CS 61A Structure and Interpretation of Computer Programs
Fall 2018
INSTRUCTIONS
• You have 3 hours to complete the exam.
Final
• The exam is closed book, closed notes, closed computer, closed calculator, except three hand-written 8.5″ × 11″ crib sheet of your own creation and the official CS 61A midterm 1, midterm 2, and final study guides.
• Mark your answers on the exam itself. We will not grade answers written on scratch paper.
Last name
First name
Student ID number
CalCentral email
TA
Name of the person to your left
Name of the person to your right
All the work on this exam is my own.
(please sign)
POLICIES & CLARIFICATIONS
• If you need to use the restroom, bring your phone and exam to the front of the room.
• You may use built-in Python functions that do not require import, such as min, max, pow, len, abs, sum, next,
iter, list, tuple, map, filter, zip, all, and any.
• You may not use example functions defined on your study guides unless a problem clearly states you can.
• For fill-in-the-blank coding problems, we will only grade work written in the provided blanks. You may only write one Python statement per blank line, and it must be indented to the level that the blank is indented.
• Unless otherwise specified, you are allowed to reference functions defined in previous parts of the same question.
• You may use the Tree, Link, and BTree classes defined on Page 2 (left column) of the Midterm 2 Study Guide.
2
1. (12 points) High Quality Air (At least one of these is out of Scope: OOP, WWPD, Lambda, Python Lists, Mutation)
For each of the expressions in the table below, write the output displayed by the interactive Python interpreter when the expression is evaluated. The output may have multiple lines. The first row is completed for you.
• If an error occurs, write Error, but include all output displayed before the error. • To display a function value, write Function.
• To display an iterator value, write Iterator.
• If an expression would take forever to evaluate, write Forever.
The interactive interpreter displays the contents of the repr string of the value of a successfully evaluated expression, unless it is None.
Assume that you have started python3 and executed the code shown on the left first, then you evaluate each expression on the right in the order shown. Expressions evaluated by the interpreter have a cumulative effect.
from operator import sub
z = (lambda x: lambda y: 2 * (y-x))(3)
def breath(f, count=1):
if count > 1:
print(count)
count += 1
return lambda x, y: f(x+1, y)
class Day:
aqi = 10
def __init__(self, aqi=0):
if aqi > self.aqi:
self.aqi = aqi
self.n = []
def mask(self, limit):
def f(aqi):
if aqi > limit:
self.n.append(aqi-limit)
return self.mask(aqi)
return f
class Week(Day):
aqi = 50
m, t = Day(), Week(199)
t.mask(200)(100)(150)(160)
Day.aqi = 140
t.aqi = 160
Expression Output
print(None) None
print(print(None), print)
z(4)
breath(breath(sub))(5, 3)
[Day().aqi, m.aqi]
[Week.aqi, t.aqi]
t.n
Name: 3 2. (8 points) Diagram Horror (At least one of these is out of Scope: Python Lists, Mutation, Environment
Diagram, Lambda)
def get(out):
out.pop()
out = scary(lambda movie: out)
return lambda: [out]
def scary(movie):
out.append(movie)
return movie(5)[:1]
out = [6]
get([7, 8])()
Fill in the environment diagram that results from ex- ecuting the code on the left until the entire program is finished, an error occurs, or all frames are filled. You may not need to use all of the spaces or frames. A complete answer will:
• Add all missing names and parent annotations to all local frames.
• Add all missing values created or referenced during execution.
• Show the return value for each local frame.
• Use box-and-pointer diagrams for lists and tuples.
Global
get
scary
func get(out) [parent=Global]
func scary(movie) [parent=Global]
f1: ___________ [parent=____________]
Return Value
f2: ___________ [parent=____________]
Return Value
f3: ___________ [parent=____________]
Return Value
f4: ___________ [parent=____________]
Return Value
4
3. (16 points) Gainz
Definition. A sequence is near increasing if each element beyond the second is larger than all elements
preceding its previous element. That is, element i must be larger than elements i − 2, i − 3, i − 4, etc.
(a) (3 pt) (All are in Scope: Python Lists, List Comprehensions, Recursion) Implement is_near, which takes
a sequence s and returns whether its elements form a near increasing sequence. def is_near(s):
“””Return whether s is a near increasing sequence.
>>> is_near([]) and is_near([1]) and is_near([1, 2]) and is_near(range(10))
True
>>> is_near([4, 2]) and is_near((1, 4, 2, 5)) and is_near((1, 2, 4, 3))
True
>>> is_near((3, 2, 1)) # 1 <= 3
False
>>> is_near([1, 4, 2, 3, 5]) # 3 <= 4
False
>>> is_near([1, 4, 2, 5, 3]) # 3 <= 4
False
>>> is_near([1, 2, 4, 2, 5]) # 2 <= 2
False
"""
return all([___________ > _______________________ for i in ___________________________])
(b) (6 pt)(All are in Scope: Exceptions, Iterators, Recursion) Implement fast_near, which takes an iterable value and returns whether its elements form a near increasing sequence. fast_near must run in Θ(n) time and Θ(1) space (not including the input itself) for an iterable input with n elements. Assume that s has a finite number of elements. You may not call is_near.
def fast_near(s):
“””Return whether the elements in iterable s form a near increasing sequence.
