CSC 207.01 2019S, Class 29: Trees and tree traversal

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• Quiz
• Tree traversal
• Lab

Preliminaries

News / Etc.

• Please make sure that you’ve installed the Google Style Sheet in Eclipse.
• If you haven’t yet submitted your epilogue for the makeup, please do so ASAP.

Upcoming work

• Readings for Wednesday
  • Osera 10.4
  • Osera 11.4
• Assignment 7 due Thursday the 18th
• No lab writeup for today.

Extra credit

Extra credit (Academic/Artistic)

• The Magic Flute, Thursday, April 18, 7:00 p.m. Sebring-Lewis
• New: Student research extravaganza (any one event)

Extra credit (Peer)

• Track and Field home on Saturday. (30 min suffice; your peer runs around noon.)

Extra credit (Wellness)

• Today: Monday, April 15, 7:30 p.m., Harris Cinema: From the Munchies to Memory Effects: What the Science Says About Cannabis/Marijuana

Extra credit (Wellness, Regular)

• Today: 30 Minutes of Mindfulness at SHACS/SHAW every Monday 4:15-4:45
• Any organized exercise. (See previous eboards for a list.)
• 60 minutes of some solitary self-care activities that are unrelated to academics or work. Your email reflection must explain how the activity contributed to your wellness.
• 60 minutes of some shared self-care activity with friends. Your email reflection must explain how the activity contributed to your wellness.

Extra credit (Misc)

• Tonight: Prairie Burn, Grimes Farm, tonight at 7:30ish. See Facebook events and random email.
• Participate in Kinetic Sculpture Competition: Saturday the 27th
  • https://bit.ly/kineticsculpture19
  • You’ll need to build your sculpture in advance
  • You get reimbursed for up to $200 in supplies, but must present to be reimbursed.
• Public speaking workshop - April 22 at 7pm in HSSC S3325, with Kathy Clemons=Beasley ‘05. “Kathy is the Global head of Leadership and Manager Development for Blackrock and has been the speaker coach for TEDxGC.”
• Clothing donation boxes in lounges. Donate!

Other good things

Questions

I hear that there are some questions about removal. Please ask them.

How do we tell what to remove?

Semantics: Remove what was returned by the most recent call to previous or next.

We must therefore have a field that stores the information about which direction we just moved.

Option 1: int dir; // Use pos for forward and neg for backwards.

Option 2: Keep a pointer to the thing we will remove or set when we call remove or set.

In (all) options, we will still need to update the state of the iterator. If we remove the previous element (because we just called next), we’ll need to decrement the position in the list.

When we remove an element in the array-based implementation, what should we do?

We’ve chosen to shift everything left.

One could try putting a null there, but that complicates the implementation.

Do we need a pos field in a linked-list iterator?

Since list iterators have prevIndex and nextIndex methods, it’s helpful to have one.

You can’t call remove twice in a row, can you?

Nope. You must always have an immediately preceding call to next or prev. (As long as we don’t move the iterator or change the list, we’re fine.)

You’ve just seen a value; your decision to remove or set is supposed to be based on that value. Historical memory is complicated, so it’s just the immediate predecessor.

What does pos represent?

That’s our choice. We tend to have it be the index that we will return with nextIndex. We could have it have a half-value and round in each direction.

What does previousIndex return at the start of the list?

The documentation says -1.

What does nextIndex return at the end of the list?

The documentation says “the list size”.

Quiz

Sam needs an editor (no, not vi). The first line of problem 1 says “each of the two primary list iterations”; it means “each of the two primary list *implementations*.

Tree traversal

Trees are like 2D lists (kinda); each node has multiple “next” elements, but only one preceding element. In binary trees, there are up to two next elements, usually designated as “left” and “right”.

How many different systematic orders can we use to traverse a tree?

Broad decisions

• Left-to-right or right-to-left: In which order do we visit children.
• Ordering
  • Preorder: Node before children
  • Inorder: Node between children
  • Postorder: Node after childen
• Depth first vs breadth first
  • Do we go deep into the tree or do we go across each level

Detailed list

• Left-to-right, depth-first, preorder
• Right-to-left, depth-first, preorder
• Left-to-right, depth-first, inorder
• Right-to-left, depth-first, inorder
• Left-to-right, depth-first, postorder
• Right-to-left, depth-first, postorder
• Left-to-right, breadth-first, preorder (top-down)
• Right-to-left, breadth-first, preorder (top-down)
• Left-to-right, breadth-first, postorder (bottom-up)
• Right-to-left, breadth-first, postorder (bottom-up)

Suppose you wanted to build an Iterator for a tree. What would it look like? (You can choose the traversal order.)

• That is, what do you need to keep track of, and how can you use that while iterating?
• “We’ll need to put the rest of the tree somewhere. Maybe a list.”
• Explore the question in lab.

Lab

Don’t spend too much time on exercise 3.

No lab writeup.