CS51A - Spring 2019 - Class 7

Example code in this lecture

   scores-lists.py
   more-lists.py

Lecture notes

  • admin
       - assignment 2

  • quick list recap
       - lists are a data structure that allows us to store multiple things in a single variable
       - we create a new list using [], e.g.
          >>> l = [100, 10, 1, 2, 5]
          >>> l
          [100, 10, 1, 2, 5]
       
       - the items in the list are ordered sequentially and we can access them by indexing into the list

          >>> l[0]
          100
          >>> l[3]
          2

          The first item in the list is at index 0, second at index 1, ...

       - We can also index from the back using negative numbers
          >>> l[-1]
          5

       - We can get a sublist, by slicing

          >>> l[0:2]
          [100, 10]
          >>> l[1:4]
          [10, 1, 2, 5]

          You specify a start:end range and you get back a *new* list including the data starting at start index up through, but *not* including end

  • looping over lists
       - We can use the for loop to iterate over each item in the list:

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

       - This is often called a "foreach" loop, i.e. for each item in the list, do an iteration of the loop

  • write a function call sum that sums up the values in a list of numbers

       def sum(numbers):
          total = 0

          for val in numbers:
             total += val

          return total

  • 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-lists.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-lists.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
          >>> 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!

       - We can look at the documentation do see what is available
          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
       - 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

  • lists are mutable
       - what does that mean?
          - we can change (or mutate) the values in a list
       
       - notice that many of the methods that we call on lists change the list itself

       - we can mutate lists with methods, but we can also change particular indices
       
          >>> my_list = [15, 2, 1, 20, 5]
          >>> my_list
          [15, 2, 1, 20, 5]
          >>> my_list[2] = 100
          >>> my_list
          [15, 2, 100, 20, 5]

  • sequences
       - lists are part of a general category of data structures called sequences that represent a sequence of things
       - *all* sequences support a number of shared behavior
          - the ability to index using []
          - the ability to slice using [:]
          - a number of built-in functions:
             - len
             - max
             - min
          - the ability to iterate over in with a for loop
       - We've actually seen one other sequence?
          - strings!


  • strings as sequences
       - notice that we can do all the sequence-like things with strings
          >>> s = "banana"
          >>> s[4]
          'n'
          >>> s[2:5]
          'nan'
          >>> len(s)
          6
          >>> for letter in s:
          ... print letter

          b
          a
          n
          a
          n
          a
       - strings, however, are immutable
          >>> s[4] = "c"
          Traceback (most recent call last):
           File "<string>", line 1, in <fragment>
          TypeError: 'str' object does not support item assignment

          - no matter how hard you try, you cannot mutate a string

  • What does the list_to_string function do in more-lists.py code?
       - takes as input a list
          - what is the type of the list?
             - a list of almost anything!
             - anything that we can call str() on (which turns out to be lots of things)
       - concatenates all the items in the list into a single string
          - results starts out as the empty string
          - it iterates through each item in the list and concatenates them on to the results
       - this is similar to our example before of summing up all the numbers in a list
    •