CS52 - Spring 2016 - Class 12

Example code in this lecture

   while.py
   bit.py
   number_and_bit.py

Lecture notes

  • admin
       - how did assignment 4 go?
       - assignment 5
          - out
          - should be able to start after today's class

  • high level
       - look at first two functions of while.py code
       
       - lists

       - defining classes in python: look at bit.py code

  • python basics
       - running python and interacting with the python shell
          - python is an interpreted language, like SML
          - can just type "python" at the terminal to get a shell
          - execfile
             - like "use" in SML
             - it's a function, though:

                >>> execfile("while.py")
          

       - :
          - defines a block, we'll see it a lot
          - indentation indicates blocks (the role previously reserved for curly braces)
             - indentation is critical and is not just stylistically important

       - basic control flow
          - if/elif/else
          - for
             - use of range
          - while

       - defining functions
          - def
          - no return type
          - no parameter types
       
       - comments

       - docstrings

       - print
          - using str()

       - talk about pass

  • lists
       - similar to arrays in Java
          - indexable
          - can iterate over them

       - some differences
          - actually a class, so they have methods
          - they're extensible and mutatable (sort of like ArrayList class in Java)
             - can append to them
             - can insert into them
             - can delete elements in them
       
       - creating with []
          >>> l = [1, 2, 3]
          >>> l
          [1, 2, 3]
          >>> l = ["apples", "bananas", "carrots"]
          >>> l
          ['apples', 'bananas', 'carrots']
          >>> l = [1, "apples", [1, 2]]
          >>> l
          [1, 'apples', [1, 2]]
       
       - indexing
          >>> l = [1, 2, 3, 4]
          >>> l2 = ["apples", "bananas", "carrots"]
          >>> l3 = [1, "apples", [1, 2]]

          - starts counting at 0, like in Java

          >>> l[0]
          1
          >>> l2[2]
          'carrots'
          >>> l3[1]
          'apples'

          - can index from the back using negative numbers

          >>> l[-1]
          4   

       - slicing
          - can also "slice" the list to get a sublist

          >>> l[1:3]
          [2, 3]


       - many methods (see the documentation)
          - append
             >>> l = []
             >>> l.append(2)
             >>> l.append(3)
             >>> l
             [2, 3]
          - indexing

       - iterating over
          - using for
          - talk about what range does


  • defining classes in python: look at bit.py code

  • camelcasing vs. underscore
       - in python, we use underscores to separate multi-word variables and function names (anything lowercase
       - but use camelcasing for uppercase things (like classes)

  • inheritance in python: look at number_and_bit.py code
       - What does the Number class do?
          - like the Bit class, but just stores a value (any value, in fact)
          - __str__ method:
             - like toString in Java
       
       - To inherit from a class (i.e. extend in Java) we put parenthesis after the class name and put the class we're extending from
          - NotSafeBit(Number)
             - NotSafeBit inherits from Number
             - We get all of the methods defined in Number, e.g. the constructor, set, get and __str__
       
       - When inheriting we can also define our own methods
          - if they don't exist already, they're added to the class
          - if they do exist already, they override the parent class's method

       - bit_and in NotSafeBit is a new method

       - Why is the class called NotSafeBit?
          - It's almost like the Bit class...
          - However, because we inherit from Number, we can't assume that the value will always be 0 and 1!

       - Look at the Bit class in number_and_bit.py code : How does his solve it?
          - override the constructor and the set methods to enforce 0 and 1 values

       - What will the different_types function print?
          Num: 10
          nsBits: 10, 1
          bits: 1, 1
          ns AND: 0
          bit AND: 1

          - declares a Number with value 10
          - declares two lists
          - in the nsBits list, adds two NotSafeBits constructed with 10 and 1
          - in the bits list, adds two Bits constructed with 10 and 1
          - Because the NotSafeBits are just numbers, the 10 stays as a 10 and gives us the wrong answer!
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