程序代写CS代考 SQL scheme python cache interpreter CS 61A Structure and Interpretation of Computer Programs

CS 61A Structure and Interpretation of Computer Programs
Fall 2014
INSTRUCTIONS
Final Exam
• You have 3 hours to complete the exam.
• The exam is closed book, closed notes, closed computer, closed calculator, except one hand-written 8.5” ⇥ 11”
crib sheet of your own creation and the 3 ocial 61A midterm study guides attached to the back of this exam. • Mark your answers ON THE EXAM ITSELF. If you are not sure of your answer you may wish to provide a
brief explanation.
Last name
First name
SID
Login
TA & section time
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)
For sta↵ use only
Q. 1 Q. 2 Q. 3 Q. 4 Q. 5 Q. 6 Total /14 /16 /12 /12 /18 /8 /80

2
Blank Page

1. (14 points) Representing Scheme Lists
For each row below, write the output displayed by the interactive Python interpreter when the expression is evaluated. Expressions are evaluated in order, and expressions may a↵ect later expressions.
Whenever the interpreter would report an error, write Error. You should include any lines displayed before an error. Reminder: The interactive interpreter displays the repr string of the value of a successfully evaluated expression, unless it is None.
The Pair class from Project 4 is described on your final study guide. Recall that its __str__ method returns a Scheme expression, and its __repr__ method returns a Python expression. The full implementation of Pair and nil appear at the end of the exam as an appendix. Assume that you have started Python 3, loaded Pair and nil from scheme reader.py, then executed the following:
blue = Pair(3, Pair(4, nil))
gold = Pair(Pair(6, 7), Pair(8, 9))
def
def
y =
Expression
Pair(1, nil)
print(Pair(1, nil))
1/0
process(s):
cal = s
while isinstance(cal, Pair):
cal.bear = s
cal = cal.second if cal is s:
return cal else:
return Pair(cal, Pair(s.first, process(s.second)))
display(f, s):
if isinstance(s, Pair):
print(s.first, f(f, s.second)) lambda f: lambda x: f(f, x)
3
Output Expression Output
Pair(1, nil) (1) Error
process(blue.second)
print(print(3), 1/0)
print(process(gold))
print(Pair(2, blue))
gold.second.bear.first
print(gold)
y(display)(gold)

4
2. (16 points) Environments
(a) (8 pt) Fill in the environment diagram that results from executing the code below 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 during execution.
• Show the return value for each local frame.
1 2 3 4 5 6 7 8
func tattoo(heart) [parent=Global]
def tattoo(heart):
def mom():
nonlocal mom
mom = lambda: heart(2) + 1
return 3
return mom() + mom() + 4
tattoo(lambda ink: ink + 0.5)
Global frame
tattoo
f1: ___________ [parent=____________]
Return Value
f2: ___________ [parent=____________]
Return Value
f3: ___________ [parent=____________]
Return Value
f4: ___________ [parent=____________]
Return Value

(b) (6 pt) For the six-line program below, fill in the three environment diagrams that would result after executing each pair of lines in order. You must use box-and-pointer diagrams to represent list values. You do not need to write the word “list” or write index numbers.
Important: All six lines of code are executed in order! Line 3 is executed after line 2 and line 5 after line 4.
5
1 2
12
Global frame meow
cat
3 4
5 6
meow = [1, 2]
cat = [meow, [4, 5]]
cat[0][1] = cat[1][0]
cat[meow[0]][0] = meow
Global frame meow
cat
meow[0] = [cat.pop(0)]
cat.extend(cat[0][1:])
Global frame meow
cat
(c) (2 pt) Circle the value, True or False, of each expression below when evaluated in the environment created by executing all six lines above. If you leave this question blank, you will receive 1 point.
Circle True or False: meow is cat[0]
Circle True or False: meow[0][0] is cat[0][0]

6
3. (12 points) Expression Trees
Your partner has created an interpreter for a language that can add or multiply positive integers. Expressions are represented as instances of the Tree class and must have one of the following three forms:
• (Primitive) A positive integer entry and no branches, representing an integer
• (Combination) The entry ’+’, representing the sum of the values of its branches
• (Combination) The entry ’*’, representing the product of the values of its branches
The Tree class is on the Midterm 2 Study Guide. The sum of no values is 0. The product of no values is 1.
(a) (6 pt) Unfortunately, multiplication in Python is broken on your computer. Implement eval_with_add, which evaluates an expression without using multiplication. You may fill the blanks with names or call expressions, but the only way you are allowed to combine two numbers is using addition.
def eval_with_add(t):
“””Evaluate an expression tree of * and + using only addition.
>>> plus = Tree(’+’, [Tree(2), Tree(3)])
>>> eval_with_add(plus) 5
>>> times = Tree(’*’, [Tree(2), Tree(3)])
>>> eval_with_add(times)
6
>>> deep = Tree(’*’, [Tree(2), plus, times])
>>> eval_with_add(deep) 60
>>> eval_with_add(Tree(’*’)) 1
“””
if t.entry == ’+’:
return sum(________________________________________________________) elif t.entry == ’*’:
total = ___________________________________________________________ for b in t.branches:
total , term = 0, ______________________________________________ for ___________ in ____________________________________________:
total = total + term return total
else:
return t.entry

(b) (6 pt) A TA suggests an alternative representation of an expression, in which the entry is the value of the expression. For combinations, the operator appears in the left-most (index 0) branch as a leaf.
Implement transform, which takes an expression and mutates all combinations so that their entries are values and their first branches are operators. In addition, transform should return the value of its argument. You may use the calc_apply function defined below.
def calc_apply(operator , args): if operator == ’+’:
return sum(args) elif operator == ’*’:
return product(args)
def product(vals): total = 1
for v in vals: total *= v
return total
def transform(t):
“””Transform expression tree t to have value entries and operator leaves.
>>> seven = Tree(’+’, [Tree(’*’, [Tree(2), Tree(3)]), Tree(1)]) >>> transform(seven)
7
>>> seven
Tree(7, [Tree(+), Tree(6, [Tree(*), Tree(2), Tree(3)]), Tree(1)]) “””
if t.branches: args = []
for b in t.branches: args.append(__________________________________________________)
t.branches = _____________________________________________________
t.entry = ________________________________________________________ return _______________________________________________________________
7
Original format: + Alternative: 7 *1+61 23*23

