CS 2120: Class #2

Welcome back

Let’s get started right away...

Quick Activity

What did you do this weekend/week?

What did you do with Python this weekend/week?

  • Heads up: Before we get to the “super awesome fun stuff”, we’ve got to cover the basics. I understand that the basics aren’t super awesome. Don’t worry, we’ll get there. But we can’t get there without the basics.

  • For motivational purposes, here’s what Google Image Search gave me when I searched for “awesome robot”:
    • At this point I would like to remind the class that I did NOT create a lot of this material -_-
      • I’m sooooo sorry about this

What’s a program?

  • The stuff in the computers
  • A thing that does stuff
  • A recipe
  • A sequence of instructions that specifies exactly how to perform a computation


Explain to a partner how you would go about making breakfast in the morning.


Someone explain to me how to make breakfast in the morning.

  • There, that’s basically a program.

What’s debugging?

  • Mystery novel
  • A logic puzzle
  • How you fix your mistakes
  • If you’re an experimental scientist, it’s a lot like “protocol optimization”

Quick Activity

Have you seen any Python errors yet?

What were they?

Did you understand them?


  • What’s the difference between a formal, and a natural, language?
  • Why is ambiguity so important to natural language?
  • Why is ambiguity deadly for a formal language?


Do you think there is a limit to what I can describe with a formal language?

Can I describe anything? Any computation?

HINT: Is the following statement true or false: “This statement is false.”

Okay, we’re done with the background, let’s get on with the real stuff


Write a (single-line) Python program that prints a witty message, of your choice, to the console.

Values (not the family kind)

  • Values are things that a program manipulates.
    • Strings: “abcdef”
    • Integers: 7, 42, 97
    • Floating-point numbers: 3.792, 0.000000000005
  • Notice how I described the type of each value along with the value itself

  • Computers are exceptionally stupid. You must be completely explict about everything.

  • To a computer, the integer 1 is not necessarily the same thing as the floating point number 1.0... because they have different types

  • Many of the errors you will make in programming result from mixing types inappropriately.

  • Some languages (e.g., C, Fortran, Java) are very militant about types. You have to be totally explicit about them.

  • Python is a little more relaxed. You can be explicit, but you don’t have to be. Python will guess if you don’t tell it.

  • Upside: less to worry about and less clutter in your code.

  • Downside: a longer rope gives you more fun and exciting ways to hang yourself!

  • Can I ask Python to tell me its guess for the type of a value?
    >>> type(12)
    <type 'int'>
    >>> type('Witty remark')
    <type 'str'>
    >>> type(3.75)
    <type 'float'>
  • It’s kinda’ easy to tell the type of a value isn’t it?
    • Most of the time.


Give a partner a value and have them tell you the type. Pleas ask if you run into a problem here.


  • Probably the most important feature of a procedural programming language.

  • If you’re going to pay attention only once this term... now’s the time.

  • Variables let you store values in a labelled (named) location

  • You store values into variables by using the assignment operator =
    >>> a=5
    >>> m='Variables are fun'
  • For historical reasons, we’re stuck with the ‘=’ symbol for assignment, but it doesn’t really mean the same thing as the ‘=’ sign in math.

  • In math when we write ‘a = 5’ we mean that ‘5’ and ‘a’ are equivalent as they exist. We’re not asking to change anything; we’re making a statement of fact.

  • In Python when we write
    >>> a=5
  • ... we’re saying “Hey, Python interpreter! Create a variable named a and store the value 5 in it. This isn’t a statement of fact, it’s an order!

What can you do with variables?

  • Anything you can do with values.

  • For example, we can add variables:
    >>> a = 5
    >>> b = 7
    >>> a+b
    >>> b=5
    >>> a+b
  • This seems pretty lame and straightforward now, but it’s this ability to store results that will let us do all the cool stuff later.


  • Assign various values of types string, integer and float to variables.
  • Try adding variables of the same type. What happens?
  • Try adding variables of different types. What happens?
  • Try the assignment 5=a. What happens?
  • Figure out how to display the current contents of a variable.

Choosing variable names

  • You can use whatever you want, within a few restrictions set by the language.
    • Python wants variable names that begin with a letter of the alphabet and limits what non-alphanumeric characters you can use
  • A good choice is a variable name that is descriptive of what the variable is meant to contain.
    • good: density
    • less good: d
    • bad: definitely_not_density


Suppose you’re a big fan of ‘80s Arena Rock. Create two variables, named def and leppard, set them to 19 and 87 respectively, then add them.

  • What happened? (To your code, not the band!)


  • A statement is an order to Python: “do something”.

  • An instruction that can be executed by the interpreter.

  • You type in the statement, press Enter, and Python does what you asked (or at least tries to).

  • Some statements produce immediate output, some just change things ‘behind the scenes’.

  • We’ve already been using assignment statements (=), but there are lots of other kinds of statements.

  • e.g., you should already have discovered the print statement:
    >>> leppard = 87
    >>> print leppard


  • An expression is, roughly, a thing that can be crunched down to a value.

  • More precisely, an expression is a combination of:
    • values (e.g., 5)

    • variables (e.g., leppard)

    • operators (e.g., +)
      >>> leppard * 2 + 7


  • Operators are symbols that tell Python to perform computations on expressions.
    • e.g., +, -, *, /


Generate expressions to:

    1. Add two variables
    1. Multiply two variables
    1. Add a third variable to 2
    1. Divide 3 by 1


Order of Operations

  • Python doesn’t blindly evaluate expressions; it follows the usual order of operations you learned in public school math class.

  • If you want things done in some other order, you can use () to make it explicit:
    >>> 2 + 5 * 2
    >>> (2 + 5) * 2

Are operators just for numbers?

  • Nope! Values of all sorts have operators that work on ‘em.


  • Experiment with the operators you know on strings (instead of just integers).
  • Which ones work? What do they do?
  • Try mixing strings and integers with various operators. What happens there?

Doing sequences of things

  • So far we’ve just been entering one line at a time into the Python interpreter.

  • That’s not going to scale very well for most of the stuff we want to do...

  • You can store an (arbitrarily long) series of statements in a file, and then ask Python to run that file for you.

  • The Python interpreter will execute each line of the file, in order, as if you’d typed them in.

  • There are lots of ways to run scripts. Suppose you put a series of statements into a file called myprogram.py
    • from the shell: $ python myprogram.py or ipython myprogram.py
    • from the interpreter: >>> execfile('myprogram.py')
    • if you’re using Ipython: %run myprogram
  • To edit the script, you can use any text editor that you want. You’ll have an easier time with one that is “Python aware”, though.
    • Wut?
    • EPD has the IDLE editor/IDE built in. It’s not bad.
    • PythonAnywhere has their own browser-based editor.
    • If you installed your own Python, you probably already have your own favourite editor.


Consider the sentence Def Leppard is a poor substitute for Van Halen. Write a program that stores each word of that sentence in it’s own variable, and then prints the whole sentence to the screen, using only a single print statement.

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