# Structure and Interpretation of Computer Programs

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1 CS 6A Fall 206 Structure and Interpretation of Computer Programs Final Solutions INSTRUCTIONS You have hours to complete the exam. The exam is closed book, closed notes, closed computer, closed calculator, except two hand-written 8.5 pages of your own creation and the official CS 6A 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 TA Room Row & Seat Number 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)

2 2. (6 points) What Would Python Display? 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. If an error occurs, write Error, but include all output displayed before the error. Assume that expressions are evaluated in the order that they appear on the page, with the left column before the right column. The Pair class appears on page 2 of the final study guide. The reduce function appears on page of the final study guide. The first two rows have been provided as examples. Assume that you have started python and executed the following statements: swap = lambda a, b: [b, a] paws = lambda s: reduce (swap, (s), ) emit = lambda y: [x+y for x in y] time = lambda t, f: (t and f(t)) or print (t +2) mite = lambda mu: time ( mu +, print ) s, t = print, [ * x for x in range ( 8)] def ti(me ): while me: yield time (me, lambda k: [k. pop ()]) def newt ( graves ): tina = graves. pop () if graves. pop () % == 0: graves. pop () print (tina, graves. pop ()) if graves : return Pair (tina -, newt ( graves )) return Pair ( tina +, tina ) Expression Interactive Output 5*5 25 print(pair(6, )) (6. ) print(2, s(print())) None 2 None emit([[], [2]])[] [2, [], [2]] s(pair(pair(4, 5), nil)) time(2, mite) ((4. 5)) 5 4 Expression paws(range(4, 6)) tuple(ti([5, 6])) s(newt([2, 5, 8])) s(newt(t)) Interactive Output [5, [4, ]] ([6], [5]) 8 2 (9. 8) (20. 9)

3 Name: 2. (8 points) A Function By Any Other Name 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 frames. Add all missing values created or referenced during execution. Show the return value for each local frame. Use box-and-pointer notation for s. You don t need to write index numbers or the word name = [2, lambda your: name[0]] def your(name): def what(s): nonlocal name name = [name, s] return what is your(name) def your(s): s[][](name[:]) return what your(true)(name) Global frame your name f: your [parent= ] Global name what your func your(name) [parent=global] True 2 func what(s) [parent=f] func your(s) [parent=f] func λ(your) [parent=global] Return Value f2: what [parent= ] f s Return Value False your f f: [parent= ] s Return Value None f4: λ [parent= ] Global your 0 True Return Value 2 Green boxes indicate parts of the diagram that were scored during grading.

4 4. (8 points) Boxes and Arrows Fill in the environment diagram that results from executing each block of code below until the entire program is finished or an error occurs. Use box-and-pointer notation for s. You don t need to write index numbers or the word. Please erase or cross out any boxes or pointers that are not part of a final diagram. (a) t = [] t.append([t, []]) Global frame t 0 (b) t = [, 2] t.append(t[t[0]:t[t[0]]]) Global frame t (c) t = [] t.extend(t + [t] + [[t]]) Global frame t 2 0 (d) t = [, 2,, 4] t[] = t[2:4] t[2:] = t t[4] = t.pop() Global frame t

5 Name: 5 4. (6 points) A Tree or a List A flat is a containing integers and None that describes a binary tree of integers. The root is element 0. The left branch of element i is element 2 i + and the right is 2 i + 2. The None value is used to indicate an empty tree, such as an empty left branch when the corresponding right branch is not empty. None values at the end of a flat can be omitted. Four examples of trees and the flat s that describe them appear below. 4 [,, 4] 4 [, None, 4] [, ] or 6 8 [,, None] 7 [6,, 8,, None, 7] or [6,, 8,, None, 7, None] (a) (4 pt) Implement grow, which takes a flat s of integer node values (which may also contain None, as described above) and an index i that is 0 by default. When i is 0, grow returns a BTree instance described by s. The BTree class appears in the left column of page 2 of the midterm 2 study guide. def grow (s, i =0): """ Return a binary tree described by the flat S when I is 0. >>> grow ([,, 4]) BTree (, BTree (), BTree (4)) >>> grow ([, None, 4]) BTree (, BTree. empty, BTree (4)) >>> grow ([,, None ]) BTree (, BTree ()) >>> grow ([6,, 8,, None, 7]) BTree (6, BTree (, BTree ()), BTree (8, BTree (7))) """ if i >= len (s) or s[i] is None : return BTree. empty left, right = grow (s, 2*i+), grow (s, 2*i+2) return BTree (s[i], left, right ) (b) (4 pt) Implement leaves, which takes a flat s and returns a generator over the leaf values of the tree described by s. You may not call grow in your solution. def leaves (s): """ Return a generator over the leaf values in a binary tree described by S. >>> ( leaves ([,, 4])) [, 4] >>> ( leaves ([,, None ])) [] >>> ( leaves ([6,, 8,, None, 7])) [, 7] """ f = lambda x: x >= len (s) or s[x] is None for i in range ( len (s )): if s[i] is not None and f (2* i +) and f (2* i +2): yield s[i]

