In Class: Week 1, Wednesday and Friday

In your cs21 subdirectory create a directory named 'class' and cd into that directory, and then make a directory named 'week01' and cd into that directory:

$ cd                                 # cd into your cs21 subdirectory:
$ cd cs21       
$ pwd	          
  /home/your_user_name/cs21

$ mkdir class                        # make a new directory named class   
$ cd class                           # cd into it 
$ pwd		
  /home/your_user_name/cs21/class

$ mkdir week01                      # make a subdirectory named week01   
$ cd week01                         # cd into it
$ pwd           
  /home/your_user_name/cs21/class/week01

Now copy over all the files from my public/cs21/week01 directory into your week01 directory (remember to add the dot as the destination of the cp command). From your week01 directory:

$ cp ~turnbull/public/cs21/week01/* .
$ ls

We are going to start by looking at the firstProg.py program together, so go ahead and open it in vim:
```
$ vim firstProg.py  
```
This is the command to run the program in the python interpreter:
```
$ python firstProg.py
```
Try modifying the firstProg.py in vim to print out a different string, and run it.
Next we will look at add.py. This program follows the general pattern that all (most) programs follow:
1. input phase: get data values for program variables
2. compute phase: do something with the program's data (some operations on variables and values)
3. output phase: display the results to the user
Finally, we will look at loop.py. This is an example of a program that uses a for loop. A loop construct tells the python interpreter to repeat a set of instructions some number of times.

Running Python Programs

You can run your python program in the python interpreter two different ways:

Have the interpreter run your program to completion:
```
$ python firstProg.py
  Hello There
$
```
Run the interpreter in interactive mode on your program (the python interpreter will continue to run after running your program, and you can enter new python commands at the prompt >>>:
```
$ python -i firstProg.py
  Hello there
  >>> print "I love computer science!"
  I love computer science!
  >>> print 13 + 7 + 11
  31
  >>>
```

To exit the python interpreter hold down the control and the D keys together (CNTL-D)

You can also just start the python interpreter in interactive mode without giving it a python code file to run:

  $ python
  >>>