CS150 - Fall 2013 - Class 2

  • exercises

  • admin
       - Videos
       - Lab
          - requires a door code (which I gave you in class)
             - don't give it to other students not in the class
          - be careful with food, etc.
          - If you have problems you can e-mail me and/or cshelp@cs.middlebury.edu
       - First lab-prep
          - available online
          - due before class on Friday
          - shouldn't take very long
       - Laptops
          - We only have 22 computers in the lab. I will need some of you to work on your laptops on Friday.
          - There are instructions on the course web page for how to install python and the Wing IDE
          - Colby Horn will be available W 1:30-2:30pm and Th 1:30-3:30pm in MBH 505/506 to help out if you have problems.

  • programs in Python
       - besides interacting with the shell, we can write statements in a file and then run them
       - the top 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
  • bbq code: quick review
       - Python supports a variety of mathematical operators (+ - / * ** %)
       - Python is a "strongly-typed" language. Everything has a type.
       - We can create variables to store values
          <variable_name> = <value>

          - when python evaluates this:
             - it evaluates the expression on the right hand side
             - then stores that value into the variable name
          - this is assignment NOT equality
       - # is the syntax for comments
          - everything following the # sign to the end of the line is ignored by python
       - Wing IDE

  • 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

  • functions
       - how did we convert an int to a float?
       - another way is to tell the shell you want the value represented by that expression to be a float

          >>> float(11)
          >>> float(11)/2

       - float is a "function". What is a function in mathematics?
       - A function in Python
          - has a name
          - has zero or more parameters
             - how many parameters does float have?
          - generally does something
          - often gives us a value
             - what type of value does float return?
                - a float :)
          - Python has many built-in functions. What are some that might be useful from a math standpoint?
             - abs (absolute value)
             - round
             - int (throws away the decimal part)
             - many more...

             >>> abs(10)
             >>> abs(-10)
             >>> round(10.4)
             >>> round(10.5)

          - Another interesting function: type
             - gives you the type of the expression
             >>> type(11)
             <type 'int'>
             >>> type(11/2)
             <type 'int'>
             >>> type(11.0/2)
             <type 'float'>

  • writing your own functions
       - the important components for a function are
          - the name of the function
          - how many parameters (or arguments) the function takes
          - what value it returns/gives you when it's done
       - We can define our own functions using the following syntax

          def function_name(parameter1, parameter2, ...) :
             return expression # this is optional
          - function_name is the name of the function (i.e. what you want it to be called)
          - parameters are the list of parameters/arguments that are expected
             - you can use the parameters in the program like variables
             - when you call the function, the number of parameters specifies the number you must supply in the function call
          - the spacing indicates which statements are within a function
             - the tab key in WingIDE gives us 4 spaces
          - the "return" statement is the value that we want to return to whoever called us. We won't always have a return statement.

  • look at simple-functions.py code
       - Anything new here?
          - We have a third type, a string
          - strings are denoted by quotes and generally represent text
       - What do these three functions do?
          - dog_years: calculates the number of dog years, given human years
          - dog_stats: prints out some information about a dog
          - interest_calculator: calculates the amount of interest earned for a given amount of money at a particular rate
       - If we "run" this file, what do you think will happen?
          - nothing gets printed out
          - but we've now defined 3 new functions that we can use:

          >>> dog_years(7)
          >>> dog_stats()
          Fido is the name of the dog
          He is a good dog
          >>> interest_calculator(1000, 3)
       - notice that the number of parameters defines how many arguments we must specify
       - when a function is called
          - we evaluate each of the arguments
          - these values are associated with the parameters
          - then we execute line by line of the function
          - if there is a return statement, where the original function call was made is replaced by the value returned
       - notice if we try to call one of our functions with the wrong number of arguments, we get an error

          >>> interest_calculator(1000)
          Traceback (most recent call last):
          File "<string>", line 1, in <fragment>
          TypeError: interest_calculator() takes exactly 2 arguments (1 given)

          - the first two lines don't mean much, but the last line tells us it takes 2, but we only gave it 1
       - if we knew that we wanted to run some of these functions, we could also add this to the end of the file and then that would get executed when we run it

  • strings
       - strings are another type (just like int of float)
       - They represent text
       - You can either use double quotes "this is a string" or single quotes 'this is also a string'
       - but you can't mix the two for any given string 'this is not a valid string"
       - if you want to concatenate two strings you can us the plus sign

          >>> "this is one string " + "plus another one"
          'this is one string plus another one'
       - If you want to include an int or a float in a string, you need to convert it to a string (just like int to float)

          >>> x = 4
          >>> "The value of x is " + str(4)
          'The value of x is 4'

  • advanced_dog_stats
       - has a single parameter
       - say we call the function as:
          >>> advanced_dog_stats(7)

       - The following will happen
          - first, we'll evaluate the argument to the function and get 7
          - 7 will then get associated with the parameter year
          - the first statement in the function calls our other function, dog_stats(), which will print out it's text
          - then to figure out the print statement
             - dog_years is called with years, which has the value 7
             - we get back the value 35 from this function call
             - it will get converted into a string from the str call
             - then all three strings will get concatenated together and the final string printed

       - this will result in the following being printed

          Fido is the name of the dog
          He is a good dog
          He is 49 years old