+
+Discussion 8: Linked Lists
+
+
+
+
+ - disc08.pdf
+
+
+
+
+ If there are fewer than 3 people in your group, feel free to merge your group with another group in the room.
+ +Now switch to Pensieve:
+ +-
+
- Everyone: Go to pensieve.co, log in with your @berkeley.edu email, and enter your group number (which was in the email that assigned you to this lab). +
Once you're on Pensieve, you don't need to return to this page; Pensieve has all the same content (but more features). If for some reason Penseive doesn't work, return to this page and continue with the discussion.
+ + +Getting Started
+ + +Everybody say your name and your birthday and then tell the group about your +favorite birthday party you've attended (either for your birthday or someone +else's).
+ +Pro tip: Groups tend not to ask for help unless they've been stuck for a +looooooong time. Try asking for help sooner. We're pretty helpful! You might +learn something.
+ + +Linked Lists
+ + +A linked list is a Link object or Link.empty.
You can mutate a Link object s in two ways:
-
+
- Change the first element with
s.first = ...
+ - Change the rest of the elements with
s.rest = ...
+
You can make a new Link object by calling Link:
-
+
Link(4)makes a linked list of length 1 containing 4.
+ Link(4, s)makes a linked list that starts with 4 followed by the elements of linked lists.
+
class Link:
+ """A linked list is either a Link object or Link.empty
+
+ >>> s = Link(3, Link(4, Link(5)))
+ >>> s.rest
+ Link(4, Link(5))
+ >>> s.rest.rest.rest is Link.empty
+ True
+ >>> s.rest.first * 2
+ 8
+ >>> print(s)
+ <3 4 5>
+ """
+ empty = ()
+
+ def __init__(self, first, rest=empty):
+ assert rest is Link.empty or isinstance(rest, Link)
+ self.first = first
+ self.rest = rest
+
+ def __repr__(self):
+ if self.rest:
+ rest_repr = ', ' + repr(self.rest)
+ else:
+ rest_repr = ''
+ return 'Link(' + repr(self.first) + rest_repr + ')'
+
+ def __str__(self):
+ string = '<'
+ while self.rest is not Link.empty:
+ string += str(self.first) + ' '
+ self = self.rest
+ return string + str(self.first) + '>'Drawing Time. Pick a way for your group to draw diagrams. Paper, a +whiteboard, or a tablet, are all fine. If you don't have anything like that, ask +another group in the room if they have extra paper.
+ + +Q1: Sum Two Ways
+ + +Implement both sum_rec and sum_iter. Each one takes a linked list of numbers
+s and a non-negative integer k and returns the sum of the first k elements
+of s. If there are fewer than k elements in s, all of them are summed. If
+k is 0 or s is empty, the sum is 0.
Use recursion to implement sum_rec. Don't use recursion to implement
+sum_iter; use a while loop instead.
s.first to the sum of the first k-1 elements in s.rest. Your base case
+condition should include s is Link.empty so that you're checking whether s
+is empty before ever evaluating s.first or s.rest.
+total, then repeatedly (in a while loop) add
+s.first to total, set s = s.rest to advance through the linked list, and reduce k by one.
+Discussion time: When adding up numbers, the intermediate sums depend on the
+order. (1 + 3) + 5 and 1 + (3 + 5) both equal 9, but the first one makes 4
+along the way while the second makes 8 along the way. For the same linked list
+s and length k, will sum_rec and sum_iter both make the same
+intermediate sums along the way?
Q2: Overlap
+ + +Implement overlap, which takes two linked lists of numbers called s and t
+that are sorted in increasing order and have no repeated elements within each
+list. It returns the count of how many numbers appear in both lists.
This can be done in linear time in the combined length of s and t by
+always advancing forward in the linked list whose first element is smallest
+until both first elements are equal (add one to the count and advance both) or
+one list is empty (time to return). Here's a
+lecture video clip
+about this (but the video uses Python lists instead of linked lists).
Take a vote to decide whether to use recursion or iteration. +Either way works (and the solutions are about the same complexity/difficulty).
+ + + + + + Run in 61A Code + if s is Link.empty or t is Link.empty:
+ return 0
+ if s.first == t.first:
+ return __________________
+ elif s.first < t.first:
+ return __________________
+ elif s.first > t.first:
+ return __________________ k = 0
+ while s is not Link.empty and t is not Link.empty:
+ if s.first == t.first:
+ __________________
+ elif s.first < t.first:
+ __________________
+ elif s.first > t.first:
+ __________________
+ return kDocument the Occasion
+ + +Please all fill out the attendance form (one submission per person per week).
+ + +