Lab 14: Final Review

Due by 11:59pm on Wednesday, August 12.

Starter Files

Download Inside the archive, you will find starter files for the questions in this lab, along with a copy of the Ok autograder.


By the end of this lab, you should have submitted the lab with python3 ok --submit. You may submit more than once before the deadline; only the final submission will be graded. Check that you have successfully submitted your code on

Required Questions


Q1: Prune Min

Write a function that prunes a Tree t mutatively. t and its branches always have zero or two branches. For the trees with two branches, reduce the number of branches from two to one by keeping the branch that has the smaller label value. Do nothing with trees with zero branches.

Prune the tree in a direction of your choosing (top down or bottom up). The result should be a linear tree.

def prune_min(t):
    """Prune the tree mutatively from the bottom up.

    >>> t1 = Tree(6)
    >>> prune_min(t1)
    >>> t1
    >>> t2 = Tree(6, [Tree(3), Tree(4)])
    >>> prune_min(t2)
    >>> t2
    Tree(6, [Tree(3)])
    >>> t3 = Tree(6, [Tree(3, [Tree(1), Tree(2)]), Tree(5, [Tree(3), Tree(4)])])
    >>> prune_min(t3)
    >>> t3
    Tree(6, [Tree(3, [Tree(1)])])
    "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q prune_min


Q2: Split

Implement split-at, which takes a list lst and a positive number n as input and returns a pair new such that (car new) is the first n elements of lst and (cdr new) is the remaining elements of lst. If n is greater than the length of lst, (car new) should be lst and (cdr new) should be nil.

(define (split-at lst n)

Use Ok to test your code:

python3 ok -q split-at


Q3: Align Skeleton

Have you wondered how your CS61A exams are graded online? To see how your submission differs from the solution skeleton code, okpy uses an algorithm very similar to the one below which shows us the minimum number of edit operations needed to transform the the skeleton code into your submission.

Similar to meowstake_matches in Cats, we consider two different edit operations:

  1. Insert a letter to the skeleton code
  2. Delete a letter from the skeleton code

Given two strings, skeleton and code, implement align_skeleton, a function that minimizes the edit distance between the two strings and returns a string of all the edits. Each addition is represented with +[], and each deletion is represented with -[]. For example:

>>> align_skeleton(skeleton = "x=5", code = "x=6")
>>> align_skeleton(skeleton = "while x<y", code = "for x<y")

In the first example, the +[6] represents adding a "6" to the skeleton code, while the -[5] represents removing a "5" to the skeleton code. In the second example, we add in the letters "f", "o", and "r" and remove the letters "w", "h", "i", "l", and "e" from the skeleton code to transform it to the submitted code.

Note: For simplicity, all whitespaces are stripped from both the skeleton and submitted code, so you don't have to consider whitespaces in your logic.

align_skeleton uses a recursive helper function, helper_align, which takes in skeleton_idx and code_idx, the indices of the letters from skeleton and code which we are comparing. It returns two things: match, the sequence of edit corrections, and cost, the numer of edit operations made. First, you should define your three base cases:

  • If both skeleton_idx and code_idx are at the end of their respective strings, then there are no more operations to be made.
  • If we have not finished considering all letters in skeleton but we have considered all letters in code, then we simply need to delete all the remaining letters in skeleton to match it to code.
  • If we have not finished considering all letters in code but we have considered all letters in skeleton , then we simply need to add all the remaining letters in code to skeleton.

Next, you should implement the rest of the edit operations for align_skeleton and helper_align. You may not need all the lines provided.

def align_skeleton(skeleton, code):
    Aligns the given skeleton with the given code, minimizing the edit distance between
    the two. Both skeleton and code are assumed to be valid one-line strings of code. 