8
4. (12 points) Lazy Sunday
(a) (4 pt) A flat-map operation maps a function over a sequence and flattens the result. Implement the flat_map
method of the FlatMapper class. You may use at most 3 lines of code, indented however you choose. class FlatMapper:
“””A FlatMapper takes a function fn that returns an iterable value. The flat_map method takes an iterable s and returns a generator over all values in the iterables returned by calling fn on each element of s.
>>> stutter = lambda x: [x, x] >>> m = FlatMapper(stutter)
>>> g = m.flat_map((2, 3, 4, 5)) >>> type(g)

>>> list(g)
[2, 2, 3, 3, 4, 4, 5, 5] “””
def __init__(self, fn): self.fn = fn
def flat_map(self, s): ______________________________________________________________________ ______________________________________________________________________ ______________________________________________________________________
(b) (2 pt) Define cycle that returns a Stream repeating the digits 1, 3, 0, 2, and 4. Hint: (3+2)%5 equals 0. def cycle(start=1):
“””Return a stream repeating 1, 3, 0, 2, 4 forever.
>>> first_k(cycle(), 12) # Return the first 12 elements as a list [1, 3, 0, 2, 4, 1, 3, 0, 2, 4, 1, 3]
“””
def compute_rest ():
return ___________________________________________________________
return Stream(_____________________________ , _________________________)

(c) (4 pt) Implement the Scheme procedure directions, which takes a number n and a symbol sym that is bound to a nested list of numbers. It returns a Scheme expression that evaluates to n by repeatedly applying car and cdr to the nested list. Assume that n appears exactly once in the nested list bound to sym.
Hint: The implementation searches for the number n in the nested list s that is bound to sym. The returned expression is built during the search. See the tests at the bottom of the page for usage examples.
(define (directions n sym) (define (search s exp)
; Search an expression s for n and return an expression based on exp. (cond ((number? s) ____________________________________________________)
((null? s) nil)
(else (search-list s exp))))
(define (search -list s exp)
; Search a nested list s for n and return an expression based on exp.
(let ((first __________________________________________________________) (rest __________________________________________________________))
(if (null? first) rest first)))
(search (eval sym) sym))
(define a ’(1 (2 3) ((4)))) (directions 1 ’a)
; expect (car a) (directions 2 ’a)
; expect (car (car (cdr a))) (define b ’((3 4) 5)) (directions 4 ’b)
; expect (car (cdr (car b)))
(d) (2 pt) What expression will (directions 4 ’a) evaluate to?
9

10
5. (18 points) Basis Loaded
notices that any positive integer can be expressed as a sum of powers of 2. Some examples:
11 = 8 + 2 + 1
23 = 16 + 4 + 2 + 1 24 = 16 + 8
45 = 32 + 8 + 4 + 1
2014 = 1024 + 512 + 256 + 128 + 64 + 16 + 8 + 4 + 2
A basis is a linked list of decreasing integers (such as powers of 2) with the property that any positive integer n can be expressed as the sum of elements in the basis, starting with the largest element that is less than or equal to n.
(a) (4 pt) Implement sum_to, which takes a positive integer n and a linked list of decreasing integers basis. It returns a linked list of elements of the basis that sum to n, starting with the largest element of basis that is less than or equal to n. If no such sum exists, raise an ArithmeticError. Each number in basis can only be used once (or not at all). The Link class is described on your Midterm 2 Study Guide.
def sum_to(n, basis):
“””Return elements of linked list basis that sum to n.
>>> twos = Link(32, Link(16, Link(8, Link(4, Link(2, Link(1)))))) >>> sum_to(11, twos)
Link(8, Link(2, Link(1)))
>>> sum_to(23, twos)
Link(16, Link(4, Link(2, Link(1)))) >>> sum_to(24, twos)
Link(16, Link(8))
>>> sum_to(45, twos)
Link(32, Link(8, Link(4, Link(1)))) “””
if _______________________________________________________________________: return Link.empty
elif _____________________________________________________________________: raise ArithmeticError
elif basis.first > n:
return sum_to(n, basis.rest)
else:
return _______________________________________________________________

(b) (6 pt) Cross out as many lines as possible in the implementation of the FibLink class so that all doctests pass. A FibLink is a subclass of Link that contains decreasing Fibonacci numbers. The up_to method returns a FibLink instance whose first element is the largest Fibonacci number that is less than or equal to positive integer n.
class FibLink(Link):
“””Linked list of Fibonacci numbers.
>>> ten = FibLink(2, FibLink(1)).up_to(10) >>> ten
Link(8, Link(5, Link(3, Link(2, Link(1))))) >>> ten.up_to(1)
Link (1)
>>> six, thirteen = ten.up_to(6), ten.up_to(13)
>>> six
Link(5, Link(3, Link(2, Link(1))))
>>> thirteen
Link(13, Link(8, Link(5, Link(3, Link(2, Link(1)))))) “””
successor = self.first + self.rest
@property
def successor ():
def successor(self):
return first + rest.first
return self.first + self.rest.first
def up_to(n):
def up_to(self, n):
while self.first > n:
self = self.rest.first
self = rest
self.first = self.rest.first
if self.first == n: return self
elif self.first > n:
return self.up_to(n) return self.rest.up_to(n)
elif self.successor > n: elif self.first < n: return else: return return return return self FibLink(self.successor(self), self).up_to(n) FibLink(self.successor , self).up_to(n) FibLink(self.successor(self), self.rest).up_to(n) FibLink(self.successor , self.rest).up_to(n) (c) (2 pt) Circle the ⇥ expression below that describes the number of calls made to FibLink.up_to when evaluating FibLink(2, FibLink(1)).up_to(n). The constant is 1+p5 = 1.618... 2 ⇥(1) ⇥(log n) ⇥(n) ⇥(n2) ⇥(n) 11 12 23*23 (d) (2 pt) Alyssa P. Hacker remarks that Fibonacci numbers also form a basis. How many total calls to FibLink.up_to will be made while evaluating all the doctests of the fib_basis function below? Assume that sum_to and FibLink are implemented correctly. Write your answer in the box. def fib_basis (): """Fibonacci basis with caching. >>> r = fib_basis() >>> r(11)
Link(8, Link(3)) >>> r(23)
Link(21, Link(2))
>>> r(24)
Link(21, Link(3))
>>> r(45)
Link(34, Link(8, Link(3))) “””
fibs = FibLink(2, FibLink(1)) def represent(n):
nonlocal fibs
fibs = fibs.up_to(n) return sum_to(n, fibs)
return represent
(e) (4 pt) Implement fib_sums, a function that takes positive integer n and returns the number of ways that n
can be expressed as a sum of unique Fibonacci numbers. Assume that FibLink is implemented correctly. def fib_sums(n):
“””The number of
>>> fib_sums(9) 2
>>> fib_sums (12) 1
ways n can be expressed as a sum of unique Fibonacci numbers. # 8+1, 5+3+1
# 8+3+1
# 13, 8+5, 8+3+2
“””Ways n can be expressed as a sum of elements in fibs.””” if n == 0:
return 1
elif _________________________________________________________________:
return 0
a = __________________________________________________________________
b = __________________________________________________________________
return a + b
return sums(n, FibLink(2, FibLink(1)).up_to(n))
>>> fib_sums(13)
3
“””
def sums(n, fibs):

