CS21: Quiz 3 Study Guide

In addition to all concepts from Quiz 1 and Quiz 2...

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

• stack diagrams
• scope of a variable
• function definition
• function call
• function parameters: defining and calling
• definite for loop
• indefinite while loop
• python in (and not in) operator used on lists and strings
• mutable and immutable types
• list manipulation (accumulators, indexing, etc)
• random library functions: randrange(start, stop), choice(<sequence>)

Practice problems:

1. Write a program which asks the user for 4 words and then prints them in reverse order.

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 (without using the built-in max(..) function). For example, maxList([7,4,8,1]) should return 8. Write a main program to test the function.

4. Write a program which generates random integers (in the range 0 to 1000) until it encounters a number that is divisible by 3.

5. 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 i in range(len(word)):
        if word[i] == letter:
            x = x+1

    # draw stack here, just before return
    return x

main()

• Draw the function stack as it would look just prior to returning from helper.
• What variables are in scope at this same point in the code?
• 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?

Question 4 Answer Key

5. Write a program that reads in student grades from the user, until the user enters a -1. After reading in the -1, the program should compute and display the average grade.

    Please enter your grades below.
    Enter a -1 when you are all done...

    grade  1: 98
    grade  2: 87
    grade  3: 65
    grade  4: 95
    grade  5: 80
    grade  6: -1

    The average of those 5 grades is 85.000

6. Write a function called results(np, nc) that takes two parameters, the number of problems, and the number correct. The function should print an appropriate message to the user, based on the percentage the user got correct. For example, if they got 100% correct, print "Super!"; if they got 80% correct, print "Good job."; and so on. Here's a few examples:

• calling results(10,10): You got 10 out of 10 correct. Super!
• calling results(5,1): You got 1 out of 5 correct. Try again...
• calling results(8,6): You got 6 out of 8 correct. Not bad.

7. Analyze the following code:

m = ["t","i","m","e"]
s = "time"
m[1] = "a"
s[1] = "a"

What happens when this code is executed? What are the values of m and s afterward?

8. Consider the following program.

def double(lst):
    total = 0
    for i in range(len(lst)):
      lst[i] = lst[i] * 2
      total += lst[i]

    # Q1: What is the function stack here?
    return total

def main():
    values = [-3, 4, 8, -5]
    print(values)
    test = double(values)
    print("Sum of doubles:", test)
    print(values)
    # Q2: What is the function stack here?

main()

• Draw the function stack as it would look just prior to returning from double.
• Draw the function stack as it would look just prior to returning from main.
• What variables are in scope at these same points in the code?
• What is the program's output (i.e. what does it print when the program is run)?
• What does the double function compute?

Use Python Tutor to check your answer! (Keeping in mind that Python tutor does not order the functions on the stack correctly, and that values should all be on the heap.)

9. Consider the following program.

def swap(i, j, lst):
    temp = lst[i]
    lst[i] = lst[j]
    # Q1: What is the stack here?
    lst[j] = temp

def mystery(lst):
    firstIdx = 0
    lastIdx = len(lst)-1
    while firstIdx < lastIdx:
        swap(lastIdx, firstIdx, lst)
        lastIdx = lastIdx - 1
        firstIdx = firstIdx + 1

def main():
    wordList = ["whose", "woods", "these", "are"]
    print("before:", wordList)
    mystery(wordList)
    print("after:", wordList)

main()

• Draw the function stack as it would look just prior to returning from swap for the first time.
• What is the program's output (i.e. what does it print when the program is run)?
• What does the mystery function do?

Use Python Tutor to check your answer! (Keeping in mind that Python tutor does not order the functions on the stack correctly, and that values should all be on the heap.)