In addition to the concepts below, you should also know the concepts from Quiz 1, Quiz 2, and Quiz 3.

You should be able to define the following terms:

• formal parameter
• actual parameter (also known as an argument)
• function definition
• function call
• call stack
• scope of a variable
• top-down design
• stubbed-out function
• pseudocode

Practice problems:

1. Write a function called getLetter() that asks the user for a letter and returns the given letter. Your function should check to make sure the user enters just a single letter. If the user enters anything other than a single letter, the function should ask again, until it receives valid input.
2. Write a function called avgList that takes a list of numbers as input and returns their average. For example, avgList([5, 10, 5, 4]) should return 6. Write a main program to test the function.
3. Write a function called maxList that takes a list of numbers as input and returns the largest number in the list. For example, maxList([7,4,8,1]) should return 8. Write a main program to test the function.
4. Consider the following program.

   def main():
       print("in main")
       test = "whee"
       check = "e"
       answer = helper(test, check)
       print("the answer is %d" % (answer))
   
   def helper(word, letter):
       print("in helper")
       print("word: %s" % (word))
       print("letter: %s" % (letter))
       x = 0
       for ch in word:
           if ch == letter:
               x = x+1
   
       # draw stack here, just before return
       return x
   
   main()
   

   • Draw the call stack as it would look just prior to returning from helper.
   • What is the program's output (i.e. what does it print when the program is run)?
   • What does the helper function compute?
   • What would helper("moobaalalala", "a") return?
   • What would helper("zoo", "a") return?
5. Write a program that reads in the lines from a file called infile.txt and creates an acronym from the first letter of each line. For example, if the input file is:

   Thanks
   Goodness
   It's
   Friday
   

   Your program will create and print out the string "TGIF"