6. (8 points) Sequels
Assume that the following table of movie ratings has been created.
create table ratings as
select “The Matrix” as title , select “The Matrix Reloaded”, select “The Matrix Revolutions”, select “Toy Story”,
select “Toy Story 2”,
select “Toy Story 3”,
select “Terminator”,
select “Judgment Day”,
select “Rise of the Machines”, 5;
Correct output
Judgment Day Terminator The Matrix Toy Story Toy Story 2 Toy Story 3
9 as rating 7
5
8
8 9 8 9
union union union union union union union union
13
The correct output table for both questions below happens to be the same. It appears above to the right for your reference. Do not hard code your solution to work only with this table! Your implementations should work correctly even if the contents of the ratings table were to change.
(a) (2 pt) Select the titles of all movies that have a rating greater than 7 in alphabetical order.
(b) (6 pt) Select the titles of all movies for which at least 2 other movies have the same rating. The results should appear in alphabetical order. Repeated results are acceptable. You may only use the SQL features introduced in this course.
with
groups(name, score, n) as (
select _____________ , _________________ , ___________ from ratings union
select _____________ , _________________ , ________ from groups , ratings
where ______________________________________________________________ )
select title from ________________________________________________________ where __________________________________________________________________ order by _______________________________________________________________;

14
Appendix: Pair and nil Implementations
This page does not contain a question. These classes were originally defined in scheme reader.py.
class Pair:
“””A pair has two instance attributes: first and second. For a Pair to be a well-formed list, second is either a well-formed list or nil. Some methods only apply to well-formed lists.
>>> s = Pair(1, Pair(2, nil)) >>> s
Pair(1, Pair(2, nil))
>>> print(s)
(1 2)
“””
def __init__(self, first, second):
self.first = first self.second = second
def __repr__(self):
return “Pair({0}, {1})”.format(repr(self.first), repr(self.second))
def __str__(self):
s = “(” + str(self.first) second = self.second
while isinstance(second , Pair):
s += ” ” + str(second.first)
second = second.second if second is not nil:
s += ” . ” + str(second) return s + “)”
class nil:
“”” The empty list “””
def __repr__(self): return “nil”
def __str__(self): return “()”
def __len__(self): return 0
def __getitem__(self, k): if k < 0: raise IndexError("negative index into list") raise IndexError("list index out of bounds") def map(self, fn): return self nil = nil() # Assignment hides the nil class; there is only one instance Scratch Paper 15 16 Scratch Paper Scratch Paper 17 18 Scratch Paper CS 61A Midterm 1 Study Guide – Page 1 Import statement Name Value Assignment statement Code (left): Statements and expressions Red arrow points to next line.  Gray arrow points to the line just executed Binding Frames (right): A name is bound to a value In a frame, there is at most one binding per name 208 mul(add(2, mul(4, 6)), add(3, 5)) mul 26 8 add(2, mul(4, 6)) add(3, 5) add 2 24 mul(4, 6) mul 4 6 add 3 5 Pure Functions -2 abs(number): 2 2, 10 pow(x, y): Non-Pure Functions -2 print(...): display “-2” 1024 None Intrinsic name of function called Formal parameter bound to argument Local frame Return value is  not a binding! User-defined function Built-in function Defining: Formal parameter >>> def square( x ):
return mul(x, x)
(return statement) Call expression: square(2+2) operand: 2+2
argument: 4
operator: square function: func square(x)
Calling/Applying: 4 square( x ):
Argument Intrinsic name
return mul(x, x) 16 Return value
Compound statement
Clause

:

