Unit 5 Lab 6 Teacher Guide

Page 1: Review Your Tic-Tac-Toe Project
- Learning Goal:
  - Review the previous Tic-Tac-Toe project and the functionality of critical procedures.
- Tips:
  - You may find it helpful to have students write out their description of these six blocks and how they work.
  - Students must understand the idea of a triple in this program. It's not any arbitrary set of three squares; it's a set of three squares that count as a win: a row, a column, or a diagonal.
  - The last problem is at the heart of the actual program; consider having a whole-class discussion about it. Board is the user interface's representation of who has moved where on the board, a straightforward list of nine squares, each of which can be X, O, or the square number to indicate empty. Status of all winning triples represents the same board status, so in a sense it's redundant, but it presents that information in a form that's structured for the decision of the computer's move. One could say that board is for the conversation between the user and the program, while status of all winning triples is for the program thinking about its strategy.
Page 2: Creating a Computer Player
- Learning Goal:
  - Use the broadcast command, when I receive event, and message reporter to implement a computer player.
- Tips:
  - This page is a turning point for students. They have now written a program that has goal-directed behavior, trying to win at Tic-Tac-Toe. Don't let them be too caught up in coding details to appreciate what they've done!
Page 3: Developing a Gameplay Strategy
- Learning Goal:
  - Make the shift from the student's intuitive, informal tic-tac-toe strategy to an explicit, formal, algorithmic set of rules.
- Tips:
  - On this page, students explore their tic-tac-toe strategy using the program they have built, but this could also be done on paper.
  - On this page and the next, students must alternate between two different points of view: (1) thinking about tic-tac-toe as a game, articulating the rules they use themselves when playing; (2) thinking about the problem of embodying those rules in a computer program. We try to make this explicit on the page, but be on the lookout for students who confuse these two levels of abstraction.
  - Creating the winning triple and winning square blocks are the most challenging parts of this page. Help students prepare by making sure they:
    - Debug everything that comes before developing these blocks
    - Think through the approach they will take (as suggested on the page just before they begin building winning triple)
    - Review the hint about winning triple and the page where they first learned about keep
    Encourage pairs who have done all of these things and are still stuck to talk with other pairs.
    You may also choose to have them each write out their approach to 'determining if a player can win on their next turn' individually after they discuss it and before they start coding.
Page 4: Making the Computer Play Strategically
- Learning Goals:
  - Practice using higher-order functions in an extended project.
- Tips:
  - The first Take It Further problem is about strategy; the second Take It Further problem is about programming.

If students have been told that a computer program is "a sequence of steps," it may take some intellectual effort to learn to think instead of a program as built out of functions. But the effort pays off in vastly reducing the work of writing the actual code. For example, students are asked to write winning square for, a function whose input is "X" or "O" and whose return value is a number from 1 to 9, or 0 if there is no winning square on this move. Some students will want to jump right into coding, and will have an impossibly long sequence of if board=(some list) report (some number). (There are 3⁹=19,683 possible board configurations, so it would be a very long procedure if they try to cover every one.) But finding a winning triple is just find first from the list of winning triples, where the predicate function is, e.g., "this triple has two O squares and no X squares." And then the actual winning square is find first looking for a number in the winning triple. The resulting program isn't quite a one-liner because it has to check for the case in which there is no winning triple, but it's hardly any code compared to the sequence of if statements.

Tic-Tac-Toe with a Computer Player

Pacing

Prepare

Lab Pages