CS150 - Fall 2013 - Class 9

  • admin
       - schedule for the next two weeks
          - lab this Friday (10/12)
             - will be a partnered lab
          - later this week I'll make available your first test program
             - take home programming exam
             - no collaboration and no help from tutors
             - will be due Wednesday 10/24 (though I encourage you to finish before the break)
          - lab next week will be a work session
          - midterm next week on Thursday night

  • An aside... other values besides True/False can be interpreted in a bool context
       if "banana":
          print "How did that work"

       - Besides True/False, most values can also be used in a boolean context
          - 0, 0.0 and "" are all equivalent to False
          - Everything else is True            
       
          >>> 10 or False
          True
          >>> 0 or False
          False
          >>> "blah, blah" or False
          True
          >> "" or False
          False

  • run scores-lists.py code
       - First, prompts user to enter a list of scores one at a time
          - how is this done?
             - while loop
             - what is the exit condition?
                - checks to see if the line is empty
                   while line:
                      ...

                   notice that the empty string counts as False
                - could have made this more explicit with:

                   while line != ""
                
       - then, calculate various statistics based on what was entered
       - how are we calculating these statistics?
          - average?
             - could keep track of the sum and the number of things entered
             - divide at the end
          - max?
             - keep track of the largest seen so far
             - each time a new one is entered, see if it's larger, if so, update the largest
          - min?
             - same thing
          - median?
             - the challenge with median is that we can't calculate it until we have all of the scores
             - need to sort them and then find the middle score

       - why can't we do this using int/float variables?
          - we don't know how many scores are going to be entered
          - even if we did, if we had 100 students in the class, we'd need 100 variables!

  • lists
       - lists are a data structure in Python
          - what is a data structure?
             - a way of storing and organizing data

       - lists allow us to store multiple values with a single variable

  • creating lists: we can create a new list using the square brackets
       >>> [7, 4, 3, 6, 1, 2]
       [7, 4, 3, 6, 1, 2]
       >>> 10 # not a list
       10
       >>> [10]
       [10]
       >>> l = [7, 4, 3, 6, 1, 2]
       >>> l
       [7, 4, 3, 6, 1, 2]
       >>> type(l)
       <type 'list'>
          
       lists are a type and represent a value, just like floats, ints, bools and strings. We can assign them to variables, print them, etc.

       - what do you think [] represents?
          - empty list
          >>> []
          []
       
  • accessing lists: lists support most of the same functionalities of strings
       - we can get at particular values in the list

          >>> l[3]
          6
          >>> l[0]
          7
          >>> l[-1]
          2
          >>> type(l[3])
          <type 'int'>
          
          - notice that the values here are ints (unlike when we use indexing for strings, where the values are strings)
       - we can also "splice" lists, just like we did with strings

          >>> l[2:4]
          [3, 6]

       - draw the list representation
          - a list is a contiguous set of spaces in memory
          - we can store anything in each of these spaces

          >>> ["this", "is", "a", "list", "of", "strings"]
          ['this', 'is', 'a', 'list', 'of', 'strings']
          >>> list_of_strings = ["this", "is", "a", "list", "of", "strings"]
          >>> list_of_strings[0]
          'this'
          >>> list_of_strings[0][1]
          'h'
          >>> list_of_strings[1].toupper()
          'IS'
          >>> list_of_strings[1:4]
          ['is', 'a', 'list']
          >>> [1, 5.0, "my string"]
          [1, 5.0, 'my string']
          >>> l = [1, 5.0, "my string"]
          >>> type(l[0])
          <type 'int'>
          >>> type(l[1])
          <type 'float'>
          >>> type(l[2])
          <type 'str'>

          in general, you should try and avoid non-homogenous lists since there are often better ways of putting things of different types together

  • range function
       - we've used the range function in loops to iterate over numbers, e.g.

          for i in range(10):
             # do something

       - what does the range function actually do?
          >>> range(10)
          [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
          >>> range(15,20)
          [15, 16, 17, 18, 19]
          >>> range(-5, 0)
          [-5, -4, -3, -2, -1]

  • looping over lists
       - all of our for loops so far have actually just been looping over lists

          for i in range(10):
             # do something

          is really the same as:
       
          some_list = range(10)

          for i in some_list:
             # do something

       - the way to read this list is:
          - for each element in the list, do something
          - for each iteration of the loop, i (or whatever variable you put there) will get the next value in the list

       >>> my_list = [4, 1, 8, 10, 11]
       >>> for i in my_list:
       ...    print i
       ...
       4
       1
       8
       10
       11

  • back to our stats program... how could we write average given what we know so far, that is a function that takes a list as a parameter and calculates the average?
       - look at the inelegant_average function in scores-list.py code
          - loop over each of the elements in the list
          - accumulate a sum
          - accumulate a count
          - divide the sum by the count
       - look at the average function in scores-list.py code

  • built-in functions over lists: there are also some built-in functions that take a list as a parameter
       - we can get the length of a list, similar to strings
          >>> len(l)
          3
          >>> len([1, 2, 3, 4, 5])
          5
          >>> len([])
          0
       - max
          >>> l = [5, 3, 2, 1, 10]
          >>> max(l)
          10

       - min
          >>> min(l)
          1
       - sum
          >>> sum(l)
          21

  • lists are objects and therefore have methods. Any guesses?
       - append: add a value on to the end of the list
          >>> my_list = [15, 2, 1, 20, 5]
          >>> my_list.append(100)
          >>> my_list
          [15, 2, 1, 20, 5, 100]

          - notice that append does NOT return a new list, it modifies the existing list!

       - try some out on your own:
          http://docs.python.org/tutorial/datastructures.html
       
          - pop: remove a value off of the end of the list and return it
             >>> my_list.pop()
             100
             >>> my_list
             [15, 2, 1, 20, 5]
          
             - notice that it both modifies the list and returns a value
             - if you want to use this value, you need to store it!
                >>> x = my_list.pop()
                >>> x
                5
             - pop also has another version where you can specify the index
       
                >>> my_list = [15, 2, 1, 20, 5]
                >>> my_list.pop(2)
                1
                >>> my_list
                [15, 2, 20, 5]
          - insert: inserts a value at a particular index
             >>> my_list = [15, 2, 1, 20, 5]
             >>> my_list.insert(2, 100)
             >>> my_list
             [15, 2, 100, 1, 20, 5]

             - again, lists are mutable, so insert does not return a new list, but modifies the underlying one
          - sort
             >>> my_list = [15, 2, 1, 20, 5]
             >>> my_list.sort()
             >>> my_list
             [1, 2, 5, 15, 20]
             >>> my_list = ["these", "are", "some", "words", "to", "sort"]
             >>> ["these", "are", "some", "words", "to", "sort"].sort()
             >>> my_list = ["these", "are", "some", "words", "to", "sort"]
             >>> my_list.sort()
             >>> my_list
             ['are', 'some', 'sort', 'these', 'to', 'words']

  • back to our grades program: look at scores-lists.py code
       - there is a function called get_scores. That gets the scores and returns them as a list. How?
          - starts with an empty list
          - uses append to add them on to the end of the list
          - returns the list when the loop finishes
       - average function
          - has a single parameter, but this parameter will represent a list
          - inelegant_average
             - calculates the sum and divides by the number of entries
                - uses a for loop to iterate over the values
                - often, we'll use something besides "i" as a variable name that makes our program more readable
          - is there a better way to do this?
             - look at fancy_average
                - us the sum function over lists
       - median function
          - sorts the values
             - notice again that sort does NOT return a value, but sorts the list that it is called on
          - returns the middle entry
    •