Suite
…
:
… 
…
!
2 boolean  contexts
def abs_value(x):
if x > 0: return x
elif x == 0: return 0
else:
return -x
1 statement, 3 clauses, 
3 headers,
3 suites,
2
A name evaluates to
the value bound to
that name in the
earliest frame of
the current 1 environment in which
that name is found.
2
Error
1
•An environment is a sequence of frames
•An environment for a non-nested function (no def within def) consists of one local frame, followed by the global frame
Evaluation rule for call expressions:
1.Evaluate the operator and operand subexpressions. 2.Apply the function that is the value of the operator
subexpression to the arguments that are the values of the operand subexpressions.
Applying user-defined functions:
1.Create a new local frame with the same parent as the function that was applied.
2.Bind the arguments to the function’s formal parameter names in that frame.
3.Execute the body of the function in the environment beginning at that frame.
Execution rule for def statements:
1.Create a new function value with the specified name, formal parameters, and function body.
2.Its parent is the first frame of the current environment. 3.Bind the name of the function to the function value in the
first frame of the current environment.
Execution rule for assignment statements:
1.Evaluate the expression(s) on the right of the equal sign. 2.Simultaneously bind the names on the left to those values,
in the first frame of the current environment.
Execution rule for conditional statements:
Each clause is considered in order.
1.Evaluate the header’s expression.
2.If it is a true value, execute the suite, then skip the remaining clauses in the statement.
Evaluation rule for or expressions:
1.Evaluate the subexpression .
2.If the result is a true value v, then the expression
evaluates to v.
3.Otherwise, the expression evaluates to the value of the
subexpression .
Evaluation rule for and expressions:
1.Evaluate the subexpression .
2.If the result is a false value v, then the expression
evaluates to v.
3.Otherwise, the expression evaluates to the value of the
subexpression .
Evaluation rule for not expressions:
1.Evaluate ; The value is True if the result is a false value, and False otherwise.
Execution rule for while statements:
1. Evaluate the header’s expression.
2. If it is a true value, execute the (whole) suite, then return to step 1.
B AB
A call expression and the body of the function being called are evaluated in different environments
1
A
2
1
def fib(n): hof.py
“””Compute the nth Fibonacci number, for N >= 1.”””
pred, curr = 0, 1 # Zeroth and first Fibonacci numbers
k = 1 # curr is the kth Fibonacci number return total
pred, curr = curr, pred + curr
k=k+1
def identity(k):
return curr return k
while k < n: def cube(k): return pow(k, 3) Function of a single argument (not called term) A formal parameter that will be bound to a function def summation(n, term): """Sum the first n terms of a sequence. >>> summation(5, cube)
225
“””
total, k = 0, 1 while k <= n: The cube function is passed as an argument value total, k = total + term(k), k + 1 return total The function bound to term gets called here 0+13 +23 +33 +43 +55 def pi_term(k): return 8 / (k * 4 − 3) / (k * 4 − 1) # Local function definitions; returning functions Def statement Body “y” is not found “y” is not found Return expression Higher-order function: A function that takes a function as an argument value or returns a function as a return value Nested def statements: Functions defined within other function bodies are bound to names in the local frame def make_adder(n): CS 61A Midterm 1 Study Guide – Page 2 square = lambda x,y: x * y Evaluates to a function.  square = lambda x: x * x VS def square(x): return x * x • Both create a function with the same domain, range, and behavior. • Both functions have as their parent the environment in which they were defined. • Both bind that function to the name square. • Only the def statement gives the function an intrinsic name. When a function is defined: A function No "return" keyword! with formal parameters x and y that returns the value of "x * y" Must be a single expression Can refer to names in the enclosing function A function that returns a function def make_adder(n): """Return a function that takes one argument k and returns k + n. ! The name add_three is bound to a function 1. Create a function value: func () 2. Its parent is the current frame. 
>>> add_three = make_adder(3)
>>> add_three(4) 7 
“””
def adder(k): return k + n
return adder
A local def statement
 
3. Bind to the function value in the current frame 
(which is the first frame of the current environment).
When a function is called:
1. Add a local frame, titled with the of the function being called.
2. Copy the parent of the function to the local frame: [parent=] 3. Bind the to the arguments in the local frame.
4. Execute the body of the function in the environment that starts with
the local frame.
•w
Is fact implemented correctly?
1. Verify the base case.
2. Treat fact as a functional abstraction!
3. Assume that fact(n-1) is correct.
4. Verify that fact(n) is correct,
assuming that fact(n-1) correct.
•Every user-defined function has a parent frame (often global) •The parent of a function is the frame in which it was defined •Every local frame has a parent
frame (often global)
•The parent of a frame is the
parent of the function called 3
2
1
A function’s signature has all the information to create a local frame
def curry2(f):!
“””Returns a function g such that g(x)(y) returns f(x, y).”””! def g(x):!
def h(y):!
return f(x, y)!
return h! return g
Anatomy of a recursive function:
• The def statement header is similar to other functions • Conditional statements check for base cases
• Base cases are evaluated without recursive calls
• Recursive cases are evaluated with recursive calls
def sum_digits(n):!
“””Return the sum of the digits of positive integer n.”””! if n < 10:! Currying: Transforming a multi-argument function into a single-argument, higher-order function. return n! else:! all_but_last, last = n // 10, n % 10! return sum_digits(all_but_last) + last • Each cascade frame is from a different call to cascade. • Until the Return value appears, that call has not completed. • Any statement can appear before or after the recursive call. • Recursive decomposition: finding simpler instances of a problem. • E.g., count_partitions(6, 4) • Explore two possibilities: •Useatleastone4 • Don't use any 4 • Solve two simpler problems: • count_partitions(2, 4) • count_partitions(6, 3) • Tree recursion often involves exploring different choices. from operator import floordiv, mod def count_partitions(n, m):! ifn==0:! return 1! elifn<0:! return 0! elif m == 0:! return 0! else:! with_m = count_partitions(n-m, m) ! without_m = count_partitions(n, m-1)! return with_m + without_m 1  12  123  1234  def 123  12  1 def inverse_cascade(n): grow(n) print(n) shrink(n) f_then_g(f, g, n): ifn:f(n) g(n) n: 0,1,2,3,4,5,6, 7, 8, fib(n): 0,1,1,2,3,5,8,13,21, def fib(n):! if n == 0:! return 0! elif n == 1:! return 1! else:! return fib(n-2) + fib(n-1) def divide_exact(n, d): """Return the quotient and remainder of dividing N by D. Multiple assignment  to two names Multiple return values, separated by commas ! >>> >>> q
q, r = divide_exact(2012, 10)
grow = lambda n: f_then_g(grow, print, n//10) shrink = lambda n: f_then_g(print, shrink, n//10)
201
>>> r
2
“””
return floordiv(n, d), mod(n, d)
Nested def

