Class Notes: Getting Started

1. Logistics and Preliminaries
2. Running Python
3. Hello World in Python
4. Python Comments
5. Basic Console Output
6. Syntax, Runtime, and Logical Errors
7. Basic Console Input
8. Importing Modules

Logistics and Preliminaries

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• Course web site (http://www.cs.cmu.edu/~112)
• Course Objectives / TP Gallery
• Programming vs Computer Science
• Course Policies / Syllabus
• Course Schedule

Running Python

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Note: Your TA's will be happy to help with any of these steps!
• Install Python 3.10.x and VSCode (see the Getting Started with VSCode notes here)
• Run VSCode (the default Python text editor / IDE)
• Edit your Python file
• Run your code (command-b or control-b in VSCode once you followed our setup instructions)

Hello World in Python

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• Command typed into shell
  

  print("Hello World!")

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• File edited in VSCode
  Download helloWorld.py, edit it in VSCode, and run it.

Python Comments

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• Comments are for the benefit of human readers
  

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

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Basic Console Output

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• Basic print function

  print("Carpe")
  print("diem")

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• Print on same line

  # You can separate multiple values with commas
  print("Carpe", "diem")
  
  # You can also use end="" to stay on the same line
  print("Carpe ", end="")
  print("diem")

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• Print using f strings

  x = 42
  y = 99
  # Place variable names in {squiggly braces} to print their values, like so:
  print(f'Did you know that {x} + {y} is {x+y}?')

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Syntax, Runtime, and Logical Errors

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• Syntax Errors (Compile-Time Errors. Python never starts running.)

  print("Uh oh!) # ERROR!  missing close-quote
  
  # Python output:
  #   SyntaxError: EOL while scanning string literal

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• Runtime Errors ("Crash." Python starts running but encounters a problem.)

  print("We started running...")
  print(1/0) # ERROR!  Division by zero!
  print("...we never reach this line")
  
  # Python output:
  #   ZeroDivisionError: integer division or modulo by zero

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• Logical Errors (Compiles and runs, but is wrong!)

  print("2+2=5") # ERROR!  Untrue!!!
  
  # Python output:
  #   2+2=5

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Basic Console Input

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• Input a string

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

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• Input a number (error!)

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

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• Input a number with int()

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

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Importing Modules

  video Python has many different features already implemented, but not available immediately. To use these features, you must import a module. You can find these modules by reading the Python documentation online.


• Call without importing

  print(math.factorial(20))  # we did not first import the math module
  
  # Python output:
  #   NameError: name 'math' is not defined

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• Call with importing

  import math
  print(math.factorial(20))  # much better...
  
  # Note that the module name is included before the function name, separated by a .
  # A function called from a module like this is more precisely known as a "method."
  # We'll explain functions this week, and more about methods in a future week!

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