8 8 6. (0 points) I Scheme for Ice Cream The built-in append procedure is equivalent in behavior to the following definition. ( define ( append s t) (if ( null? s) t ( cons ( car s) ( append ( cdr s) t )))) (a) ( pt) Circle True or False: The recursive call to append in the definition above is a tail call. (b) (4 pt) Implement atoms, which takes a Scheme expression. It returns a of the non-nil atoms contained in the expression in the order that they appear. A non-nil atom is a number, symbol, or boolean value. scm > ( atoms ) () scm > ( atoms (+ 2 )) (+ 2 ) scm > ( atoms (+ (* 2 ) 4)) (+ * 2 4) scm > ( atoms (* (+ (* 2 )) (+ 4 5))) (* + * ) ( define ( atoms exp ) ( cond (( null? exp ) nil ) (( atom? exp ) ( exp )) ( else ( append ( atoms ( car exp )) ( atoms ( cdr exp )))))) (c) (5 pt) If Scheme had only numbers and two-argument procedures, parentheses would be unnecessary. To demonstrate, implement tally, which takes the of atoms in a Scheme expression. It returns a whose first element is the value of the original expression. Assume that the original expression consists only of numbers and call expressions with arithmetic operators (such as + and *) and exactly two operands. Hint: tally is similar to the built-in eval procedure: (eval (+ (* 2 ) 4)) evaluates to 0. scm > ( car ( tally ())) ; atoms in scm > ( car ( tally (+ 2 ))) ; atoms in (+ 2 ) 5 scm > ( car ( tally (+ * 2 4))) ; atoms in (+ (* 2 ) 4) 0 scm > ( car ( tally (* + * ))) ; atoms in (* (+ (* 2 )) (+ 4 5)) 6 ( define ( tally s) (if ( number? ( car s)) s ( let (( first ( tally ( cdr s )))) ( let (( second ( tally ( cdr first )))) ( cons ( eval ( ( car s) ( car first ) ( car second ))) ( cdr second ))))))

9 Name: 9 7. (2 points) Hailstoned The following two tables of integers from 0 to 99 (including 99) and to 99 are used in the questions below. create table ns as with t( n) as ( select 0 union select n + from t where n < 99 ) select * from t; create table ps as select n from ns where n > 0; (a) ( pt) Create a one-column table of all integers from 2000 to 9999 (including 9999). Do not use recursion. select a. n * 00 + b. n from ns as a, ns as b where a. n >= 20; (b) (5 pt) Create a table with a row for each pair of different positive integers x and y below 00. The three columns contain x, y, and the greatest common divisor of x and y. For example, one row contains (20, 50, 0) because 0 is the largest n that evenly divides both 20 and 50. Another row contains (50, 20, 0). select a.n as x, b.n as y, max (c.n) as gcd from ps as a, ps as b, ps as c where a.n <> b.n and a.n % c.n = 0 and b.n % c.n = 0 group by a.n * 00 + b.n; (c) (4 pt) A hailstone sequence begins with a positive m. If m is even, divide it by 2. If m is odd, triple it and add. Repeat until is reached. For example, the sequence starting at is, 0, 5, 6, 8, 4, 2,. Create a two-column table with positive integer m in the first column and the length of the hailstone sequence starting at m in the second column. For example, one row contains (, 8) because the hailstone sequence starting at has 8 elements. This table should have one row for each m, but only include m for which the entire hailstone sequence starting at m consists of numbers below 00. with hailstone ( m, length ) as ( select, union select n, length + from hailstone, ps where (n %2==0 and m == n /2) or (n > and n %2== and m == *n+) ) select * from hailstone ;

10 0 8. (0 points) Draw! (Optional) Compare Python, Scheme, and SQL in a picture.

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