>>> fast_near([2, 5, 3, 6, 6, 7, 7, 9, 8])
True
“””
t, s = iter(s), None # Do not refer to s below this line.
try:
largest, last = _________________________________, ________________________________
except StopIteration:
return ____________________________________________________________________________
for x in t:
if ________________________________________________________________________________:
return False
largest, last = _________________________________, ________________________________
return True
Name: 5
Alternative Definition. (Equivalent to the one on the previous page, but stated in a more useful way for the problem below.) A sequence is near increasing if each element but the last two is smaller than all elements following its subsequent element. That is, element i must be smaller than elements i + 2, i + 3, i + 4, etc.
(c) (6 pt)(All are in Scope: Lambda, Recursion) Implement near, which takes a non-negative integer n and returns the largest near increasing sequence of digits within n as an integer. The arguments smallest and d are part of the implementation; you must determine their purpose. You may not call is_near or fast_near. You may not use any values except integers and booleans (True and False) in your solution (no lists, strings, etc.).
def near(n, smallest=10, d=10):
“””Return the longest sequence of near-increasing digits in n.
>>> near(123)
123
>>> near(153)
153
>>> near(1523)
153
>>> near(15123)
1123
>>> near(11111111)
11
>>> near(985357)
557
>>> near(14735476)
143576
>>> near(812348567)
1234567
“””
if n == 0:
return ____________________________________________________________________________
no = near(n//10, smallest, d)
if smallest > _________________________________________________________________________:
yes = _____________________________________________________________________________
return ___________________________________________________________________(yes, no)
return ________________________________________________________________________________
(d) (1 pt) What is the largest possible integer that could ever be returned from the near function? Note: In general, integers in Python can be arbitrarily large.
__________________________________________________________________________________________
6
4. (11 points) Tree Time
Definition. A runt node is a node in a tree whose label is smaller than all of the labels of its siblings. A
sibling is another node that shares the same parent. A node with no siblings is a runt node.
(a) (7 pt)(All are in Scope: Tree Recursion, Tree Class, HOFs) Implement runts, which takes a Tree instance t in which every label is different and returns a list of the labels of all runt nodes in t, in any order. Also implement apply_to_nodes, which returns nothing and is part of the implementation. Do not mutate any tree. The Tree class is on the Midterm 2 Guide.
def runts(t):
“””Return a list in any order of the labels of all runt nodes in t.
>>> sorted(runts(Tree(9, [Tree(3), Tree(4, [Tree(5, [Tree(6)]), Tree(7)]), Tree(2)])))
[2, 5, 6, 9]
“””
result = []
def g(node):
if _____________________________________________________________________________:
result.append(______________________________________________________________)
apply_to_nodes(_____________________________________________________________________)
return _____________________________________________________________________________
def apply_to_nodes(f, t):
“””Apply a function f to each node in a Tree instance t.”””
____________________________________________________________________________________
for b in t.branches:
________________________________________________________________________________
Name: 7 (b) (4 pt)(All are in Scope: Tree Recursion, Tree Class, Lambda) Implement max_label, which takes a Tree t
and returns its largest label. Do not mutate any tree. def max_label(t):
“””Return the largest label in t.
>>> max_label(Tree(4, [Tree(5), Tree(3, [Tree(6, [Tree(1), Tree(2)])])]))
6
“””
def f(node):
________________________________________________________________________________
________ max(____________, ____________, key=lambda n: _________________________)
apply_to_nodes(f, t) # Assume that apply_to_nodes above is implemented correctly.
return t.label
8
5. (9 points) Run, Program, Run (All are in Scope: Scheme Lists)
Implement runs, a Scheme procedure that takes a list of integers s and returns a list of non-empty lists of integers t. Together, the lists in t should contain all elements of s in order. The first element in each list in t must be less than the last element in the previous list, if there is one. The rest of the elements in each list in t must be greater than or equal to the previous element.
Also implement and use next-run in your solution, which takes a non-empty list of integers s and returns a pair of lists: the longest non-decreasing prefix of s and the rest of s. Use the provided pair data abstraction. Your implementation should be correct even if the pair implementation were to change.
;; Return a list of non-decreasing lists that together contain the elements of s.
;; scm> (runs ‘(3 4 7 6 6 8 1 2 5 5 4))
;; ((3 4 7) (6 6 8) (1 2 5 5) (4))
;; scm> (runs ‘(4 3 2 3))
;; ((4) (3) (2 3))
(define (runs s)
(if (null? s) _______________________________________________________________________
(let ((p (next-run s)))
_____________________________________________________________________________)))
;; A data abstraction for a pair of a first run and the rest of a list.
