15-112: Fundamentals of Programming and Computer Science
Class Notes: Getting Started

Logistics and Preliminaries
Running Python
Hello World in Python
Python Comments
Syntax, Runtime, and Logical Errors
Basic Console Output
Basic Console Input
Importing Modules

Logistics and Preliminaries
- 15-112 vs 15-110 (Are you in the right course?) (Really?)
- Programming vs Computer Science
- Course Objectives (eg, student project gallery)
- Waitlist policy
- Course web site (http://www.cs.cmu.edu/~112) and backup site.
- Course Policies / Syllabus
- Your well-being and happiness
- Course Resources
- Optional Textbooks (use them!)
- Course Schedule
Running Python
Note: Your CA's will be happy to help with any of these steps!
- Install Python
  If Python is not yet installed (as it already is, say, on most Mac and Linux machines), download and install Python 2.7.x from python.org's download page
- Run IDLE
  IDLE is a text editor just for code, and it is available in nearly all Python installations. In many cases, you will have an icon you can click to run IDLE. Otherwise, you can run IDLE from the command line with:
  python -m idlelib.idle
- Or... Run another code editor
  Everyone is expected to be able to use IDLE on the cluster computers (where you have recitation, and where we could take computer-based quizzes on those same machines). Beyond that, you do not have to use IDLE to study or do your homework. This is especially important on Macs, where IDLE is known to be unstable (though you still need to be able to use IDLE even on Macs). In general, we recommend Sublime, but you can use Sublime, or Komodo Edit, or PyCharm, or Wing, or many others. Each is set up a bit differently, so get your CA to help you if needed. Also, note that we disrecommend Eclipse for 15-112 (it's great for experts, but too complicated for novices). Note: if you do use Sublime, we recommend that you follow the steps in this file to help set it up as an IDE and not just a standalone editor.
- Edit your Python file
  In IDLE (or whatever editor you are using), you need to open a Python file or create a new Python file, which then places you in the editing window. That is where you write your code. Be sure to save your code in a file ending in .py (like: foo.py) before running it!
- Run your code
  Each IDE has its own way to run code. In IDLE, it is F5 ("Run Module"). In Sublime, if you followed our instructions above, it is command-b on Macs and control-b on Windows. Either way, this loads and runs your code in the Python Shell (the interpreter that runs Python code, instead of editing it).
Hello World in Python
- Command typed into shell
```
print "Hello World!"
```
- Function typed into shell
```
def helloWorld():
    print "Hello World!"

helloWorld()
```
- File edited in IDLE
  Download helloWorld.py, edit it in IDLE, and run it.

Python Comments

print "Hello World!" # This is a comment
# print "What will this line do?"

Syntax, Runtime, and Logical 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

Basic Print Statement
```
print "Carpe"
print "diem"
```

Print with Comma
```
print "Carpe",
print "diem"
```

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

String with raw_input()

name = raw_input("Enter your name: ")
print "Your name is:", name

Number with raw_input() (error!)

x = raw_input("Enter a number: ")
print "One half of", x, "=", x/2 # Error!

Number with raw_input() and int()

x = int(raw_input("Enter a number: "))
print "One half of", x, "=", x/2

Strings and Numbers with input()

x = input("Try this with a string and then a number: ") # input() is not recommended!
print 2*x

A more robust example

def inputInteger(prompt):
    # We won't use while or try/except until later,
    # but they are handy here to keep asking for input
    # until we get the kind of input we are looking for
    while True:
        userInput = raw_input(prompt + " --> ")
        try:
            return int(userInput)
        except:
            print "That was not an integer. Please try again."

# Try entering a non-integer age...
age = inputInteger("Enter your age in years")
print "In 10 years, you will be", (age+10), "years old! Wow!"

Importing Modules

Calling without importing

print math.factorial(20)  # we did not first import the math module

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

Calling with importing

import math
print math.factorial(20)  # much better...

What does a module export?
- Use dir() and help()
```
import math
dir(math)
help(math)
help(math.factorial)
```
- Use online documentation (usually better)
  Try here: https://docs.python.org/2/library/math.html

15-112: Fundamentals of Programming and Computer Science Class Notes: Getting Started

15-112: Fundamentals of Programming and Computer Science
Class Notes: Getting Started