... 
  >>> third_element
  6

• There is an easier way
• All elements in a list are arranged in order
• One right after the other
• Each element's place in line can be specified by an integer
• The integer that specifies an element's place is called an index
• Unfortunately, index numbers do not start at 1
• Indexes start at 0
• This is called zero-based indexing
• This means that the first element in a list has an index of 0
• The second has an index of 1
• And so on
• The index of the last element is one less than the length

• To get a particular element we use the [ ] operator
• We put this operator right after the name of the list variable
• The index goes inside the square brackets

  >>> even_numbers[0]
  2
  >>> even_numbers[1]
  4
  >>> even_numbers[2]
  6

• If you use an index that is too big
• You will get an IndexError exception

  >>> even_numbers[5]
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  IndexError: list index out of range

Changing Elements Using Indexes

• Lists are said to be mutable
• That means they can be changed
• This is usually done using indexes
• If we have a list of numbers

  numbers = [1, 4, 6, 8]

• We can change the first number like this

  >>> numbers[0] = 2
  >>> numbers
  [2, 4, 6, 8]

• We can use indexes in a for loop ...
• to change every element in a list

  >>> for i in range(len(numbers)):
  ...   numbers[i] += 1
  ... 
  >>> numbers
  [3, 5, 7, 9]

Slices

• Let's say we define the following list

  >>> days = ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"]

• What if I wanted to create a new list with just weekdays?
• That is, all the days except Sunday and Saturday
• Python gives us a easy way to do this using a slice
• A slice is new, smaller, list created from another list ...
• by selecting certain index values
• To create a slice you use the following format

  LIST_VARIABLE[FIRST_INDEX:ONE_MORE_THAN_LAST_INDEX]

• So you need to have two indexes to create a slice
• The first index is the index in the original list ...
• of the element you want to be first in the new list
• The second index is trickier
• The second index is not the index of the last element ...
• in the original list
• Its the index of the element after the last element you want
• Let's say we wanted to create a new list weekdays
• The first element we want in our slice is "Monday"
• Which has an index of 1 in the original list
• The last element we want is "Friday"
• Which has an index of 5 in the original list
• Adding 1 to 5 we get 6
• So we create our new list like this

  >>> weekdays = days[1:6]
  >>> weekdays
  ['Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday']

• Here is another list

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

• To get a slice from 2 to 8 we would write

  >>> digits[2:9]
  [2, 3, 4, 5, 6, 7, 8]

Leaving Out Indexes in a Slice

• Python allows you to create a slice using only one index
• If you leave out the first index ...
• Python assumes the value is 0
• In other words, the index of the first element

  >>> days
  ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
  >>> days[:4]
  ['Sunday', 'Monday', 'Tuesday', 'Wednesday']
  >>> digits
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
  >>> digits[:5]
  [0, 1, 2, 3, 4]

• If you leave out the second index ...
• Python uses the length of the list
• Which is one more than the index of the last element

  >>> days[4:]
  ['Thursday', 'Friday', 'Saturday']
  >>> digits[5:]
  [5, 6, 7, 8, 9]

• You can even leave out both indexes ...
• as long as you keep the :
• In this case Python will return a slice that consists of the entire list

  >>> days[:]
  ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']
  >>> digits[:]
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

• This is a great way to make a copy of a list

  >>> daynames = days[:]
  >>> daynames
  ['Sunday', 'Monday', 'Tuesday', 'Wednesday', 'Thursday', 'Friday', 'Saturday']

Skipping Elements in a Slice

• You can also skip elements when creating a slice
• It works just like the 3rd argument to the range function
• To use this feature you add a second : ...
• followed by an integer

  LIST_VARIABLE[FIRST_INDEX:ONE_MORE_THAN_LAST_INDEX:STEP_VALUE]

• The STEP_VALUE works just like in the range function
• It is what we add to the index of the previous element ...
• to get the index of the next element
• It is the difference between the index of any two elements ...
• in the original list
• If we wanted to create a slice of all the even digits ...
• we could do it like this

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

• Actually, I could leave out the second index

  >>> digits[0::2]
  [0, 2, 4, 6, 8]

• And even the first

  >>> >>> digits[::2]
  [0, 2, 4, 6, 8]

The in Operator

• Operators are special characters that perform some action
• The only operator we have seen so far have been symbols
  • +
  • -
  • *
  • /
  • //
  • **
  • %
• But operators can also be words
• in is an operator that works on objects ...
• that are a collection of values
• We use it like this

  VALUE in LIST

• The in operator returns True if the LIST contains VALUE
• Otherwise it returns False

  >>> digits
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 0]
  >>> 1 in digits
  True
  >>> 11 in digits
  False

• Is is possible for a list to contain the same value many times

  >>> l = [1, 2, 3, 3, 3, 4, 5, 6]

