Add detail to their undertsanding of the update-world function
Identify possible sub-domains which require different behavior of the function
Students will use Cond in their update-world functions
Students will identify circumstances in which the functions should behave differently
Students will define these circumstances - and the desired behavior - in code, as different Cond branches
Materials and Equipment:Preparation:
Language Table Posted
Seating arrangements: ideally clusters of desks/tables
Write the Ninja World version of update-world towards the bottom of the board, with room to transform it into a cond branch under the function header.
Introduction(Time 5 minutes)
Students should have Ninja World (with dogX, rubyX and catY defined) open on the computer.
Let’s return to our Ninja World...it’s slowly turning into a finished game!
- Look at the code for update-world. Raise your hand if you can tell me - in English - what this function does.
Take a volunteer or two.
What is dogX when the dog is in the center of the screen? 320. According to the code, what will the next dogX be?
What is dogX when the dog is on the right-hand edge? 640. What will the next dogX be? And the next? And the next? It disappears and never comes back!
Protecting the Boundaries(Time 20 minutes)
Just like in Bootstrap 1, we need to write a function that checks whether the dog has gone off the right side of the screen.
Turn to Page 33. Let’s work through the first function together. Write a function called off-right?, which returns true if a coordinate is greater than...what number? 640.
We’re going to go through the design recipe, just like always: what’s the first step? Contract and purpose statement. What’s the name of the function? Domain? Range? Purpose statement?
Alright, let’s pick a few example coordinates for our examples: What is an x coordinate that would put our dog on the center of the screen? How do we check whether it’s off the right hand side? We said that anything greater than 640 is off the right side of the screen.
Excellent! Take two seconds and write another example for a coordinate that is off the screen on the right, circle what changes, and write your definition.
Pencils down. Excellent.We’ve written a function to check whether an object has run off the right side of the screen! But do we care whether the ruby goes off the right side? It’s moving to the left! Take two minutes to finish the next design recipe page for off-left?
Now we have a way to check whether something has gone off the right or the left of the screen, but we still haven’t told our game what to do when it does. Think back to our dog, in Ninja World. Suppose we’d like it to reappear on the left-hand side. In that situation, what would the next dogX be after 640? ZERO.
We want to change update-world, so that it behaves the old way most of the time...but it sets dogX to zero when dogX is greater than 640. What can we use that makes a function behaves one way for some inputs, but another way for different inputs? Our old friend: cond!
We know that we’ll have two conditions right now: when dogX is greater than 640, and then the rest of the time. Let’s work on the code for this:
(define (update-world w) (cond [...test... ...result...] [else (make-world (+ (world-dogX w) 10) (world-rubyX w) (world-catY w))]))
- We still want our original code to be there - it’s now going to be used in the else clause, because when dogX is not off the right side of the screen we want the world to update normally.
Walk the students through the transformation.
Now let’s look at the first condition. What is the test that tells us if a number is greater than 640?
But how do we check if the dog is off the right? We’ll need to pull the dog out of the world! What function do we use for that? world-dogX. So what will our input to off-right? be? (world-dogX w)
Let’s add this to the definition.
(define (update-world w) (cond [(off-right? (world-dogX w)) ...result...] [else (make-world (+ (world-dogX w) 10) (world-rubyX w) (world-catY w))]))
What should we have for our result?
Well, we know that we need to return a World, since the range of the function is World. That means we can immediately write (make-world...):
(define (update-world w) (cond [(off-right? (world-dogX w)) (make-world ...dogX... ...rubyX... ...catY...)] [else (make-world (+ (world-dogX w) 10) (world-rubyX w) (world-catY w))]))
How should dogX change in this condition? We want it to be zero! Since we want to move the dog back to the left side of the screen, we’ll just replace it with zero!
Does rubyX change? No. Does catY change? No.
(define (update-world w) (cond [(off-right? (world-dogX w)) (make-world 0 (world-rubyX w) (world-catY w))] [else (make-world (+ (world-dogX w) 10) (world-rubyX w) (world-catY w))]))
Now let’s think about the ruby. Instead of checking if rubyX was off the right side of the screen, what would I be checking?
How do I need to start changing update-world? By making a new cond branch! Can you walk me through it?
(define (update-world w) (cond [(off-right? (world-dogX w)) (make-world 0 (world-rubyX w) (world-catY w))] [(off-left? (world-rubyX w)) (make-world (world-dogX w) 640 (world-catY w))] [else (make-world (+ (world-dogX w) 10) (world-rubyX w) (world-catY w))]))
Tests and Results(Time 45 minutes)
Now open your own game file.
The first thing to do is to reformat your update-world function so that it uses cond, with your current code inside the else clause. You have 5 minutes - GO!
Next copy and paste your off-left? and off-right? functions from Ninja World into your game. Take one more minute - GO!
5, 4, 3, 2, 1...MONITORS OFF!
Think about the things in your game that fly offscreen. Do they fly off the left? The right? The top or bottom? Do you need to write off-top? or off-bottom?
On Page 34, make a list of the tests that you need to do in the left hand column, to decide whether each thing flies offscreen. For example, with the dog we said (off-right? (world-dogX w)). On the right, figure out which world you need to make, so that the thing you’re testing re-appears on screen once it’s flown off.
Work in small groups to complete the workbook page.
Branches in update-world(Time 15 minutes)
Look at our cond example on the board, for Ninja World. Notice that for each branch, we need a test and a result. This is exactly what you’ve written in your workbook. All you need to do now is surround each row of your table with square brackets, and type it into your game.
Adapt update-world so that that each thing re-appears on screen once it’s flown off.
Closing(Time 5 minutes)
Have the students show each other their games!
Bootstrap by Emmanuel Schanzer is licensed under a Creative Commons 3.0 Unported License. Based on a work at www.BootstrapWorld.org. Permissions beyond the scope of this license may be available at schanzer@BootstrapWorld.org.