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Question 1. [6 marks]
Beside each code fragment below, show the output that it would create. If it would generate an error say so, and give the reason why.
Part (a) [1 mark]
print -10 / 4
Part (b) [1 mark]
print 24 - 12 / 4 - 2
Part (c) [1 mark]
z = "over" + "and" print "..." + z * 3 + "..."
Part (d) [1 mark]
avg = 87. print "Average is %.2f percent", % (avg)
Part (e) [1 mark]
s = "ya" for c in s: piece = "" for i in range(4): piece = piece + c * i + ":" print piece
Part (f) [1 mark]
s = "no" for i in range(4): piece = "" for c in s: piece = piece + c * i + ":" print piece
Solution:
(a) - (b) 19 (c) ...overandoverandoverand... (d) Average is 87.28 percent (e) :y:yy:yyy: :a:aa:aaa: (f) :: n:o: nn:oo: nnn:ooo:
Question 2. [4 marks]
Assume we have a game board consisting of n positions, and two objects: the hero and the enemy. (The game board has only one row.) At any point in time, the hero and the enemy will be a certain number of positions apart. Consider the number of spaces that the hero must move to land on the enemy’s position, as a fraction of the most the hero might have to move, n − 1. That fraction is the distance between the hero and the enemy. It is positive if the enemy is to the right of the hero, negative if the enemy is to the left of the hero, and exactly 0 if the hero and enemy are on the same position. For example, consider the following game board where n = 6:
hero enemy
The distance here is 3/5, which is 0.6. For this game board with n = 5:
enemy hero
The distance is − 0 .25. Write the following function according to its docstring.
def distance (game_board_size, hero, enemy): ’’’game_board_size is a positive int giving the number of positions on a game board. The positions are numbered from 0 to game_board_size-1. hero and enemy are ints giving valid hero and enemy positions on the game board, i.e., they are between 0 and game_board_size-1. Return a float between -1. and 1.0 giving the distance between hero and enemy.’’’
Solution:
def distance (game_board_size, hero, enemy): ’’’game_board_size is a positive int giving the number of positions on a game board. The positions are numbered from 0 to game_board_size-1. hero and enemy are ints giving valid hero and enemy positions on the game board, i.e., they are between 0 and game_board_size-1. Return a float between -1. and 1.0 giving the distance between hero and enemy.’’’
return (enemy - hero) / float(game_board_size - 1)
Lots of options: +String with a u +String with all b’s +String with one b and one u +String with one g and one u
Part (b) [3 marks]
Identify a function that would be a useful helper for best_slice. Define it here, including a docstring.
Part (c) [8 marks]
Now write function best_slice. You do not need to repeat the docstring.
def best_slice (s, k):
Solution:
def goodness(s): if s.count (’u’) > 0: return 0 else: return s.count(’g’)
def best_slice (s, k): ’’’s is a str, k is an integer such that 0 <= k <= len(s). Return the starting index of the length-k slice of s with the highest goodness. If k is zero, return -1.’’’
stop = len(s) - k best_start = - best_goodness = 0 for i in range(stop + 1): cur_slice = s[i:i+k] slice_goodness = goodness(cur_slice) if slice_goodness > best_goodness: best_start = i best_goodness = slice_goodness return best_start
Question 5. [8 marks]
We can use a Python list to represent a table or matrix with rows and columns as follows. Each element of the list is itself a list, containing the items from one row of the table. For example, this table
5 10 15 1 2 3 10 20 30 2 4 6
is represented by this list in Python: [[5, 10, 15], [1, 2, 3], [10, 20, 30], [2, 4, 6]]. Notice that all the sublists have the same length.
Write the following function according to its docstring.
def sums(L): ’’’L is a list of lists of ints. All of L’s sublists have the same length. Return a new list that contains the sums of all the "columns" of L. For example, for the list [[5, 10, 15], [1, 2, 3], [10, 20, 30], [2, 4, 6]], return [18, 36, 54].’’’
Solution:
ans = [] for column in range(len(L[0])): sum = 0 for row in range(len(L)): sum += L[row][column] ans.append(sum) return ans
Question 6. [8 marks]
A question bank is a text file consisting of one or more multiple choice questions. Each multiple choice question consists of the following lines, in order:
- A line containing only the characters MC
- One line, containing the question
- One or more lines, containing possible answers to the question
For example, here is a question bank with two questions. Notice that the number of answers to each question may be different.
MC The instructor that has taught CSC108 the most is: Diane Dan Steve MC The funniest CSC108 instructor is: Dan Daniel Dan Z Daniel Z
We want a function that will read such a file and produce a list of strings, each of which contains the full text of one of the multiple-choice questions (including the answers). For example, the list produced from the file above would be:
[’The instructor that has taught CSC108 the most is:\nDiane\nDan\nSteve\n’, ’The funniest CSC108 instructor is:\nDan\nDaniel\nDan Z\nDaniel Z\n’]
(Notice that the newlines are preserved.) Such a list could be used to choose random questions and generate a test, although you will not be writing any such code today. On the following page, write function read_mcqs according to its docstring.
