Computer Science 15-112, Spring 2013
Class Notes: Getting Started in 15-112

Logistics and Preliminaries
Running Python
Hello World in Python
Python Comments
Errors
Basic Console Output
Basic Console Input
Importing Modules
The vim editor
The Linux Shell
Getting Started with Variables
Getting Started with Writing Functions

Getting Started in 15-112

Logistics and Preliminaries
1. 15-112 vs 15-110 (Are you in the right course?) (Really?)
2. Programming vs Computer Science
3. Course Objectives (eg, student project gallery)
4. Waitlist policy
5. My CMU web page (http://www.cs.cmu.edu/~koz/)
6. Course web site (http://www.cs.cmu.edu/~112) and backup site here.
7. Course Policies / Syllabus
8. Your well-being and happiness
9. Course mailing list + archives
  https://lists.andrew.cmu.edu/mailman/private/15-112/2013/date.html
10. Course schedule
Running Python
1. via TryPython or repl.it
  1. Simply go to: http://www.trypython.org/
  2. Or go to: http://repl.it (and choose Python, of course)
2. Skulpt + HTML5/Canvas: A hint of things to come...
3. via Windows/Mac Python install
  1. Install Python
    from: http://www.python.org/download/releases/2.7.2/
    Note: probably already installed on Macs!
    Note: be sure to use appropriate 32-bit or 64-bit install!
  2. Add the python directory to your system path (also probably not required on Macs, here are some instructions for Windows):
    1. First, you have to find where Python installed itself. You might try to look for it in one of these locations:
      - c:\program files\python27
      - c:\program files (x86)\python27
      - c:\python27
    2. Once you found it, you have to tell the Command Shell where to find it (silly, but true), as such:
      1. Go to Start / Control Panel / System / Advanced System Settings / Environment Variables
      2. On bottom half of window (System Variables), select Path and click Edit button
      3. In the variable value textbox, go to the end and type a semicolon, then...
      4. Type the path to the directory where you found the Python install (back in step 1 above)
      5. Click ok as needed (three times, probably) to finish the procedure.
      6. Changes will be visible the next time you open a Command Prompt (or shell or terminal)
  3. Run a Command Prompt or Command Shell or Terminal
  4. Type python or type python myscript.py
    (If it says that python is not recognized or found, it means you did not set your path correctly above. Please see a course staff member for assistance!)
  5. To run IDLE (Python GUI): either run IDLE from Start menu, or type: python -m idlelib.idle
  6. Also suggested: Install Komodo Edit (http://www.activestate.com/komodo-edit) (Note: Komodo Edit is free, Komodo IDE is not. Use Komodo Edit.)
    1. How to run Python in Komodo Edit
      1. Mac OSX:
        1. go to Toolbox -> Add -> New Command...
        2. in the top field enter the name 'Run Python file'
        3. in the 'Command' field enter this text: %(python) -u %F
        4. click Ok.
      2. Windows:
        1. go to Toolbox -> Add -> New Command...
        2. in the top field enter the name 'Run Python file'
        3. in the 'Command' field enter this text: "%(python)" -u "%F"
        4. click Ok.
    2. Even better, how to save-and-run Python in Komodo Edit
      This solves the problem that Komodo Edit will run the last-saved version, not the current version, of your file.
      1. First add the Run-Python command from above
      2. Be sure you can see the Run-Python command in the Toolbox (probably on the right-hand side)
      3. Load a Python file and modify it but don't save it
      4. Select Tools/Macros/StartRecording
      5. Select File/Save
      6. Double-click on the Run-Python Command
      7. Select Tools/Macros/StopRecording
      8. Select Tools/Macros/SaveToToolbox, name your Macro Save-and-Run Python (or some such)
      9. Right-click on the Save-and-Run Python command in the toolbox, and select properties
      10. Select Key Binding, then click in New Key Sequence box, enter F5 or ctrl-2 or whatever you like, then select Add (to the right of the box) and then OK (at the bottom).
      11. That's it! Now when you press F5 (or whatever binding you used), you'll save-and-run the Python file.
4. via GHC Cluster
  1. Physically go to a GHC cluster (say, 5201 or 5205)
  2. Login using your andrew id and password
  3. Run SystemTools/Terminal
  4. Type python or type python myscript.py
  5. To run IDLE (Python GUI): in an xterm, type: python -m idlelib.idle &
5. via ssh
  1. Install ssh client (recommended: PuTTY (for ssh) and SSH Tectia Client (for file transfer) from https://my.cmu.edu/, go to Computing, then Download Software)
    Note: probably already installed on Macs!
  2. Run ssh client connected to linux.andrew.cmu.edu
    On Windows, run PuTTY or "SSH Tectia - Terminal" from Start menu, select "Quick Connect", enter linux.andrew.cmu.edu
    On Mac/Linux, in a terminal just type: ssh andrewid@linux.andrew.cmu.edu
    (of course, replace andrewid with your actual andrew id).
  3. Or run ssh client connected to ghcN.ghc.andrew.cmu.edu
    1. Choose N from 10 to 80 (to load balance, choose randomly, maybe add one each time)
  4. Type python or type python myscript.py
Hello World in Python
1. Interactive
  1. in python, type:
    print "Hello World!"
2. From a script (via ssh)
  1. Do this outside of Python, running from an ssh shell (that's the whole point here)
  2. create the script (type this, exactly (maybe copy-paste it), into the ssh shell):
    echo 'print "Hello World!"' > myscript.py
  3. Then type this into the ssh shell:
    python myscript.py
Python Comments
print "Hello World!" # This is a comment
# print "What will this line do?"

