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Spring 2019 CS 1110 Prelim 1 Solutions
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There are 7 problems. Make sure you have the whole exam.
You have 90 minutes to complete 90 points. Use your time accordingly.
Question Points Score 1 8 2 12 3 17 4 15 5 13 6 14 7 11 Total: 90
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- Start Your Engines
(a) [2 points] Python is a dynamically typed language. This means that a variable can hold values of any type and a variable can hold different types at different times. Write 2 lines of Python code that illustrate this. x = 1 x = "hi" (b) [2 points] A type is defined as “a set of values and operations on these values”. The meaning of an operator can change depending on the type. Give an example of an operator that has two distinct meanings when used in the context of two different types. What are these two meanings?
- means addition for integers and concatenation for strings (c) [2 points] If Python has just executed line 6 and is about to execute line 7, what does the call stack look like?
1 def f3(): 2 print("f3") 3 4 def f2(): 5 print("f2") 6 f3() 7 f3() 8 f3() 9 10 def f1(): 11 print("f1") 12 f2() 13 14 f1() Correct Answer: D
(d) [2 points] After the following lines of code have been executed: x = [0, 1, 2, 3, 4] z = x y = x[1:4] z[y[1]] = y[2] What does x[2] evaluate to? Correct Answer: 3
- Above your pay grade. iClicker software creates a list of iClicker scores for each student. Each score is the iClicker participation points for one lecture. If a student is absent, the software does not enter a zero; the student receives no score for that day, making the length of the scores list shorter than the total number of lectures. To calculate each student’s average iClicker score, Professor Bracy wants a zero added to the scores list for each missed lecture. She implements a function make complete scores(scores, num lectures) which takes scores, a (possibly empty) list of floats with values > 0.0 but <= 2.0 that represent a student’s iClicker grades, and num lectures, a non-negative integer that represents the current number of lectures in the semester. num lectures >= the length of scores. (Since each lecture is worth at most 2 points, the ordering of the scores list doesn’t matter.) It returns a list of scores of length num lectures, with zeroes explicitly present at the end of the list for missed lectures. (a) [9 points] Write 3 conceptually distinct test cases for make complete scores(scores, num lectures). Make sure your input values are ordered scores, num lectures.
There were many correct answers. Here are some we came up with. Test case # Input: [], 5 Output: [0.0, 0.0, 0.0, 0.0, 0.0] Rationale: student absent for the whole time
Test case # Input: [2.0, 1.0, 2.0], 4 Output: [2.0, 1.0, 2.0, 0.0] Rationale: student with some iclicker points
Test case # Input: [2.0, 2.0, 1.5], 3 Output: [2.0, 2.0, 1.5] Rationale: student with full attendance
(b) [6 points] The famous clock maker, Timex, would like to donate an alarm clock to anyone who seems to be struggling to make it to class. To identify students who would benefit from an alarm clock, the head TA for CS 1110 writes a function needs alarm(complete scores, num lectures). For this function, complete scores will represent a student’s iClicker scores fully constructed from make complete scores(). The second parameter num lectures is identical to that used in make complete scores(). This function will return a bool; True if the student has missed between 1/3 and 2/3 (exclusive) of the num lectures and False otherwise. (If they skip 2/3 or more of the lectures, they probably just need more sleep, not an alarm clock.) Implement the function as described, ignoring the need for any preconditions for now. def needs_alarm(complete_scores, num_lectures): """checks to see if the student could benefit from an alarm clock Returns: True if the student has missed between 1/3 and 2/3 of all lectures. (a missed lecture is indicated by a score of 0) Otherwise, returns False"""
num_absent = complete_scores.count(0) skip_rate = num_absent/num_lectures if skip_rate > 1/3 and skip_rate < 2/3: return True return False
(c) [2 points] What is one precondition you should add to the specification of the function above? In other words, what condition (if violated) would cause your implementation to either behave incorrectly or raise an error?
len(complete scores) == num lectures or else the skip rate will be incorrectly calculated
num lectures > 0 or else you’ll have a divide by zero error
- What’s the frequent problem, Kenneth? Fix the errors in the code below. If you change a function definition, please update the calls to that function, as necessary.