Question 7. [6 marks]
Define the dictionary difference between two dictionaries to be a new dictionary containing keys (and their values) that occur in one but not both of the dictionaries. If any key is in both original dictionaries, but with different values, it does not appear in the new dictionary.
For example, if we have these two dictionaries:
d1 = {1: ’a’, 2: ’b’, 3: ’c’, 9: ’h’} d2 = {1: ’k’, 6: ’f’, 2: ’g’, 5: ’e’, 8: ’c’}
their difference is {3: ’c’, 9: ’h’, 6: ’f’, 5: ’e’, 8: ’c’}.
Write the following function according to the docstring and the definition above.
def dict_diff(d1, d2): ’’’d1 and d2 are dicts. Return a new dict which is the dictionary difference between d1 and d2.’’’
Solution:
d3 = {} for k1 in d1: if k1 not in d2: d3[k1] = d1[k1] for k2 in d2: if k2 not in d1: d3[k2] = d2[k2] return d
Question 8. [8 marks]
In some games there are more than two dice, and the number of dice to roll can vary. We need a program that asks the user for a positive integer indicating how many dice to roll. If 0 is entered, the program stops; otherwise, the specified number of dice are rolled and the process is repeated. Here is a sample execution.
Roll how many dice (0 to stop)? 4 2 2 4 5 Roll how many dice (0 to stop)? 2 1 6 Roll how many dice (0 to stop)? 1 5 Roll how many dice (0 to stop)? 0
This question continues on the next page.
Below are 9 lines of code in random order. Your task is to use each of these lines at least once, and add the proper indentation, to arrive at a program that implements the above description. Do not add new code; simply copy and rearrange the given code.
- counter = counter + 1
- counter = 0
- while num_dice > 0:
- num_dice = int(raw_input ("Roll how many dice (0 to stop)? "))
- while counter < num_dice:
- print random.randint (1, 6),
- print ""
- if name == ’main’:
- import random
Assemble the complete program here:
Solution:
import random
if name == ’main’: num_dice = int(raw_input ("Roll how many dice (0 to stop)? ")) while num_dice > 0: counter = 0 while counter < num_dice: print random.randint (1, 6), counter = counter + 1 print "" num_dice = int(raw_input ("Roll how many dice (0 to stop)? "))
Question 10. [6 marks]
Throughout this question, assume that we are sorting lists into non-descending order. Do not guess on the yes/no questions. There is a one-mark deduction for incorrect answers.
Part (a) [2 marks]
We are partly through sorting a list, and have completed 4 passes through the data. The list currently contains [10, 20, 30, 40, 16, 94, 8, 22] Could we be doing selection sort? Circle one.
yes no
Solution: No. Selection sort is “selective” (or picky), so on each pass it chooses the next smallest item to put into place. The first four items would thus be the overall smallest four items, which they are not.
Suppose it is insertion sort that we are doing. We have completed those 4 passes through, and the list con- tains: [10, 20, 30, 40, 16, 94, 8, 22] Show the state of the list after the next (5th) pass through it.
Solution: [10, 16, 20, 30, 40, 94, 8, 22]
Part (b) [2 marks]
We are partly through sorting a different list, and have completed 4 passes through the data. The list currently contains [11, 22, 33, 44, 56, 81, 48, 72] Could we be doing insertion sort? Circle one.
yes no
Solution: Yes.
Suppose it is selection sort that we are doing. We have completed those 4 passes through, and the list con- tains: [11, 22, 33, 44, 56, 81, 48, 72] Show the state of the list after the next (5th) pass through it.
Solution: [11, 22, 33, 44, 48, 56, 81, 72]
Part (c) [2 marks]
We are partly through sorting a different list, and have completed 4 passes through the data. The list currently contains [0, 1, 2, 3, 60, 70, 20, 40, 50, 30] Could we be doing selection sort? Circle one.
yes no
Solution: Yes.
Suppose it is bubble sort that we are doing. We have completed those 4 passes through, and the list contains: [0, 1, 2, 3, 60, 70, 20, 40, 50, 30] Show the state of the list after the next (5th) pass through it.
Solution: [0, 1, 2, 3, 20, 60, 70, 30, 40, 50]
Part (a) [2 marks] Create a Member variable called m1. His name is “Danny Z”, his status is “torturing
108 students” and he has one friend named “Diane”.
Solution:
m1 = Member("Dan") m1.update_status("torturing 108 students") m1.add_friend("Diane")
Part (b) [2 marks] Write the following new method, to be added to the class.
def friendliness(self): ’’’Return the number of friends this member has.’’’