Errors

Syntax Errors (Compile-Time Errors)

print "Uh oh! # ERROR!  missing close-quote

Python output:
   SyntaxError: EOL while scanning string literal

Runtime Errors ("Crash")

print 1/0 # ERROR!  Division by zero!
Python output:
   ZeroDivisionError: integer division or modulo by zero

Logical Errors (Compiles and Runs, but is Wrong!)
print "2+2=5" # ERROR! Untrue!!! Python output:
2+2=5

Basic Console Output
1. Basic Print Statement
  print "Carpe"
  print "diem"
2. Print with Comma
  print "Carpe",
  print "diem"
3. Print Multiple Items
  print "Carpe", "diem"
  
  Another Example:
  # Compute the hypotenuse of a right triangle
  a = 3
  b = 4
  c = ((a**2) + (b**2))**0.5
  print "side a =", a
  print "side b =", b
  print "hypotenuse c =", c
Basic Console Input
1. String with raw_input()
  name = raw_input("Enter your name: ")
  print "Your name is:", name
2. Number with raw_input() (error!)
  x = raw_input("Enter a number: ")
  print "One half of", x, "=", x/2 # Error!
3. Number with raw_input() and int()
  x = int(raw_input("Enter a number: "))
  print "One half of", x, "=", x/2
4. Strings and Numbers with input()
  x = input("Try this with a string and then a number: ") # input() is not recommended!
  print 2*x

Importing Modules

Fails:
print math.sqrt(5)

Python output:
   NameError: name 'math' is not defined

Works:
import math
print math.sqrt(5)

Python output:
   2.2360679775

The vim editor

After creating myscript.py in the previous step, type this into the ssh shell:
vim myscript.py
You start vim in command mode (so you cannot enter text yet!)

Some vim commands:

Key	Command
h or Left	Left
l or space or Right	Right
k or Up	Up
j or Down	Down
0	Start of line
$	End of line
nG or :n	Go to line number n
i	Insert at cursor
a	Insert (append) after cursor
I	Insert at beginning of line
A	Insert (Append) at end of line
ESC	Exit insert mode (switch back to command mode)
x	Delete current cursor position
dd	Delete current line
ndd	Delete n lines
p	Paste ("push" last text cut/deleted) at the cursor
u	Undo last change
ctrl-r	Redo
/text	Search for text
n	Search for next occurrence of text
:w	Write file
:w!	Overwrite file (ignoring warnings)
:wq	Save (if necessary) and quit
:q	Quit if saved (or warn if unsaved)
:q!	Quit without saving (be careful!)

