To Lecture Notes

IT 211 -- Feb 11, 2019

Review Exercises

1. What does it mean to say that a computer can only add two numbers at a time?
   Ans: Even if a Python statement adds three numbers like this:
   

   n = 1 + 2 + 3
   

   the addition proceeds from left to right, the first two numbers are added; then the sum is added to the third: 1 + 2 == 3; 3 + 3 == 6. Even if 100 numbers are added in this loop:
   

   sum = 0.0
   for n in 1..100
     sum += n
   end
   print sum
   

   n is added to sum to produce the updated sum.
2. Translate this pseudocode into Python. This pseudocode is supposed to compute the product of a list of numbers, terminated by a blank line.
   

   initialize product
   input a line
   while line is not blank
       convert line to float value
       update product
       input another line
   end
   output product
   
   Ans:
   product = 1.0
   line = input("Enter a float value: ")
   while line != "":
       value = float(line)
       product *= value
       line = input("Enter a float value: ")
       
   print("Product: ", product)
   

3. Convert your script from Exercise 4 to a script that reads the floating point values from the input file values.txt.
   

   Ans: 
   product = 1.0
   fin = open("values.txt", "r")
   line = readline( )
   while line != "":
       value = float(line.strip( ))
       product *= value
       line = readline( )
       
   print("Product: ", product)
   

Sequential Processing Practice Problems

Fix up the the Employees Example to work these problems. Use the input file employees.txt.
Here are the answers to the following sequential processing problems.

1. What is the average salary of all employees?
2. The average salary for the women.
3. The average age of the male employees in Chicago.
4. The average salary of the desired city, where the desired city is entered from the keyboard.

User Defined Methods

• A method is a named block of code that can be invoked (called) by using the name of the method in a script for a standalone method. (We will see how to define and use object methods later.)
• Other names for a method are function, subroutine, or procedure.
• A method definition starts with the def keyword.
• Here is an example of a short method:
  

  # define method
  def say_moo( ):
      print("moooooooo...")
  
  # Invoke method three times.
  say_moo( )
  say_moo( )
  say_moo( )
  
  # Output:
  moooooooo...
  moooooooo...
  moooooooo...
  

Nested for Loops

• How many dollar signs are printed?
  

  for i in range(0, 10):
      for j in range(0, 10):
          print("$")
  

• Write a Python script to draw a rectangle of stars (*) of width 20 and height 12 like this:
  

  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  *************************
  

  The restriction is that your print statement is not allowed to print more than one * at a time.
• Draw triangles like this with double for loops. Use write statements that only print one * or space at a time.
  

  *           *****           *           **
  **           ****          **          ****
  ***           ***         ***         ******
  ****           **        ****        ********
  *****           *       *****       **********
  Triangle 1  Triangle 2  Triangle 3  Triangle 4
  

  Ans: here is a script for drawing triangles 1, 2, and 3..

Project 4

• Look at the Project 4 Description
• Look at the LetterH and LetterZ Examples.