Solution:
return len(self.friends)
Part (c) [4 marks] Write the cmp method below, also to be added to the class. Define it so that
if we call sort on a list of members, they will come out in order from least friendly (having the fewest friends) to most friendly (having the most friends).
def cmp(self, other):
Solution:
def cmp(self, other):
if self.friendliness() < other.friendliness(): return - elif self.friendliness() == other.friendliness(): return 0 else: return 1
Part (d) [1 mark] Assume both functions, friendliness and cmp, have been implemented
correctly. Suppose you have another variable of type Member called m2. His name is Steve, and you don’t know how many friends he has. Write an expression that is True if Dan is more friendly than Steve and False otherwise.
Solution:
print m1 > m
Question 12. [7 marks]
Don’t guess. There is a 1-mark deduction for wrong answers to parts (a) through (c).
Part (a) [2 marks]
def f1(s): for i in range(len(s)): for j in range(10000): print s[i] * j
Let n be the length of the string s passed to this function. Which of the following most accurately describes how the runtime of this function grow as n grows? Circle one.
(a) It grows linearly, like n does. (b) It grows quadratically, like n^2 does.
(c) It grows less than linearly. (d) It grows more than quadratically.
Part (b) [2 marks]
def f2(n): sum = 0 while n > 0: sum = sum + n ** 2 n = n / 2 return sum
Let n be the positive int passed to this function. Which of the following most accurately describes how the runtime of this function grow as n grows? Circle one.
(a) It grows linearly, like n does. (b) It grows quadratically, like n^2 does.
(c) It grows less than linearly. (d) It grows more than quadratically.
Short Python function/method descriptions: builtins: len(x) -> integer Return the length of the list, tuple, dict, or string x. max(L) -> value Return the largest value in L. min(L) -> value Return the smallest value in L. open(name[, mode]) -> file object Open a file. Legal modes are "r" (read), "w" (write), and "a" (append). range([start], stop, [step]) -> list of integers Return a list containing the integers starting with start and ending with stop - 1 with step specifying the amount to increment (or decrement). If start is not specified, the list starts at 0. If step is not specified, the values are incremented by 1. raw_input([prompt]) -> string Read a string from standard input. The trailing newline is stripped. cPickle: dump(obj, file) Write an object in pickle format to the given file. load(file) --> object Load a pickle from the given file dict: D[k] --> value Return the value associated with the key k in D. k in d --> boolean Return True if k is a key in D and False otherwise. D.get(k) -> value Return D[k] if k in D, otherwise return None. D.keys() -> list of keys Return the keys of D. D.values() -> list of values Return the values associated with the keys of D. D.items() -> list of (key, value) pairs Return the (key, value) pairs of D, as 2-tuples. file (also called a "reader"): F.close() Close the file. F.read([size]) -> read at most size bytes, returned as a string. If the size argument is negative or omitted, read until EOF (End of File) is reached. F.readline([size]) -> next line from the file, as a string. Retain newline. A non-negative size argument limits the maximum number of bytes to return (an incomplete line may be returned then). Return an empty string at EOF. float: float(x) -> floating point number Convert a string or number to a floating point number, if possible. int: int(x) -> integer Convert a string or number to an integer, if possible. A floating point argument will be truncated towards zero. list: x in L --> boolean Return True if x is in L and False otherwise.
L.append(x) Append x to the end of the list L. L.index(value) -> integer Returns the lowest index of value in L. L.insert(index, x) Insert x at position index. L.remove(value) Removes the first occurrence of value from L. L.reverse() Reverse IN PLACE L.sort() Sorts the list in ascending order. random: randint(a, b) Return random integer in range [a, b], including both end points. str: x in s --> boolean Return True if x is in s and False otherwise. str(x) -> string Convert an object into its string representation, if possible. S.count(sub[, start[, end]]) -> int Return the number of non-overlapping occurrences of substring sub in string S[start:end]. Optional arguments start and end are interpreted as in slice notation. S.find(sub[,i]) -> integer Return the lowest index in S (starting at S[i], if i is given) where the string sub is found or -1 if sub does not occur in S. S.index(sub) -> integer Like find but raises an exception if sub does not occur in S. S.isdigit() -> boolean Return True if all characters in S are digits and False otherwise. S.lower() -> string Return a copy of the string S converted to lowercase. S.lstrip([chars]) -> string Return a copy of the string S with leading whitespace removed. If chars is given and not None, remove characters in chars instead. S.replace(old, new) -> string Return a copy of string S with all occurrences of the string old replaced with the string new. S.rstrip([chars]) -> string Return a copy of the string S with trailing whitespace removed. If chars is given and not None, remove characters in chars instead. S.split([sep]) -> list of strings Return a list of the words in S, using string sep as the separator and any whitespace string if sep is not specified. S.strip() -> string Return a copy of S with leading and trailing whitespace removed. S.upper() -> string Return a copy of the string S converted to uppercase.
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