COMP 150 Lab 1 -- Starting With Python

Task 0, Set up for DH 342

• If you are at your own computer with you, you can set up a folder for your Python programs anywhere you like.  I suggest you create a dedicated folder, not just the Desktop.   You might call it python.  Create it and open that folder. This will be your Python folder in later discussion.  You can ignore instructions in future labs to copy in old work or save new work.  You should have installer Python to continue. 
• If you have a flash drive:  plug it into the computer.  On the machines in 342 DH, there is one USB port in the front and many on the back of the computer.  Unfortunatelt the one on the front is well hidden.  Open the front plastic cover saying Dell, see the retangular projection.  Underneath is a plug, at almost a vertical angle.  Plig in your flash drive carefully.  (Again I strongly suggest a long string connected to belongings you will not forget.)  Open My Computer (on the desktop) to see what where the flash drive is mounted, and open that drive.  Make a directory for your Python programs -- I suggest calling it python.  Change to that directory and leave the folder open.  This will be your Python folder in later discussion.
• If you do not have a flash drive and you are at a lab computer:  Open My Computer from the desktop, and then D:.  Create a folder with your name or initials to make it easy to save and remove things.  Change to that folder.  This will be your (temporary) Python folder in the later discussion.   

Task 1, Start IDLE

In Windows, go to your Python folder, and you should now see the shortcut you downloaded.  Double click on the shortcut.  After this the instructions are the same in any operating environment.

Python comes with a tool called IDLE.  Idle can be thought of as a shell running an interpreter, as an editor or as an integrated development environment (IDE.).  Depending on the configuration, Idle can start up showing either of two windows.  You could see the Python Shell, looking something like

Look at the Python Shell.  The >>> is the prompt, telling you IDLE is waiting for you to type something.

Task 2, Using Python to do arithmetic

Try entering the following literal values at the prompt. (Hit ENTER after each)

Something odd should occur. Describe it on paper.

Class discussion should eventually provide an explanation. For now, remember, Computers cannot precisely represent all decimal numbers.

Now try entering the following arithmetic expressions using only a single operator each. Note the response from the interpreter in each case:

Task 3, Assigning values to variables, the print statement, and the id function

To create a variable in Python we assign a value to the name. Python does the rest.

 =  is the Python assignment operator. To use the value stored in a variable we simply use the variable in an expression. Try the following, noting the output.

Use the above results to understand the following points:

1. print is a way to tell the interpreter to display one or more values, with multiple values separated by commas.
2.  = is assignment. Even if we read x=5 as "x equals 5", the meaning is "assign a memory location to the name x and assign the value 5 to that memory location". This is not the same = as in algebra; 5=x makes no sense at all in Python.
3.  To use a value stored in a variable, like 5 stored in x, just refer to x in an expression
4.  id(x) invokes the built-in id function on variable x. The function returns the memory location (address, unique internal identifier) Python assigned to the name x. We will not often use this function, but it will become important to understand.
5. When we assign a new value to a variable, e.g. x = 7, Python assigns a new memory location to the variable x.  See that after the value of x is changed, its id changes.  (Fortunately Python takes care of reclaiming unused memory so we do not have to worry about it. This is called garbage collection.)

Task 4, Python Identifiers

Recall (or look up) the rules for Python identifiers on p. 29, the start of section 2.3.1.   Figure out which of the following is legal from the rules, and then test  to see if you are correct.  The expressions on the right side of the equals signs are all legal.  They define various sorts of data that we will discuss as we go on:  integers, floating point numbers, strings, and tuples in parentheses.

mean = 43.5
my_score = 1
my_score = "hello there teddy bear"
the letter = "a test on one line"
LoyolaU = 6
C6H0= "the big game"
2hot = 'summer'
christmasday = (12, 25)
independence-day = (7, 4)