Unit 3Structures and Worlds

Agenda

• 10 minReview: The Party Struct

• 80 minYour Bakery

• 5 minClosing

Product Outcomes:

• Students define a new data structure: a party

• Students will write functions that access fields of a Cake, and produce new Cakes

Standards and Evidence Statements:

Standards with prefix BS are specific to Bootstrap; others are from the Common Core. Mouse over each standard to see its corresponding evidence statements. Our Standards Document shows which units cover each standard.

Length: 95 minutes

Glossary:

• contract: a statement of the name, domain, and range of a function

• domain: the type of data that a function expects

• dot-accessors: A way to extract values from a data structure

• name: how we refer to a function or value defined in a language (examples: +, *, star, circle)

• purpose statement: a brief description of what a function does

• range: the type of data that a function produces

• variable: something that changes

Materials:

• The Party Planner file preloaded on students’ machines

• Pens/pencils for the students, fresh whiteboard markers for teachers

• Class poster (List of rules, design recipe, course calendar)

• Editing environment (Pyret Editor)

• Language Table

• The Bakery file used in the previous unit

Preparation:

• Seating arrangements: ideally clusters of desks/tables

Types	Functions
Number	+ - * / num-sqr num-sqrt num-expt
String	string-append string-length
Image	rectangle circle triangle ellipse star text scale rotate put-image
Cake	cake .flavor .color .message .layers .is-iceCream

Review: The Party Struct

Overview

Learning Objectives

• Write complex functions that consume, modify and produce structures

• Deepen their understanding of structures, constructors and accessors by being introduced to a new data structure.

Evidence Statements

Product Outcomes

• Students define a new data structure: a party

Materials

• The Party Planner file preloaded on students’ machines

• Pens/pencils for the students, fresh whiteboard markers for teachers

• Class poster (List of rules, design recipe, course calendar)

• Editing environment (Pyret Editor)

• Language Table

Preparation

• Seating arrangements: ideally clusters of desks/tables

Review: The Party Struct (Time 10 minutes)

• In the last lesson you learned about a new kind of data structure, called a Cake. However, a Cake isn’t the only kind of structure - we can create any kind we want! Let’s define another one. In this exercise, you’ll be a party planner. Data structures will be a useful way to represent each Party that you’re planning, keeping track of its location, theme, and number of guests.

  • What datatype could be used to represent the location of a Party?

  • What about the Party’s theme? (This could be something like "50s" or "laser tag".)

  • How about the number of guests?

  Fill out the Party structure definition on Page 13 in your workbook.

   # a Party is a location, theme, and number of guests data Party: | party(location :: String, theme :: String, guests :: Number) end Once the Party structure is defined, you have access to new pieces of code: a function to make a Party, and three dot-accessors to get the location, theme, and number of guests out of the Party.

  • What is the Name of the function that creates a Party?

  • What is the function’s Domain? (What kinds of things are part of a Party?)

  • What is the Range of this function?

  • On Page 13, use the constructor function to define two new Parties of your own.

  • Right below your new Parties, list how you would access the fields of party2. How would you get the location out of party2? (Think about how you got the model or color out of a Car.)

• • Open the Party Planner file. Take a look at the first four lines in the definitions area. Do they match what you have written in your workbook for the data definition of Party?

  • Now define two new Party structures of your own. No matter what party you’re planning, make sure that your party has the right inputs in the right order.

  As with the Cake structure, repetition is key: have students identify the fields of each of their parties, and ask them lots of questions: How would you get the theme out of Halloween? How would you get the number of guests out of JulyFourth?

Your Bakery

Overview

Learning Objectives

• Students will write complex functions that consume, modify and produce structures

Evidence Statements

Product Outcomes

• Students will write functions that access fields of a Cake, and produce new Cakes

Materials

• The Bakery file used in the previous unit

Preparation

Your Bakery (Time 80 minutes)

• In the last lesson you learned about a new data structure, called a Cake.

  • What is a Cake? What information is part of a Cake structure?

  • What function creates a Cake?

  • How do you get the flavor out of a Cake? The message? The color?

  As part of running a bakery, we’ll take orders for Cakes and bake them to the customers specifications. Let’s figure out how to write functions to help us do that.

• Turn to Page 14 in your workbooks. Write a function called change-flavor, which takes in a Cake and a flavor, and returns a new Cake that is mostly the same as the original, but is now the given flavor.