    >>> align_skeleton(skeleton="", code="")
    >>> align_skeleton(skeleton="", code="i")
    >>> align_skeleton(skeleton="i", code="")
    >>> align_skeleton(skeleton="i", code="i")
    >>> align_skeleton(skeleton="i", code="j")
    >>> align_skeleton(skeleton="x=5", code="x=6")
    >>> align_skeleton(skeleton="return x", code="return x+1")
    >>> align_skeleton(skeleton="while x<y", code="for x<y")
    >>> align_skeleton(skeleton="def f(x):", code="def g(x):")
    skeleton, code = skeleton.replace(" ", ""), code.replace(" ", "")

    def helper_align(skeleton_idx, code_idx):
        Aligns the given skeletal segment with the code.
        Returns (match, cost)
            match: the sequence of corrections as a string
            cost: the cost of the corrections, in edits
        if skeleton_idx == len(skeleton) and code_idx == len(code):
            return _________, ______________
        if skeleton_idx < len(skeleton) and code_idx == len(code):
            edits = "".join(["-[" + c + "]" for c in skeleton[skeleton_idx:]])
            return _________, ______________
        if skeleton_idx == len(skeleton) and code_idx < len(code):
            edits = "".join(["+[" + c + "]" for c in code[code_idx:]])
            return _________, ______________

        possibilities = []
        skel_char, code_char = skeleton[skeleton_idx], code[code_idx]
        # Match
        if skel_char == code_char:
            possibilities.append((_______, ______))
        # Insert
        possibilities.append((_______, ______))
        # Delete
        possibilities.append((_______, ______))
        return min(possibilities, key=lambda x: x[1])
    result, cost = ________________________
    return result

Use Ok to test your code:

python3 ok -q align_skeleton

Tree Recursion

Q4: Num Splits

Given a list of numbers s and a target difference d, how many different ways are there to split s into two subsets such that the sum of the first is within d of the sum of the second? The number of elements in each subset can differ.

You may assume that the elements in s are distinct and that d is always non-negative.

Note that the order of the elements within each subset does not matter, nor does the order of the subsets themselves. For example, given the list [1, 2, 3], you should not count [1, 2], [3] and [3], [1, 2] as distinct splits.

Hint: If the number you return is too large, you may be double-counting somewhere. If the result you return is off by some constant factor, it will likely be easiest to simply divide/subtract away that factor.

def num_splits(s, d):
    """Return the number of ways in which s can be partitioned into two
    sublists that have sums within d of each other.

    >>> num_splits([1, 5, 4], 0)  # splits to [1, 4] and [5]
    >>> num_splits([6, 1, 3], 1)  # no split possible
    >>> num_splits([-2, 1, 3], 2) # [-2, 3], [1] and [-2, 1, 3], []
    >>> num_splits([1, 4, 6, 8, 2, 9, 5], 3)
    "*** YOUR CODE HERE ***"

Use Ok to test your code:

python3 ok -q num_splits

Linked Lists

Q5: Insert

Implement a function insert that takes a Link, a value, and an index, and inserts the value into the Link at the given index. You can assume the linked list already has at least one element. Do not return anything -- insert should mutate the linked list.

Note: If the index is out of bounds, you can raise an IndexError with:

raise IndexError
def insert(link, value, index):
    """Insert a value into a Link at the given index.

    >>> link = Link(1, Link(2, Link(3)))
    >>> print(link)
    <1 2 3>
    >>> insert(link, 9001, 0)
    >>> print(link)
    <9001 1 2 3>
    >>> insert(link, 100, 2)
    >>> print(link)
    <9001 1 100 2 3>
    >>> insert(link, 4, 5)
    if ____________________:
    elif ____________________:

Use Ok to test your code:

python3 ok -q insert


Q6: Switch

Define the macro switch, which takes in an expression expr and a list of pairs, cases, where the first element of the pair is some value and the second element is a single expression. switch will evaluate the expression contained in the list of cases that corresponds to the value that expr evaluates to.

scm> (switch (+ 1 1) ((1 (print 'a))
                      (2 (print 'b))
                      (3 (print 'c))))

You may assume that the value expr evaluates to is always the first element of one of the pairs in cases. You can also assume that the first value of each pair in cases is a value.

(define-macro (switch expr cases)
	(cons _________
		(map (_________ (_________) (cons _________ (cdr case)))

Use Ok to test your code:

python3 ok -q switch


Make sure to submit this assignment by running:

python3 ok --submit