To Lecture Notes

IT 211 -- Jan 9, 2019

Review Exercises

1. What are the four ways to create a Python script and execute it?
   
   1. Run a script file called a module from the Idle IDLE.
   2. Enter and run Python statements interactively in an IDLE Shell Window
   3. Enter and run Python statements using interactive python. Enter py to start Interactive Python.
   4. Run the script test.py from a Command/Terminal Window like this:
      

      > py test.py
      

2. What is the exponentiation operator?
   Ans: **
3. Translate these formulas into Python:
   

   a = x + 2y x - 3y

   b = 1 + 3x-5n

   Ans: a = (x + 2 * y) / (x - 3 * y)
        b = 1 + 3 * x ** (3 * n)
   

   
   Create a Python script that computes a and b for the values x = 3, y = 5, and n = 2. In the output, round the value of a to four digits after the decimal; round the value of b to six digits after the decimal. Ans: Here are the script and output:
   

   x = 3
   y = 5
   n = 2
   a = (x + 2 * y) / (x - 3 * y)
   b = 1 + 3 * x ** (-5 * n)
   print("Output:", round(a, 4), round(b, 6))
   
   # Output:
   Output: -1.0833 1.000051
   

4. Download and run the script from the FrenchFlag Example and display it in a computer lab using Adobe PhotoShop.
   For your Windows computer, you can download IrfanView
   For your Mac computer, you can download Toy Viewer for free. It requires operating system 10.8+.
    
5. What does this PPM image look like?
   PPM means Portable Pixel Map, represented by the magic number P3.
   

   P3 5 5 255
   128 128 128  128 128 128  128 128 128  128 128 128  128 128 128
   128 128 128  255 128   0  255 128   0  255 128   0  128 128 128
   128 128 128  255 128   0    0 128 128  255 128   0  128 128 128
   128 128 128  255 128   0  255 128   0  255 128   0  128 128 128
   128 128 128  128 128 128  128 128 128  128 128 128  128 128 128
   

   The header line P3 5 5 255 indicates the following:
   

   P3:  Portable Pixel Map (ppm)
   5:   Image is 5 pixels wide
   5:   Image is 5 pixels high
   255: Component value range is 0 to 255.
   

   Pixels contain red, green, and blue components.
   Ans: 128 128 128 represents the color median dark gray; 0 255 128 represents orange because it is between red = 255 0 0 and yellow = 255 128 0; 0 128 128 is dark bluegreen, which is teal. Here is the image: tiny.ppm.

DOS/Unix Commands

• A reference of DOS/Unix Commands.
• Here is a more extensive table of Unix commands: Unix for DOS Users.

Constants, Datatypes, and Variables

• See the document Constants, Datatypes, and Variables

Operators

• See this Python/JavaScript Operator Dictionary
• Commonly used arithmetic operators are
  
  

  Operator	Meaning
  +	Addition Concatenation
  -	Subtraction
  *	Multiplication Addition
  /	Floating Point Division
  //	Integer Point Division
  **	Exponentiation
  %	Remainder from Floating Point Division
  =	Assignment

  
  
• An example that uses the assignment operator =:
  

  n = 3 * 5
  

  This statement assigns the value on the right, which is 3 * 5 = 15 to the variable n.
• We will discuss these operators later:
  

  <  <=  >  >=  ==  !=  and  or  not
  

• Important: = is an assignment operator; == is a comparison operator.

Variable Trace Tables

• A variable trace table or variable trace shows how the values of the variables in a script change as the script executes.
• What is the output of the following script. Construct the Pythontrace to help you find the output.
  

  a = 5
  b = 7
  a = 3
  print(a, b)
  
  Ans:
  Variable Trace:   a     b
                  -----+-----
                    5  |  7
                    3  |
  Output: 3 7
  

  The latest value of a variable is always the one used. This is why a 3 is printed for a instead of 5
  There is no space between the 3 and 7 in the output because there is no space between a and b in the print statement.
• The value of a variable that is used in the execution of a script is always the most recent value of that variable.
• Look at the Trace1 Example. Ans:
  

  # Trace1 Exampl
  # Perform variable trace and predict output. 
  
  a = 3
  b = 7
  c = 5
  a = 2 * a + 2 * b + c
  b = a + c
  c = 2 * c + 1
  a = a + 2
  b = b + 3
  c = c + 4 
  
  Variable Trace:
  
    a     b     c    
  -----+-----+-----
    3     7     5
   25    30    11    a = 2 * a + 2 * b + c
   27    33    15      = 2 * 3 + 2 * 7 + 5  
  # Output:            = 6 + 14 + 5
  Output: 27 33 15     = 25
  

• A variable trace is used to desk check a script to verify that it is producing the correct output.

Input Validation

• Practice Problems:
  
  1. Modify the Overtime Example so that only positive values are accepted as input for the hours worked and hourly salary. This example contains statements to validate hours_worked and hourly_salary to insure that they are non-negative.

Project 1

• Look at Project 1.
• See this Guide to Submitting Projects.