CS 61A Midterm 2 Study Guide — Page 1
Numeric types in Python:
List comprehensions:
[

for in if ]
Short version: [

for in ]
A combined expression that evaluates to a list using this
evaluation procedure:
1. Add a new frame with the current frame as its parent 2. Create an empty result list that is the value of the
expression
3. For each element in the iterable value of :
A. Bind to that element in the new frame from step 1 B. If evaluates to a true value, then add
List & dictionary mutation:
!
>>> type(2)
!
>>> type(1.5)
!
>>> type(1+1j)
Represents integers exactly
Represents real numbers approximately
>>> a = [10] >>> b = a >>> a == b True
>>> a.append(20) >>> a == b
True
>>> a
[10, 20]
>>> b [10, 20]
>>>a=[10] >>>b=[10] >>>a==b True
>>> b.append(20) >>> a
[10]
>>> b
[10, 20]
>>>a==b False
Functional pair implementation: def pair(x, y):
“””Return a functional pair.”””
the value of

to the result list apply_to_all(map_fn, s):
>>> nums = {‘I’: 1.0, ‘V’: 5, ‘X’: 10} >>> nums[‘X’]
10
>>> nums[‘I’] = 1
>>> nums[‘L’] = 50
>>> nums
{‘X’: 10, ‘L’: 50, ‘V’: 5, ‘I’: 1}
>>> sum(nums.values())
66
>>> dict([(3, 9), (4, 16), (5, 25)])
{3: 9, 4: 16, 5: 25}
>>> nums.get(‘A’, 0)
0
>>> nums.get(‘V’, 0)
5
>>> {x: x*x for x in range(3,6)}
{3: 9, 4: 16, 5: 25}
>>> suits = [‘coin’, ‘string’, ‘myriad’] >>> original_suits = suits
>>> suits.pop()
‘myriad’
>>> suits.remove(‘string’)
>>> suits.append(‘cup’)
>>> suits.extend([‘sword’, ‘club’])
>>> suits[2] = ‘spade’
>>> suits
[‘coin’, ‘cup’, ‘spade’, ‘club’]
>>> suits[0:2] = [‘heart’, ‘diamond’] >>> suits
[‘heart’, ‘diamond’, ‘spade’, ‘club’] >>> original_suits
[‘heart’, ‘diamond’, ‘spade’, ‘club’]
Identity:
is
evaluates to True if both and evaluate to the same object Equality:
==
evaluates to True if both and evaluate to equal values
Identical objects are always equal values
You can copy a list by calling the list constructor or slicing the list from the beginning to the end.
Constants: Constant terms do not affect the order of growth of a process
def get(index): if index == 0:
return x elif index == 1: return y
This function represents a pair
def
!
def
!
def
“””Apply map_fn to each element of s.
>>> apply_to_all(lambda x: x*3, range(5)) [0, 3,6,9,12]
“””
return [map_fn(x) for x in s]
keep_if(filter_fn, s):
“””List elements x of s for which filter_fn(x) is true.
0, 1, 2, 3, 4
λx: x*3
0, 3, 6, 9, 12
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
λx: x>5 6, 7, 8, 9
return get def select(p, i):
Constructor is a higher-order function
“””Return element i of pair p.”””
return p(i)
>>> p = pair(1, 2)
>>> select(p, 0) 1
>>> select(p, 1) 2
Lists:
Selector defers to the object itself
>>>
[6,
“””
keep_if(lambda x: x>5, range(10)) 7,8,9]
return [x for x in s if filter_fn(x)]
reduce(reduce_fn, s, initial):
“””Combine elements of s pairwise using reduce_fn,  
starting with initial.
“””
r= initial
for xins: pow 64 4
>>> digits = [1, 8, 2, 8] >>> len(digits)
16,777,216
4
>>> digits[3] 8
digits
r = reduce_fn(r, x) returnr
pow 4 3
>>> [2, 7] + digits * 2
[2, 7, 1, 8, 2, 8, 1, 8, 2, 8]
>>> pairs = [[10, 20], [30, 40]] >>> pairs[1]
reduce(pow, [1, 2, 3, 4], 2)
pow 2
pow 2 1
2
[30, 40]
>>> pairs[1][0] 30
pairs
Executing a for statement:
for in :
1. Evaluate the header , which must yield an iterable value (a sequence)
2. For each element in that sequence, in order:
A. Bind to that element in the current frame
B. Execute the
Unpacking in a  A sequence of  
for statement: fixed-length sequences
⇥(n) ⇥(500·n) ⇥( 1 ·n) 500
>>> >>>
>>> … … ! >>> 2
pairs=[[1, 2], [2, 2], [3, 2], [4, 4]] same_count = 0
A name for each element in a fixed-length sequence
for x, y in pairs: if x == y:
same_count = same_count + 1 same_count
Logarithms: The base of a logarithm does not affect the order of growth of a process
⇥(log2 n) ⇥(log10 n) ⇥(ln n)
Nesting: When an inner process is repeated for each step in an outer process,multiply the steps in the outer and inner processes to find the total number of steps
def overlap(a, b): count = 0
for item in a:
if item in b:
count += 1 return count
Outer: length of a
Inner: length of b
Type dispatching: Look up a cross-type implementation of an operation based on the types of its arguments
Type coercion: Look up a function for converting one type to another, then apply a type-specific implementation.
(bn )
⇥(n2 ) (n)
(log n) (1)
Exponential growth. Recursive fib takes
(n) steps, where = 1 + 5 1.61828 2
Incrementing the problem scales R(n) by a factor
Quadratic growth. E.g., overlap Incrementing n increases R(n) by the problem size n
Linear growth. E.g., factors or exp Logarithmic growth. E.g., exp_fast Doubling the problem only increments R(n) Constant. The problem size doesn’t matter
The parent frame contains the balance of withdraw
Every call decreases the same balance
>>> withdraw = make_withdraw(100) >>> withdraw(25)
75
>>> withdraw(25)
50
def make_withdraw(balance): def withdraw(amount): nonlocal balance
if amount > balance: return ‘No funds’
balance = balance – amount
return balance return withdraw
-2, -1, 0, 1,
…, -3,
2, 3, 4, …
If a and b are both length n,
then overlap takes ⇥(n2)steps
Lower-order terms: The fastest-growing part of the computation dominates the total
⇥(n2) ⇥(n2 +n) ⇥(n2 +500·n+log2 n+1000) Effect
Create a new binding from name “x” to number 2 in the first frame of the current environment
Re-bind name “x” to object 2 in the first frame of the current environment
Re-bind “x” to 2 in the first non-local frame of the current environment in which “x” is bound
SyntaxError: no binding for nonlocal ‘x’ found SyntaxError: name ‘x’ is parameter and nonlocal
range(-2, 2)
Length: ending value – starting value
Element selection: starting value + index
Status
x = 2
>>> list(range(-2, 2)) [-2, -1, 0, 1]
List constructor
•No nonlocal statement •”x” is not bound locally
•No nonlocal statement •”x” is bound locally •nonlocal x
•”x” is bound in a
non-local frame •nonlocal x
•”x” is not bound in a non-local frame
•nonlocal x
•”x” is bound in a  
non-local frame
•”x” also bound locally
!>>> list(range(4)) [0, 1, 2, 3]
Membership:
Range with a 0 starting value
Slicing:
Strings as sequences:
>>> city = ‘Berkeley’ >>> len(city)
8
>>> city[3]
‘k’
>>> ‘here’ in “Where’s Waldo?”
True
>>> 234 in [1, 2, 3, 4, 5] False
>>> [2, 3, 4] in [1, 2, 3, 4] False
>>> digits = [1, 8, 2, 8] >>> digits[0:2]
>>> 2 in digits
True
>>> 1828 not in digits True
[1, 8]
>>> digits[1:] [8, 2, 8]
Slicing creates a new object
[
[
R(n) = (f(n))
means that there are positive constants k1 and k2 such that k1 ·f(n)R(n)k2 ·f(n) for all n larger than some m