Printable list of vim commands: vimCommands.doc
More vim commands (Google vim tutorials, pick your favorite!)
Also, try: /usr/bin/vimtutor (or just vimtutor)
Your .vimrc file
1. In ssh shell, type this (maybe copy-paste it to be sure to exactly match):
  vim ~/.vimrc
2. Then, enter input (or view) mode (say, by the command A)
3. Then, copy these lines into that file:
  set tabstop=4
  set shiftwidth=4
  set expandtab
  set autoindent
  set number
  syntax on
4. Finally, save the file and exit. Changes take effect next time you run vim.
Copy-pasting code (say, from the course website) into vim
1. The problem: vim will try to autoindent already-indented code, leading to a mess
2. The solution: first switch vim into paste mode, with this command:
      :set paste
  Then do the paste. Then switch out of paste mode with this command:
     :set nopaste
  That�ll do it! You can also try to add this to your .vimrc file:
     set pastetoggle=<F12>
  Then, when you press the F12 key, you should automatically switch between paste and nopaste modes. Unfortunately, this did not work for me when ssh�ing via Tectia, but it did work in all other cases (including ssh�ing via Putty)

The Linux Shell

Some Linux shell commands (enough to get started):

Command	Description
whoami	Print login id of current user
hostname	Print name of current host (computer)
users	Print user names of all users on current host
pwd	Print present working directory (where am I?)
cd dir	Change directory to dir
cd ~	Change directory to user's home directory (~)
ls	List files in the current directory
ls -alst	List files with lots of useful information
ls dir	List files in the directory dir
mkdir dir	Make a directory named dir
rmdir dir	Remove the directory named dir (must be empty)
vim file	Use vim to edit the given file
cat file	Print (concatenate) the file(s) to standard output
more file	Print the file, one page at a time
cp file1 file2	Copy file1 to file2
cp -r dir1 dir2	Copy (recursively) directory dir1 to dir2
mv file1 file2	Move (rename) file1 to file2 (may be directories)
rm file	Remove the file
man cmd	Display the manual pages for cmd
history	Print recent history of shell commands
up/down arrows	Navigate history of shell commands (also ctrl-p/ctrl-n)
cmd	Run the command cmd eg: /afs/cs.cmu.edu/academic/class/15112-f11/bin/hello or: /usr/bin/vimtutor
./cmd	Run the command cmd in the current directory
ln -s cmd	Make a symbolic link to cmd in the current directory
alias name cmd	Make an alias with the given name for the given command eg: alias hi /afs/cs.cmu.edu/academic/class/15112-f11/bin/hello
alias	List all aliases
unalias name	Remove the alias with the given name
clear	Clear the terminal screen
date	Display the current date and time
finger user	Print information about user (use Andrew id for user)
ping machine	Test a network connection to the given machine (ctrl-c to stop)
exit	Exit the shell (and logout if in a login shell)
logout	Logout of a login shell

More Linux shell commands (Google Linux shell tutorials, pick your favorite!)
That said, here's a nice one (among many): http://www.ee.surrey.ac.uk/Teaching/Unix/
Your .cshrc file
1. In ssh shell, type this (maybe copy-paste it to be sure to exactly match):
  vim ~/.cshrc
2. Then, append these lines to the end of that file:
  set path = ($path ~/bin /afs/cs.cmu.edu/academic/class/15112-f12/bin)
  alias a alias
  a hi hello
  mesg n
3. Finally, save the file and exit. Changes take effect next time you login.

Getting Started with Variables

Define a variable and use it
x = 3
print x
Same, but with a nicer UIx = 3
print "x =", x

Use a variable without defining it
print "yikes =", yikes # ERROR! No such variable as yikes

Assigning and re-assigning
x = 3
print "x =", x
x = 4
print "x =", x

Using two variables
x = 3
y = 4
print "x =", x
print "y =", y
print "x + y =", (x+y)

Getting Started with Writing Functions
1. See Notes on Getting Started with Writing Functions

carpe diem - carpe diem - carpe diem - carpe diem - carpe diem - carpe diem - carpe diem - carpe diem - carpe diem