Quiz 4 Study Guide

Quiz Study Guides are provided as a courtesy. You may work with other students on the questions, and ask questions about the guide during evening ninja help sessions, on EdSTEM, or during meetings with faculty and staff. We do not provide full solutions to the quiz guides.

You are responsible for all material covered through the end of Week 9.

In addition to all concepts from Quiz 3, you should understand the following:

Python concepts

  • lists and strings as objects

  • list methods (e.g., append)

  • string methods (e.g., split, strip, isdigit, isalpha)

  • top-down design

  • file I/O (e.g., open, readlines)

  • Searching: linear vs binary search

  • Analysis of algorithms, and run times:

    • \(\log N\) — logarithmic (e.g. binary search)

    • \(N\) — linear (e.g. linear search)

Practice problems

  • NOTE: The binary search worksheet we did in class is viewable in your directory as the file `cs21/inclass/vasanta/10/searchWorksheet.txt'.

    1. How many steps would it take to do binary search on a list of size 1 million, when the item you are searching for is NOT in the list?

    2. Binary search is much faster than linear search, so why don’t we use it for every search problem?

    3. For each iteration of the loop in binarySearch(x, L), show the index values for low, high, and mid, and the value stored at location mid. What will be returned by this function call?

      x = 67
L = [10, 12, 14, 21, 37, 44, 45, 46, 58, 67, 99]
index 0   1   2   3   4   5   6   7   8   9  10

low | high | mid | L[mid]
- - - - - - - - - - - - -
    |      |     |
    |      |     |
    |      |     |
    |      |     |

    4. For each iteration of the loop in binarySearch(y, L), show the index values for low, high, and mid, and the value stored at location mid. What will be returned by this function call?

      y = 4
L = [10, 12, 14, 21, 37, 44, 45, 46, 58, 67, 99]
      0   1   2   3   4   5   6   7   8   9  10

low | high | mid | L[mid]
- - - - - - - - - - - - - -
    |      |     |
    |      |     |
    |      |     |
    |      |     |

    5. True/False questions:

      • Linear search requires a number of steps proportional to the size of the list being searched (T/F)

      • The python in operator performs a binary search (T/F)

      • Binary search is an O(N) algorithm (T/F)

      • The number of times N can be divided by 2 before reaching 1 is the log (base 2) of N (T/F)

    6. Write a function called isVowel(letter) that has one parameter, letter. This function should return True if the letter is a vowel (upper or lowercase), False if not. For example, calling isVowel("i") should return True, and isVowel("Q") should return False.

    7. 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 main():

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

################
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

main()

    8. The following get_numbers(n) function doesn’t work. Find and fix the bug in the code below.

      def get_numbers(n):
    """
    Purpose: Read n numbers from the user and return them as a list
    Parameters: n -- the number of values to read from the user (integer)
    Return: a list of the numbers entered by the user
    """
    for i in range(n):
    	value_list = []
	value = int(input("Enter a number: "))
	value_list.append(value)
    return value_list

    9. Assume you have these two functions already written:

      • getPick() asks the user for "r,p,s?" and returns:

        • "rock" if they enter r

        • "paper" if they enter p

        • "scissors" if they enter s

      • winner(user,comp) returns:

        • "user" if user won the game (e.g., user="rock", comp="scissors")

        • "comp" if comp won the game (e.g., user="rock", comp="paper")

        • "tie" if it’s a tie game (e.g., user="rock", comp="rock")

      Write a main() function that uses the above functions to play one round of rock-paper-scissors. Here are a few sample runs of the program:

      $ python3 rps.py
r,p,s?: r
Computer chose rock
tie...
$ python3 rps.py
r,p,s?: r
Computer chose paper
Computer wins!
$ python3 rps.py
r,p,s?: r
Computer chose scissors
You win!

    10. Write the stub for the winner(user, comp) function.

    11. Write the implementation for the getPick() function. Your getPick() function should only accept r, p, or s from the user. If they enter anything else, print an error message and ask again.

      r,p,s?: w
please enter r, p, or s!!!
r,p,s?: zebra
please enter r, p, or s!!!
r,p,s?: r

    12. Given the assignments for S, L, and N, what is the value and type of each expression?

      S = "CS21 Rocks"
L = ["May", "the Force", "be", "with you."]
N = "".join(L)

                                 VALUE            TYPE

    L

    len(L)

    len(S)

    "a" in L[2]

    "cs" in S

    L[1]

    len(L[1])

    s.split()

    N

    13. Given an input file named "numbers.txt" that consists of some number of lines, each with a single float value. For example, the first few lines of the file might look like:

      4.5
-0.5
32.1
-8
54.1
12.3

      Complete the program below to implement the functions fix_list and find_max:

      def main():

  infile = open("numbers.txt", "r")
  num_list = []
  for line in infile:
      line = line.strip("\n")
      num_list.append(line)

  fix_list(num_list)

  max = find_max(num_list)

  print("Max value in list of %d values is %f" % (len(num_list), max))
  infile.close()

########################

# implement the fix_list function, which should take in a list of strings
# and convert it to a list of ints


# implement the find_max function, which should look through a list
# and return the maximum value

########################
main()