CS 61A Midterm 2 Study Guide — Page 2
Linked list data abstraction:
Python object system:
Idea: All bank accounts have a balance and an account holder;  
the Account class should add those attributes to each of its instances A new instance is >>> a = Account(‘Jim’)
empty = ’empty’
!
def link(first, rest):
return [first, rest] !
def first(s): return s[0]
!
def rest(s): return s[1]
!
def len_link(s): x = 0
while s != empty:
s, x = rest(s), x+1
return x !
def getitem_link(s, i): while i > 0:
s, i = rest(s), i – 1 return first(s)
!
def partitions(n, m):
“””Return a linked list of partitions of n using parts of up to m.
Each partition is a linked list.
“””
if n == 0:
returnlink(empty,empty) elif n < 0: return empty elif m == 0: return empty else: # Do I use at least one m? yes = partitions(n-m, m) no = partitions(n, m-1) add_m = lambda s: link(m, s) yes = apply_to_all_link(add_m, yes) return extend(yes, no) created by calling a >>> a.holder class ‘Jim’
>>> a.balance Whenaclassiscalled: 0
An account instance
1.A new instance of that class is created:
2.The __init__ method of the class is called with the new object as its first
argument (named self), along with any additional arguments provided in the call expression.
class Account:
def __init__(self, account_holder): self.balance = 0
self.holder = account_holder def deposit(self, amount):
self.balance = self.balance + amount
return self.balance bound to an instance of def withdraw(self, amount):
__init__ is called a constructor
self should always be
def extend(s, t):
assert is_link(s) and is_link(t) if s == empty:
return t else:
return link(first(s), extend(rest(s), t))
!
def apply_to_all_link(f, s): if s == empty:
return s else:
return link(f(first(s)), apply_to_all_link(f, rest(s)))
the Account class or a subclass of Account
if amount > self.balance: return ‘Insufficient funds’
self.balance = self.balance – amount return self.balance
>>> type(Account.deposit)
>>> type(a.deposit)
>>> Account.deposit(a, 5) 10
>>> a.deposit(2)
12
Dot expression
str and repr are both polymorphic; they apply to any object repr invokes a zero-argument method __repr__ on its argument
Account class attributes
Instance attributes of jim_account
>>> jim_account = Account(‘Jim’) >>> tom_account = Account(‘Tom’) >>> tom_account.interest
0.02
>>> jim_account.interest 0.02
>>> Account.interest = 0.04 >>> tom_account.interest 0.04
>>> jim_account.interest 0.04
Instance attributes of tom_account
>>> jim_account.interest = 0.08 >>> jim_account.interest
0.08
>>> tom_account.interest
0.04
>>> Account.interest = 0.05 >>> tom_account.interest 0.05
>>> jim_account.interest 0.08
Functioncall:all arguments within parentheses
Method invokation: One object before the dot and other arguments within parentheses
The can
The must be a
Evaluates to the value of the attribute looked up by in the object that is the value of the .
To evaluate a dot expression:
1. Evaluate the to the left of the dot, which yields
the object of the dot expression
2. is matched against the instance attributes of that object;
if an attribute with that name exists, its value is returned 3. If not, is looked up in the class, which yields a class
attribute value
4. That value is returned unless it is a function, in which case a
bound method is returned instead
Assignment statements with a dot expression on their left-hand side affect attributes for the object of that dot expression
• If the object is an instance, then assignment sets an instance attribute • If the object is a class, then assignment sets a class attribute
Call expression
A linked list is a pair
link(1, link(2, link(3, link(4, empty) 
represents the sequence
1 2 3 4
“empty” represents the empty list
.
be any valid Python expression. simple name.
The 0-indexed element of the pair is the first element of the linked list
The 1-indexed element of the pair is the rest of the linked list
The result of calling repr on a value is what Python prints in an interactive session
The result of calling str on a value is   what Python prints using the print function
>>> 12e12 12000000000000.0
>>> print(repr(12e12)) 12000000000000.0
>>> print(today) 2014-10-13
>>> today.__repr__() ‘datetime.date(2014, 10, 13)’
class Link: Some zero empty=() lengthsequence
>>> today.__str__() ‘2014-10-13’
def __init__(self, first, rest=empty): self.first = first
self.rest = rest
def __getitem__(self, i): if i == 0:
return self.first else:
return self.rest[i-1] def __len__(self):
return 1 + len(self.rest)
Yes, this call is recursive
class CheckingAccount(Account):
“””A bank account that charges for withdrawals.””” withdraw_fee = 1
interest = 0.01
def withdraw(self, amount):
return Account.withdraw(self, amount + self.withdraw_fee) or
>>> ch = CheckingAccount(‘Tom’) # Calls Account.__init__ >>> ch.interest # Found in CheckingAccount
0.01
>>> ch.deposit(20) # Found in Account
20
>>> ch.withdraw(5) # Found in CheckingAccount 14
class Tree:
def __init__(self, entry, branches=()):
self.entry = entry
for branch in branches:
assert isinstance(branch, Tree) self.branches = list(branches)
Built-in isinstance function: returns True if branch has a class that is or inherits from Tree
class BinaryTree(Tree):
empty = Tree(None)
empty.is_empty = True
def __init__(self, entry, left=empty, right=empty): 1
To look up a name in a class:
2. Otherwise, look up the name in the base class, if there is one.
Tree.__init__(self, entry, (left, right))
self.is_empty = False @property
def left(self):
return self.branches[0] t = Bin(3, Bin(1),
@property
def right(self): returnself.branches[1]
E
9
E: An empty tree
Bin = BinaryTree
Bin(7, Bin(5),
11
3
7 E 5
E E E
return super().withdraw(
Bin(9, Bin.empty, Bin(11))))E E
amount + self.withdraw_fee)
1. If it names an attribute in the class, return the attribute value.
balance: 0 holder: ‘Jim’
Memoization:
def memo(f): cache = {}
def memoized(n):
if n not in cache:
cache[n] = f(n) return cache[n]
return memoized
interest: 0.02 0.04 0.05 (withdraw, deposit, __init__)
balance: 0 holder: ‘Jim’ interest: 0.08
balance: 0 holder: ‘Tom’
Sequence abstraction special names:
__getitem__ Element selection [] __len__ Built-in len function