1 def fight(attack): 2 d = attack.damage 3 if d <= n_alien: 4 return d 5 6 def check_victory(): 7 if n_alien <= 0: 8 return True 9 return False 10 11 def defend_universe(cap, n_alien): 12 kills = 0 13 if n_alien >= 2: 14 kills = fight(cap.attack1) 15 else: 16 kills = fight(cap.attack2) 17 n_alien = n_alien - kills 18 victory = check_victory() 19 if victory: 20 print("WE WON!") 21 22 n_alien = 3 23 a1 = Attack("tackle", 2) 24 a2 = Attack("bodyslam", 1) 25 c = Captain("Dan", a1, a2) 26 defend_universe(c, n_alien)
The code from the previous page is copied here for your convenience. (a) [4 points] The function fight sometimes triggers a Python error. Explain why and fix the problem by modifying the definition of fight. The return statement is inside a conditional so sometimes the function will return None. This will cause a problem when the return value is subtracted on line 17. The solution is to return something unconditionally on line 5. What the return value should be (d?,n alien?, 0 ?) was not specified nor graded. Fixing the error was all that matters.
(b) [4 points] Even when Captain Dan kills all the aliens, the code never prints “WE WON!”. Ex- plain why and fix the problem by modifying the definition of check victory. check victory is reading the global variable n alien which will never change from its origi- nal value (in this case 3). The solution is to pass n alien as an argument to the function. Code will need to be changed on lines 6 and the call on line 18.
1 def name(s): 2 print(s + " had a " + lamb()) 3 4 def lamb(): 5 print("little lamb") 6 7 def fleece(): 8 print("Its fleece was white as snow") 9 10 def sing(): 11 for i in range(2): 12 name(person) 13 if i == 0: 14 for j in range(2): 15 lamb() 16 fleece() 17 18 person = "Mary" 19 sing()
(c) [5 points] The code to the left should print out the following lyrics: Mary had a little lamb little lamb little lamb Mary had a little lamb Its fleece was white as snow Instead, it throws an error. Explain why and fix code that causes the problem. Note: the for-loops (lines 11 and 14) are correct. lamb() has no return value but its return value (None) is being concatenated with a string in line 2. The fixes are on line 5 (have lamb() return a string instead of printing it and 15 (print(lamb()) now that returns rather than prints the string).
- [14 points] Hang in there! In the game Hangman, a player must guess a hidden word in some number of guesses. At first, each letter is shown as a ’ ’. As the player correctly guesses the letters in the word, they are revealed. Complete the function process guess(hidden, shown, guess, guesses left) below so that it obeys the following specification in support of the game hangman. (Don’t include a docstring.) Preconditions:
- hidden is a string with length >= 1 with only lower-case letters and no repeating letters
- shown is a string identical to hidden, but 1 or more (not guessed) letters are replaced by ’ ’
- guess is a lower-case character
- guesses left is an int >= 1 process guess() should: (1) print ”YOU WIN!” or ”YOU LOSE!” when applicable:
- If after this guess, the whole hidden word is now known/shown, the player has won.
- If the player didn’t just win the game and they only had 1 guesses left, then they lose. (2) return the string shown, updated in response to guess:
- If guess is not a letter in hidden, return shown.
- If guess is a letter in hidden, return shown but with the ’ ’ corresponding to that letter replaced with guess. Examples: hidden shown guess guesses_left what to print what to return
"world" "_____" 'o' 6 "o__" "world" "_o__d" 'e' 4 "_o__d" "world" "_o__d" 'o' 3 "_o__d" "world" "_o__d" 'r' 1 "YOU LOSE!" "or_d" "world" "w_rld" 'o' 1 "YOU WIN!" "world" "world" "worl" 'd' 4 "YOU WIN!" "world"
def process guess(hidden, shown, guess, guesses left): i = hidden.find(guess) new_shown = shown if i != -1: new_shown = shown[:i]+guess+shown[i+1:] if (new_shown == hidden): print("YOU WIN!") elif guesses_left == 1: print("YOU LOSE!")
return new_shown
There are many correct ways to do this. This is just one possible answer.
def address_equals(a1, a2): """ Inputs: a1 and a2 are both Address objects Preconditions: a1 and a2 should have all 4 attributes of an Address
Functionality: compares all 4 class attributes of the inputs to determine Address equality
Returns True if all 4 attributes are equal. Otherwise False. """ return (a1.num == a2.num) and (a1.street == a2.street) and
(a1.city == a2.city) and (a1.zip == a2.zip)
def live_together (c1, c2): """ c1 and c2 are Contacts with distinct names and non-empty home addresses Return True if c1 and c2's home addresses are the same, False otherwise """
return address equals(c1.home, c2.home)
def work_together (c1, c2): """ c1 and c2 are Contacts with distinct names and non-empty work addresses Return True if c1 and c2's work addresses are the same, False otherwise """
return address equals(c1.work, c2.work)