• in will tell you that value is in a list

  >>> 3 in l
  True

• But it will not tell you it is in there more than once

List Methods

• Lists are objects
• Objects usually have functions that work on the values ...
• inside the object
• These functions are called methods
• Here are some of the more useful list methods

  Method	Description
  append(item)	Adds item to the end of the list
  pop(index)	Removes the element at a given index from the list and returns the value. If called with no argument, it returns the last element and deletes it from the list.
  index(item)	Returns the index of the first element whose value is equal to item. A ValueError exception is raised if item is not found in the list.
  insert(index, item)	Inserts item into the list at the specified index. When an item is inserted into a list, the list is expanded in size to accommodate the new item. The item that was previously at the specified index, and all the items after it, are shifted by one position toward the end of the list.
  sort()	Sorts the items in the list so they appear in ascending order (from the lowest value to the highest value)
  reverse()	Reverses the order of the items in the list

The append Method

• The append method adds a value ...
• to the end of a list
• It takes one argument, the value you want to add
• Is it probably the most frequently used list method
• You can use the append method to create a list from scratch
• First you create an empty list

  >>> teams = []
  >>> teams
  []

• Then we can call the append method on this list using dot notation

  >>> teams.append("Red Sox")
  >>> teams
  ['Red Sox']

• We can call this method as many times as we like

  >>> teams.append("Orioles")
  >>> teams.append("Blue Jays")
  >>> teams.append("Rays")
  >>> teams.append("Yankees")

• Each new value is added to the end of the list

  >>> teams
  ['Red Sox', 'Orioles', 'Blue Jays', 'Rays', 'Yankees']

The pop Method

• The pop method does two things
  • Removes an element from a list
  • Returns the value of that element
• Its argument is the index of the value to be removed

  >>> numbs = [1, 2, 3, 4, 5]
  >>> n = numbs.pop(0)
  >>> n
  1
  >>> numbs
  [2, 3, 4, 5]

• If you leave out the index argument ...
• pop removes the last element

  >>> n = numbs.pop()
  >>> n
  5
  >>> numbs
  [2, 3, 4]

The index Method

• The in operator will tell us if a list contains a value
• But it won't tell us where that value is inside the list

  >>> "Red Sox" in teams
  True

• We can use the index method to do that
• The argument to index is the value we are looking for
• It will return the index of that value

  >>> teams.index("Red Sox")
  0
  >>>  teams.index("Blue Jays")
  2

• If the value we are looking for is not in the list
• We will get a ValueError exception

  >>> teams.index("Mets")
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  ValueError: 'Mets' is not in list

• If the value occurs more than once
• The index of the first element is returned

  >>> >>> l = [1, 2, 3, 3, 5]
  >>> l.index(3)
  2

The insert Method

• append always adds to the end of a list
• To insert a value somewhere else we need insert
• The method takes two arguments
  • The index where the element should be inserted
  • The value to be inserted there
• insert puts the value at the index given ...
• and moves every element to its right over one position

  >>> l1 = [1, 2, 3, 4, 5]
  >>> l1.insert(2, 7)
  >>> l1
  [1, 2, 7, 3, 4, 5]

The sort Method

• A list is mutable so we can add elements or remove them
• But we can also change the order of the elements
• This can be done with the sort method which takes no argument

  >>> l2 = [9, 1, 0, 2, 8, 6, 7, 4, 5, 3]
  >>> l2
  [9, 1, 0, 2, 8, 6, 7, 4, 5, 3]
  >>>l2.sort()
  >>> l2
  [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

The reverse Method

• The reverse method reverses the order of a list

  >>> l3 = [5, 4, 3, 2, 1]
  >>> l3.reverse()
  >>> l3
  [1, 2, 3, 4, 5]
  >>> l3.reverse()
  >>> l3
  [5, 4, 3, 2, 1]

• If you want to sort a list in descending order
• You first need to use sort

  >>> l4 = [2, 5, 9, 1, 8, 6, 3, 7, 4]
  >>> l4.sort()
  >>> l4
  [1, 2, 3, 4, 5, 6, 7, 8, 9]

• Then use reverse

  >>> l4.reverse()
  >>> l4
  [9, 8, 7, 6, 5, 4, 3, 2, 1]

The del Statement

• List methods aren't the only things you can use to work with lists
• You can delete elements from a list using the del statement
• The format of a del statement looks like this

  del LIST_VARIABLE[INDEX]

• After the keyword del comes the list variable ...
• and the index of the value to be removed ...
• inside square brackets

• >>> l5 = [0, 1, 2, 3, 4, 5]
  >>> del l5[2]
  >>> l5
  [0, 1, 3, 4, 5]

The min and max Functions

• The two built-in functions min and max ...
• can also be used with lists
• They do not change the list
• But they return information about it
• min returns the smallest value in a list

  >>> l6
  [1, 4, 5, 6]
  >>> min(l6)
  1

• max returns the largest

  max(l6)
  6

Class Exercise

Class Quiz