(define (pair a b) (lambda (c) (if c a b)))
(define (first p) (p #t))
(define (rest p) (p #f))
;; Return a pair containing the first run in s (a list) and the rest of s (another list).
;; scm> (first (next-run ‘(4 5 1 3 2)))
;; (4 5)
;; scm> (rest (next-run ‘(4 5 1 3 2)))
;; (1 3 2)
(define (next-run s)
(if (or _______________________________________________________________________________
_______________________________________________________________________________)
(pair ____________________________________ _______________________________________)
(begin
(define p (next-run (cdr s)))
(pair ________________________________ ____________________________________))))
Name: 9 6. (9 points) Generation Z
(a) (4 pt) (All are in Scope: Generators, Linked List Class) Implement rev, a generator function that takes a Link instance and yields the elements of that linked list in reverse order. The Link class appears on Page 2 of the Midterm 2 Study Guide.
def rev(s):
“””Yield the elements in Link instance s in reverse order.
>>> list(rev(Link(1, Link(2, Link(3)))))
[3, 2, 1]
>>> next(rev(Link(2, Link(3))))
3
“””
if ____________________________________________________________________________________:
___________________________________________________________________________________
yield _____________________________________________________________________________
(b) (2 pt) (At least one of these is out of Scope: Scheme Lists, Scheme Streams) Using the provided add procedure, define not-three, an infinite stream of all positive integers that are not evenly divisible by 3. The not-three stream is increasing and begins with 1, 2, 4, 5, 7, 8, 10, 11, 13.
(define (add k s) (cons-stream (+ k (car s)) (add k (cdr-stream s))))
(define not-three __________________________________________________________________________)
(c) (3 pt)(All are in Scope: Scheme Macros) Implement infix, a Scheme macro that evaluates infix expressions. An infix expression is either a number or a three-element list containing an infix expression, a procedure, and another infix expression. The value of a compound infix expression is the value of its second element applied to the values of its first and third elements. Note: The last line begins with a quasiquote. If you cross out the quasiquote and solve the problem without using quasiquote or unquote, you can receive up to 2 out of 3 points (not recommended).
;; A macro to evaluate infix expressions.
;; scm> (infix (2 * 3))
;; 6
;; scm> (infix ((1 + 1) * (1 + 2)))
;; 6
;; scm> (infix ((1 + (3 – 2)) * ((2 + 3) + 2)))
;; 14
(define-macro (infix e)
(if (number? e) e
`(____________________________________________________________________________________)))
(define (cadr x) (car (cdr x)))
(define (caddr x) (car (cdr (cdr x))))
10
7. (10 points) SQL of Course (At least one of these is out of Scope: SQL, SQL Aggregation)
The courses table describes the course name, start time hour (h) and minute (m), and length in minutes (len) for different lectures. For example, 61A starts at 13:00 and lasts 50 minutes. The locations table describes the course name and location (loc) of these courses. Assume that each course name appears exactly once in each table. Write your SQL statements so that they would still be correct if the table contents changed.
CREATE TABLE courses AS
SELECT “1” AS course, 14 AS h, 0 AS m, 80 AS len UNION
CREATE TABLE locations AS
SELECT “1” AS name, “VLSB” AS loc UNION
SELECT “2”
SELECT “8”
SELECT “10”
SELECT “50AC”
SELECT “61A”
, 13
, 12
, 12
, 13
, 13
,30 ,80 ,30 ,50 ,30 , 110 ,30 ,45 ,0 , 50;
UNION
UNION
UNION
UNION
SELECT “2”
SELECT “10”
SELECT “50AC”
SELECT “61A”
, “Dwinelle”
, “VLSB”
, “Wheeler”
, “Wheeler”;
UNION
UNION
UNION
(a) (2 pt) Select a one-column table that contains the course names of all courses that start before 13:30. SELECT course FROM courses WHERE ___________________________________________________________;
61A 8 10
(b) (4 pt) Select a two-column table with one row per location that contains the location, as well as the shortest length in minutes of any lecture held in that location.
SELECT loc, ________________________________________________________________________________
FROM _____________________________________________________________________________________
__________________________________________________________________________________________;
Dwinelle 80 VLSB 80 Wheeler 45
(c) (4 pt) Select a three-column table where each row describes an earlier course, a later course, and the amount of time in minutes between the end time of the earlier course and the start time of the later course. Only include pairs of courses where the lectures do not overlap in time. Note: There are 60 minutes in an hour.
SELECT _________________________________________ , _________________________________________
___________________________________________________________________________________ AS gap
FROM _____________________________________________________________________________________
__________________________________________________________________________________________;
61A 1 10 8 1 40 8 2 10 8 50AC 10
Name: 11 8. (0 points) Draw! (Optional) Draw a picture of some function or procedure.