# Lab 9 [Regular Sections] Solution

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## Description

Goals:

• To practice writing data definitions for trees, writing functions that consumes trees, and writing functions that produce trees.
• To practice writing data definitions and functions for mutually recursive structures

Simple Binary Trees

A Corgi is a high-class breed of dog known for its short legs, high enthusiasm, and adorable face. Some people care very strongly that they know the genetics of their Corgi, even though all dogs are good dogs. Regardless of whether you care about such things, you have been hired by a wealthy English Corgi enthusiast to write a Corgi Tracking System.

A Corgi has a name (a string), a height (a number), a mom, and a dad. Both the mom and dad of a Corgi are Corgi Family Trees.

A Corgi Family Tree is either a Corgi, the value #false (which indicates that we don’t know anything about their parent), or a string indicating the name of the Corgi (which indicates that we don’t know their height or parentage, just their name).

Problem 1A. Create a data definition for a Corgi Family Tree and a Corgi. Note that this includes the template and several examples. Model the following Corgi Family Tree as one of your examples. Note the question marks indicate a Corgi without any information (the value #false).

Problem 1B. Write a function count-corgis that consumes a Corgi Family Tree and produces the number of dogs in the tree. If a Corgi’s parent is “?”, then they are not counted towards the total (but do count parents where we only know their name).

Problem 1C. Write a function count-moms that consumes a Corgi Family Tree and produces the number of Corgi moms in the tree. Again, do not count question marks, but do count dogs with just names. In the tree above, there are 4 corgi moms.

Problem 1D. Write a function has-corgi? that consumes a Corgi Family Tree and a name, then produces whether or not the given Corgi is in the tree.

Problem 1E. Write a function find-corgis-by-height that consumes a Corgi Family Tree and a threshold height, then produces a list of all the Corgi’s names that are BELOW the given height. The order of the names is up to you. Hint: You may find it helpful to use the built-in append function, which joins two lists together (as opposed to cons, which adds an element to a list).

Calder’s Binary Mobiles

• We’ve created a data structure that refers to itself twice. But now we’ll create a data structure that has multiple parts, some of which refer to each other. This will be a Binary Mobile, like the image below.

A binary mobile consists of two branches, a left branch and a right branch. Each branch is a rod of a certain length, from which hangs an attachment, either a weight or another binary mobile.

A branch is constructed from a rod length (which must be a number) together with one attachment hanging from the end of the rod.

An attachment may be either a Number (representing a simple weight, which is a number) or another binary mobile.

Here is a more abstract representation of this idea.

Problem 2A. Write the data definition [including an example and template] for a binary mobile. Note that there are three data definitions involved here (BinaryMobile, Branch, Attachment). Thus, there need to be at least three kinds of examples, and three templates.

Problem 2B. Define a procedure total-weight that returns the total weight of a binary mobile. For this problem, assume the rod weight is negligible [If you are a real sculptor, rod weight would be

proportional to the length, of course]. Recall that any function on a BinaryMobile involves three functions (on a BinaryMobile, a Branch, and an Attachment, since they are mutually referring).

Problem 2C. Design a predicate balanced? that tests whether a binary mobile is balanced. A binary mobile is said to be balanced? if the torque applied by its left branch is equal to the torque applied by its right branch and if each of the attachments hanging off its branches are balanced?.

Note: torque = length x weight. That is, torque is the length of a rod multiplied by the weight of the attachment hanging from that rod. Torque is only well-defined on a Branch.

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