CS30 - Spring 2016 - Class 1

Example code in this lecture


Lecture notes

  • https://www.youtube.com/watch?v=WnzlbyTZsQY

  • Goals of the class

  • Course Administrative (http://www.cs.pomona.edu/~dkauchak/classes/cs30/administrivia.html)
       - Schedule
          - T/Th class with M lab
          - Roughly weekly assignments, often due on Thursday nights
       - All material will be posted on the course web page
       - We have a discussion board setup on piazza linked from the course web page
          - Use this to ask any content-based questions
          - Check it regularly (or sign up for e-mail feeds)
       - Late Policy
       - Grading
       - Honor code and collaboration (read the policy!)

  • Other misc. announcements
       - Make sure you're registered for a lab section
       - We will have our first lab session on Monday
       - First assignment (assignment 0) will be posted later today and due on Thursday

  • Style of the course
       - interactive!
          - ask lots of questions
          - participate
          - be expected to do group work in class
       - You'll be expected to be here in class and lab
       - I'll post my notes and examples online. You may still want to write some things down, but you don't have to write every word down

  • Pacing
       - I assume no prior computer science, programming or science background
       - The pacing may be a bit slow for some early, but it will get harder

  • Wing IDE 101
       - We'll be using Wing IDE for this course as our interface into Python
       - What is an IDE?
          - Integrated Development Environment
          - Incorporates a text editor with other tools for running, debugging and navigating through the code
       - When it starts, a number of different parts of the window
          - For now (and most of this course), we're only going to be using two main components:
             - Python Shell
             - Main editor window
          - You can rearrange them however you'd like, but I prefer main editor on left and Python Shell on right

  • Python
       - Python is an interpreted language
          - you can type commands and get an immediate response back
          - many programming languages require you to compile the program first and then run it
       - Python makes a great calculator
          - the ">>>" is the interpreter prompt where you type statements
          - when you hit enter/return, Python executes the statements and gives you the answer
          - Python has all of the standard mathematical operations
             - What operations might you want?
                - +, -, *, /
                - ** (power or exponentiation)
                - % (mod aka remainder)

                >>> 4+4
                >>> 15*20
                >>> 20/4
                >>> 10-20
                >>> 10+4*2
                >>> (10+4)*2
                >>> 2**10
                >>> 2**30
                >>> 2**-2
                >>> 10%4
                >>> 11 % 7

             - What is operator precedence?
             - Python follows the normal operator precedence you're used to for math
                - things in parenthesis get evaluated first
                - ** is next
                - %, * and / next
                - + and - last
             - What should be the answer for 11/2?
                >>> 11/2

             - Why 5?
          - Expressions
             - anything that represents a value (e.g. a number) is called an expression
             - contrast this with a statement, which tells the computer something to do
                - for example, "walk over there" is a statement in English
                - or, for computers, something like, "print this out" or "draw that on the screen"
             - All the things we've seen so far have been expressions
             - In Python (and many programming languages) all expressions have a type
             - 11 is an expression
             - 2 is an expression

          - types
             - the "type" of an expression determines how Python interprets and understands an expression
             - Python is a "strongly typed" language: every expression in Python has a type
             - what are the types of the expressions above?
                - they are both numbers, but Python makes a distinction between integers and floating point numbers (i.e. numbers with decimals)
          - Does this explain why 11/2 gave us 5?
             - Because both 11 and 2 are integers, the result is also an integer
             - Python rounds down (i.e. towards -infinity or to the smaller number) when doing integer division
             - What should be the answer off 11/-2?
          - We can make a number a float by adding a decimal point

             >>> 11.0
             >>> 2.0
             >>> 11.0/2.0
             >>> 11.0/2
             >>> 11/2.0

             - for a given operation, if one of the numbers is a float, then the other is converted to a float and the result will be a float
             - what is the result of
                - (11/2)*4.0?
                   - 20.0
                   - the parenthesis will get evaluated first (11/2) will give us the integer 5
                   - we'll then do floating point multiplication by 4.0, giving us a float of 20.0
                - (11.0/2)*4
                   - 22.0