CS 61A Final Exam Study Guide – Page 1
Exceptions are raised with a raise statement. raise
must evaluate to a subclass of BaseException or an instance of one.
Exceptions are constructed like any other object. E.g.,
TypeError(‘Bad argument!’)
try:
except as :
…
The is executed first.
If, during the course of executing the
, an exception is raised that is not handled otherwise, and
If the class of the exception inherits from , then
The is executed, with bound to the exception.
for in :
1. Evaluate the header , which yields an iterable object. 2. For each element in that sequence, in order:
A. Bind to that element in the first frame of the current environment.
B. Execute the .
An iterable object has a method __iter__ that returns an iterator.
class LetterIter:!
def __init__(self, start=’a’, end=’e’):!
>>> a_to_c = LetterIter(‘a’, ‘c’)! >>> next(a_to_c)!
‘a’!
>>> next(a_to_c)!
‘b’!
>>> next(a_to_c)!
Traceback (most recent call last):!
…! StopIteration!
!
>>> b_to_k = Letters(‘b’, ‘k’)! >>> first_iterator = b_to_k.__iter__()!
>>> next(first_iterator)!
‘b’!
>>> next(first_iterator)!
‘c’!
>>> second_iterator = iter(b_to_k)!
!
!
self.next_letter = start! self.end = end!
def __next__(self):!
if self.next_letter >= self.end:!
raise StopIteration!
result = self.next_letter! self.next_letter = chr(ord(result)+1)! return result!
class Letters:!
def __init__(self, start=’a’, end=’e’):!
>>> try:
x = 1/0
except ZeroDivisionError as e: print(‘handling a’, type(e)) x=0
!
handling a >>> x
0
self.start = start! self.end = end!
!
!
def letters_generator(next_letter, end):! while next_letter < end:! yield next_letter! next_letter = chr(ord(next_letter)+1) • A generator is an iterator backed by a generator function. • Each time a generator function is called, it returns a generator. def __iter__(self):! return LetterIter(self.start, self.end)!>>> second_iterator.__next__()!
‘b’!
>>> first_iterator.__next__()! ‘d’!
!
>>> for letter in letters_generator(‘a’, ‘e’):! … print(letter)!
a!
b!
c!
d
>>> counts = [1, 2, 3]! >>> for item in counts:!
print(item)!
1! 2! 3
class FibIter:!
def __init__(self):!
>>> items = counts.__iter__()! >>> try:!
while True:!
item = items.__next__()! print(item)!
except StopIteration:! pass
>>> fibs = FibIter()!
>>> [next(fibs) for _ in range(10)]! [0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
A row has a value for each column
select [expression] as [name], [expression] as [name], … ;
select [columns] from [table] where [condition] order by [order];
create table parents as
select “abraham” as parent, “barack” as child union
A table has columns and rows
A column has a name and a type
Latitude Longitude Name
38
122
Berkeley
42
71
Cambridge
45
93
Minneapolis
!
“Please don’t reference these directly. They may change.”
def __next__(self):!
result = self._next!
self._addend, self._next = self._next, self._addend + self._next! return result
self._next = 0! self._addend = 1!
A stream is a linked list, but the rest of the list is computed on demand.
Once created, Streams and Rlists can be used interchangeably using first and rest.
class Stream:
“””A lazily computed linked list.””” class empty:
def __repr__(self): return ‘Stream.empty’
empty = empty()
Streams are lazily computed linked lists
select “abraham” select “delano” select “fillmore” select “fillmore” select “fillmore” select “eisenhower”
, “clinton” , “herbert” , “abraham” , “delano”
, “grover”
, “fillmore”;
union
union E union
union
union F
union
union ADG union
union
union
union BCH
union
def __init__(self, first, compute_rest=lambda: Stream.empty): assert callable(compute_rest), ‘compute_rest must be callable.’ self.first = first
self._compute_rest = compute_rest
@property
def rest(self):
“””Return the rest of the stream, computing it if necessary.””” if self._compute_rest is not None:
self._rest = self._compute_rest()
select a.child as first, b.child as second
from parents as a, parents as b
where a.parent = b.parent and a.child < b.child; with ancestors(ancestor, descendent) as ( select parent, child from parents union select ancestor, child from ancestors, parents where parent = descendent First Second ancestor abc first Stored explicitly rest Evaluated lazily select "barack" select "clinton" select "delano" select "eisenhower" select "fillmore" select "grover" select "herbert" , "short" , "long" , "long" , "short" , "curly" , "short" , "curly"; create table dogs as select "abraham" as name, "long" as fur barack clinton abraham delano abraham grover delano grover self._compute_rest = None return self._rest def integer_stream(first=1): def compute_rest(): return integer_stream(first+1) return Stream(first, compute_rest) def filter_stream(fn, s): if s is Stream.empty: return s def compute_rest(): return filter_stream(fn, s.rest) if fn(s.first): return Stream(s.first, compute_rest) else: return compute_rest() def primes(positives): def not_divisible(x): return x % positives.first != 0 def compute_rest(): ) select ancestor from ancestors where descendent="herbert"; create table pythagorean_triples as with i(n) as ( select 1 union select n+1 from i where n < 20 ) select a.n as a, b.n as b, c.n as c from i as a, i as b, i as c where a.n < b.n and a.n*a.n + b.n*b.n = c.n*c.n; delano fillmore eisenhower def map_stream(fn, s): if s is Stream.empty: return s def compute_rest(): return map_stream(fn, s.rest) return Stream(fn(s.first), compute_rest) return primes(filter_stream(not_divisible, positives.rest)) return Stream(positives.first, compute_rest) The way in which names are looked up in Scheme and Python is called lexical scope (or static scope). Lexical scope: The parent of a frame is the environment in which a procedure was defined. (lambda ...) Dynamic scope: The parent of a frame is the environment in which a procedure was called. (mu ...) > (define f (mu (x) (+ x y)))
> (define g (lambda (x y) (f (+ x x)))) > (g 3 7)
13
Message sequence of a TCP connection
3
5
6
8
9
12
4
12
8
15
12
16
5
13
10
17
15
20
Acknowledgement & synchronization request
Acknowledgement
Data message from A to B
Acknowledgement
Data message from B to A
Acknowledgement
Termination signal
Acknowledgement & termination signal
Acknowledgement
Computer B
.. .. ..
Computer A

