EBoard 41: Shortest paths

Warning This class is being recorded (and transcribed) (assuming Teams succeeds).

Approximate overview

• Administrivia
• Questions
• Lab
• Implementing graphs (maybe)

Administrivia

• I have no scheduled office hours during Finals Week. I will be in my office as much as I can. Feel free to DM me to check if I’m there or to ask questions.
• Since we’ll be doing EOCEs in class on Friday and have fewer computers than students, it would be nice if some of you brought your laptops on Friday.

Upcoming Token activities

Academic

Cultural

• Wednesday, 2023-12-06, Noon–1:30pm, Bucksbaum, Vox Feminae Christmas Concert
• Saturday, 2023-12-09, 11:55am–2:40pm, Harris Cinema, Florencia en el Amazonas

Peer

• Friday, 2023-12-08, 1:00-2:50 pm, Psych Commons, Poster Session

Wellness

Misc

Other good things (no tokens)

Upcoming work

• MP redos: As you can do them. Last possible moment is 5pm on Dec 15th.
• LAs: As you can do them. Last possible moment is 5pm on Dec 15th.

Questions

Graphs

Administrative

Lab

• Finish implementing Prim’s.
  • See the prior eboard or the end of the prior lab for notes about some of the details.
• Implement Dijkstra’s.

Questions

What comparator might I use for the priority queue of edges?

A comparator takes two parameters and is supposed to return * A negative number, if the first parameter “comes before” the second parameter. * Zero, if the two parameters are equal. * A positive number, if the second parameter “comes after” the first parameter.

We can write comparators with lambdas.

(e1,e2) -> e1.weight() < e2.weight() ? -1 : e1.weight() == e2.weight() ? 0 : 1

(e1,e2) -> e1.weight() - e2.weight()

Could we have a sample graph?

Sure.

• 0 1 1
• 0 2 3
• 0 3 3
• 1 2 2
• 1 4 4
• 2 3 2
• 3 4 1
• 4 5 5

The MST for that graph is (0 1 1, 0 3 3, 1 2 2, 3 4 1, 4 5 5). At least I think it is.

WHat made this harder for some people?

• Treating the MST as a Graph, which required dealing with some of the subtleties of the Graph implementation. The MST is just a list of edges.
• Using extant fields in the Graph class. Those were already serving a purpose. Reusing them for other purposes is dangerous.