  • bbq party
       - you're having a party and you're trying to figure out how many hot dogs to buy. Here's what you know:
          - teran isn't a big fan of hot dogs, so he'll only eat 1
          - jasmin generally eats 2
          - chris always eats twice as many as jasmin
          - brenda eats one less than chris
          - grace eats half as many as brenda, but likes to take an extra on her way home
       - try and do this on paper
          - 13 (assuming that if someone eats half a hot dog, we still have to count the whole thing)
       - how did you do it?
       - how could you figure this out in Python?
          - might be able to do it, but would require a lot of remembering (or writing things down)

  • variables
       - variables allow us to store things and then use them in other expressions
       - a variable is storage for a value
          - it holds a value
          - we can change its value
       - we change the value of a variable using '=' (also know as assigning to a variable)
          - changing the value of a variable is a statement. It tells the interpreter to do something, but does NOT represent a value

             teran = 1
             jasmin = 2
             chris = 2 * jasmin
             brenda = chris - 1
             grace = (brenda+1)/2 + 1
             total_hotdogs = teran + jasmin + chris + brenda + grace

       - When assigning to a variable, Python evaluates what is on the right hand side of the equals sign and then assigns this value to the variable
       - If I then type jasmin = 4, what happens to chris?
          - chris stays the same

  • naming variables
          - generally you want to give good names to variables (x and y are not good names unless they represent x and y coordinates)
          - variables should be all lowercase
          - multiple words should be separated by an '_' (underscore)

  • what if you get a text from jasmin and she now says she's planning on eating 4?
       - we'd have to re-enter each of the lines (well except the first one)

  • programs in Python
       - besides interacting with the shell, we can write statements in a file and then run them
       - the file editing window in the Wing IDE allows us to do this
       - on many levels, it behaves a lot like a text editor (e.g. Word). You can:
          - create new files
          - open files
          - save files
          - edit files
  • look at bbq.py code
       - I've typed in the same code we typed into the shell, but now in the text editing section
       - I've saved it in a file called bbq.py
          - we'll use the extension .py by convention to indicate a Python program
       - Anything different from what I'd typed before into the shell?
          - In a program (vs. typing it into the shell) if you want to see the value of a variable or an expression, you need to "print" it
             - If I didn't include this, I wouldn't get any answer (in fact I wouldn't see anything)
          - I've used whitespace
             - you can use spaces without affecting how the code executes
             - we use blank lines to make the code more understandable
          - I've included "comments"

  • comments
       - comments in Python are designated using '#' (the pound sign)
       - python ignores everything from the # to the end of the line
          - you can put them on lines by themselves
          - or if you have short comments, you can add them at the end of a line
       - comments are VERY important
          - they allow you to communicate in plain English (to others and to yourself when you look at the code later)
          - you will be required to put them in your programs for this course

  • running the code
       - With an IDE, not only can you edit code, you can also run it
       - the green arrow, runs the program
       - when you run a program, you get a brand new shell session (in the python shell)
          - Any variables, etc. you may have manually created in the shell window will NO LONGER EXIST
          - it executes your program a line at a time from the top
             - it's like you typed all of those commands in the shell
          - the one difference is that you don't get line-by-line feedback
             - for example, if you put 4+4 in a program on a line by itself, you won't see anything
          - if you want to see the value of a variable or an expression, you need to "print" it
       - if we run this program, we see 13 printed out, like we would expect

  • What is the syntax of a language (say English)?
       - It describes what are valid things to say in that language
          - "I like dogs" is syntactically correct (it's a determiner followed by a verb followed by a noun)
          - "I dog like" is not syntactically correct (it's a determiner followed by a noun followed by a verb)
       - programming languages also have there own syntax
       - it describes what is valid in a language

  • when syntax fails...
       - have we seen any examples of syntax so far?
          - all of the math operations have implicit syntax
             >>> 4+
             Traceback (most recent call last):
             File "<string>", line 1, in <fragment>
             invalid syntax: <string>, line 1, pos 3
              - this big mess tells us that we had a syntax error

          - assignment as well

          >>> dave =
          Traceback (most recent call last):
          File "<string>", line 1, in <fragment>
          invalid syntax: <string>, line 1, pos 7
       - the nice thing about using the WingIDE is that when you save a file, it checks the syntax and tries (but doesn't always succeed) at highlighting where the syntax error is
          - sometimes look at the line right before it if you can't find it on that line