Expression Trees
CS 61A Final Exam Study Guide – Page 2
Scheme programs consist of expressions, which can be:
• Primitive expressions: 2, 3.3, true, +, quotient, … • Combinations: (quotient 10 2), (not true), …
Numbers are self-evaluating; symbols are bound to values. Call expressions have an operator and 0 or more operands.
A combination that is not a call expression is a special form: • If expression: (if ) • Binding names: (define )
scheme_reader.py scalc.py
• New
procedures: (define ( ) )
> (define pi 3.14) > (* pi 2)
6.28
> (define (abs x) (if (< x 0) (- x) x)) > (abs -3)
3
A basic interpreter has two parts: a parser and an evaluator A basic interpreter has two parts: a parser and an evaluator
A basic interpreter has two parts: a parser and an evaluator.
lines lines
‘(+ 2 2)’ ‘(+ 2 2)’
‘(* (+ 1′
”(* (+(1-’23)’
” ((*- 4235).’6))’ ” 10)'(* 4 5.6))’
‘ 10)’
expression expression
Pair(‘+’, Pair(2, Pair(2, nil))) Pair(‘+’, Pair(2, Pair(2, nil)))
Pair(‘*’, Pair(Pair(‘+’, …))) Pair(‘*’, Ppariirn(tPeadiars(‘+’, …))) (* (+ 1 (p-ri2nt3e)d (as* 4 5.6)) 10)
(* (+ 1 (- 23) (* 4 5.6)) 10)
value value
4 4
4 4
Lines forming
A number or a Pair with an
operator as its first element A number or a Pair with an
operator as its first element
A number A number
a Scheme Lines forming
eaxpSrcehsesmieon expression
scheme_reader.py
scalc.py
Parser Parser
Evaluator Evaluator
Lambda expressions evaluate to anonymous procedures.
A Scheme list is written as elements in parentheses:
( … ) A Scheme list
Each can be a combination or atom (primitive).
(+ (* 3 (+ (* 2 4) (+ 3 5))) (+ (- 10 7) 6))
The task of parsing a language involves coercing a string representation of an expression to the expression itself.
Parsers must validate that expressions are well-formed.
6 6
(lambda () ) Two equivalent expressions:
(define (plus4 x) (+ x 4))
(define plus4 (lambda (x) (+ x 4)))
An operator can be a combination too:
λ
((lambda (x y z) (+ x y (square z))) 1 2 3)
In the late 1950s, computer scientists used confusing names. • cons: Two-argument procedure that creates a pair
• car: Procedure that returns the first element of a pair • cdr: Procedure that returns the second element of a pair • nil: The empty list
They also used a non-obvious notation for linked lists.
• A (linked) Scheme list is a pair in which the second element is
nil or a Scheme list.
• Scheme lists are written as space-separated combinations.
• A dotted list has an arbitrary value for the second element of the
last pair. Dotted lists may not be well-formed lists.
> (define x (cons 1 2)) > x
A Parser takes a sequence of lines and returns an expression.
Lines
‘(+ 1’
‘ (- 23)’
‘ (*45.6))’
Tokens
‘(‘, ‘+’, 1
‘(‘, ‘-‘, 23, ‘)’ ‘(‘,’*’,4,5.6,’)’,’)’
Expression
Pair(‘+’, Pair(1, …))
printed as
(+1(-23)(*45.6))
Lexical analysis
Syntactic analysis
(1 . 2)
> (car x)
1
> (cdr x)
2
> (cons 1 (cons 2 (cons 3 (cons 4 nil)))) (1 2 3 4)
Symbols normally refer to values; how do we refer to symbols?
• Iterative process
• Checks for malformed tokens • Determines types of tokens • Processes one line at a time
• Tree-recursive process • Balances parentheses
• Returns tree structure • Processes multiple lines
> (define a 1) > (define b 2) > (list a b) (1 2)
No sign of “a” and “b” in the resulting value
Syntactic analysis identifies the hierarchical structure of an expression, which may be nested.
Each call to scheme_read consumes the input tokens for exactly one expression.
Base case: symbols and numbers
Recursive call: scheme_read sub-expressions and combine them
Not a well-formed list!
Quotation is used to refer to symbols directly in Lisp.
Base cases:
• Primitive values (numbers)
• Look up values bound to symbols
Eval
The structure of the Scheme interpreter
Creates a new environment each time a user- defined procedure is applied
Apply
> (list ‘a ‘b) (a b)
> (list ‘a b) (a 2)
Symbols are now values
Recursive calls:
• Eval(operator, operands) of call expressions • Apply(procedure, arguments)
• Eval(sub-expressions) of special forms
Quotation can also be applied to combinations to form lists.
> (car ‘(a b c)) a
> (cdr ‘(a b c)) (b c)
Dots can be used in a quoted list to specify the second element of the final pair.
> (cdr (cdr ‘(1 2 . 3))) 3
However, dots appe

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