# CS21: Quiz 4 Study Guide

### In addition to all concepts from Quiz 1, Quiz 2, and Quiz 3...

#### You should understand and/or be able to use the following Python concepts:

• formal parameter
• actual parameter (also known as an argument)
• function definition
• function call
• call stack
• scope of a variable
• top-down design
• bottom-up development
• stubbed-out function
• pseudocode
• file I/O

### 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, given a list of numbers, calculates 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, given a list of numbers, finds 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"

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?

1. For the following program, show the full output when the program is run to completion, and draw the stack as it would look just before the computer executes the return count statement.
def update(L, S):
count = 0
data = S.split()
for item in data:
if item not in L:
L.append(item)
count = count + 1
# draw stack here
return count

def main():
words = ['roses','are','red']
print(words)
line = 'violets are blue'
result = update(words,line)
print(result)
print(words)

main()

1. 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"