  • What is the domain for this function? We’ll need to know which Cake we’re taking in AND what flavor it should be.

  • What do you think our bakery is going to give back? What would be the range of change-flavor?

   # change-flavor: Cake String -> Cake: # consumes a Cake and produces a Cake that is the same as the original, but has the given flavor. In your first example, use the original cake1 and make it strawberry. We know our customer will expect to get a Cake back: you wouldn’t return a Cake made incorrectly to a bakery and be OK with only getting a frosting message back! But we won’t be returning the same Cake with our function - we’re making a new Cake, which must have the same message, color, layers, etc. as the given Cake, but one thing needs to be different: the flavor!

  It might not be immediately obvious to students that when a function returns a Cake, they must use the cake constructor function to produce it. This way the function does not return the same value it was given, but instead creates a Cake with the same specs as the given Cake, where only the flavor is different. By starting with a "fresh" Cake, students are forced to think about every single field in order. Thinking about what exactly makes up a Cake and going back to the contract for cake gives them lots of practice with the Cake struct and dot-accessors.

• The moment you write Cake in the function’s Range, you know that you’ll need to call the cake constructor and give it five things: the flavor, color, message, layers, and is-iceCream of that cake.  examples: change-flavor(cake1, "strawberry") is cake(....) end

  We already know what flavor this cake should be: strawberry! So we can start by giving our cake function the given String as its first input (since the first input to the cake function must be its flavor).

   examples: change-flavor(cake1, "strawberry") is cake("strawberry", ...) end

  • We also know what color this Cake should be: the same as the given Cake! So how could you access JUST the color of cake1 and use it in the cake function? Use the dot-accessor! The message of the cake doesn’t change, either. So how do you get the message out of cake1?

  • The number of layers shouldn’t change if you change the flavor. How do you get the layers out of cake1?

  Don’t forget the last field in the Cake struct: is-iceCream! The purpose statement for change-flavor doesn’t say anything about the inside make-up of the Cake changing, so how do you get the original is-iceCream out of cake1?

   examples: change-flavor(cake1, "strawberry") is cake("strawberry", cake1.color, cake1.message, cake1.layers, cake1.is-iceCream) end

  Remind students that the arguments to each function in Pyret must be separated by commas. This wasn’t necessary in Racket, but they will receive error messages if they don’t use commas correctly in Pyret code.

• • Write one more example for the function change-flavor, this time using it to make cake2 a vanilla cake.

  • Next, circle and label what changes between the two examples. How many variables will this function need? Then write the definition, using your examples to help you.

  After replacing the changing things with variables, your definition should look similar to:  fun change-flavor(a-cake, new-flavor): cake(new-flavor, a-cake.color, a-cake.message, a-cake.layers, a-cake.is-iceCream) end

  Students may be tempted to put new-flavor in quotes, because the flavor of the Cake must be a string. However, the domain of change-flavor tells us that the function will take in a Cake and a String, so whatever flavor is input will already have quotes around it. Values evaluate to themselves, so the string "new-flavor" cannot evaluate to anything other than "new-flavor". If we want new-flavor to be a variable, or shortcut for "strawberry", "vanilla", "chocolate", etc. it must be written WITHOUT quotation marks.

• Turn to Page 15 in your workbooks. Your bakery needs to know if certain Cakes need to be refrigerated. If the temperature is greater than 32 degrees AND the given cake is an ice cream cake, the function should return true.

  • Fill out the Contract and Purpose Statement for the function.

  • Write two examples for how one would use will-melt.

  • Circle and label what varies between those examples and label it with a variable name.

  • Define the function.

  Give students plenty of time to practice using dot-accessors, extracting pieces of the Cake structures and modifying them.

Closing

Overview

Learning Objectives

Evidence Statements

Product Outcomes

Materials

Preparation

Closing (Time 5 minutes)

• So far you’ve worked with three data structures: Coords, Parties, and Cakes. In the next lesson, you’ll look at a new struct as you dissect a video game, and start thinking about the struct(s) you’ll need to create for your own game!

  Have students volunteer what they learned in this lesson

Bootstrap by Emmanuel Schanzer is licensed under a Creative Commons 4.0 Unported License. Based on a work at www.BootstrapWorld.org. Permissions beyond the scope of this license may be available at schanzer@BootstrapWorld.org.