I am hacker man! I read HTDP!

I know, not UNIX, but it’s what I have in front of me. Let’s install Racket and iracket for use with Jupyter.

- Use the Windows installer
- Add
`C:\Program Files\Racket`

to the system path - Run
`raco pkg install iracket`

I’ve got a physical copy of **HTDP 2001**. A copy of this book is accessible online along with the answer key.

Chapter summaries and important notes from my own learning.

Some programs are as small as essays. Others are like sets of encyclopedias. Writing good essays and books requires careful planning, and writing good programs does, too. Small or large, a good program cannot be created by tinkering around. It must be carefully designed.

- 2.1 Numbers and Arithmetic
- 2.2 Variables and Programs
- 2.3 Word Problems
- 2.4 Errors
- 2.5 Designing Programs

Scheme uses parentheses `( )`

or round brackets to organize programs. A set of parentheses encloses an **expression**.

*Parenthesized expressions.*

```
(operator input... )
```

Expressions can take in a number of variables:

```
(define (area-of-disk r)
(* 3.14 r r))
(define (area-of-ring outer-r inner-r)
(- (area-of-disk outer-r) (area-of-disk inner-r)))
```

If it’s not in parentheses it’s an *atom.*

```
; Exercise 2.2.1. Define the program Fahrenheit->Celsius,
; which consumes a temperature measured in Fahrenheit and
; produces the Celsius equivalent. Use a chemistry or
; physics book to look up the conversion formula.
(define (fahrenheit->celsius x)
(* (- x 32) (/ 5 9)))
(convert-gui fahrenheit->celsius)
```

Scheme allows you to represent fractions directly as well:

```
(define (fahrenheit->celsius x)
(* (- x 32) 5/9 ))
```

…though I’m not a huge fan of that.

Inexact numbers are represented with an `#i`

in front.

Here’s a word problem and my solution:

Company XYZ & Co. pays all its employees $12 per hour. A typical employee works between 20 and 65 hours per week. Develop a program that determines the wage of an employee from the number of hours of work.

```
(define (wage h)
(* 12 h))
(define (tax gross-pay)
(* 0.15 gross-pay))
(define (subtract-taxes net-pay)
(- net-pay (tax net-pay)))
(define (net-pay h)
(subtract-taxes (wage h)))
```

My solution is slightly more efficient than the solution as it does not recompute the gross pay by including the `subtract-taxes`

function.

**Designing programs** is something that must be done intentionally and with care. SICP provides a **Design Recipe** which can be followed to yield reliably useful programs.

We need to determine what’s relevant in the problem statement and what we can ignore. We need to understand what the program consumes, what it produces, and how it relates inputs to outputs. We must know, or find out, whether Scheme provides certain basic operations for the data that our program is to process. If not, we might have to develop auxiliary programs that implement these operations. Finally, once we have a program, we must check whether it actually performs the intended computation.

Here is Figure 3, the example recipe:

```
;; Contract: area-of-ring : number number -> number
;; Purpose: to compute the area of a ring whose radius is
;; outer and whose hole has a radius of inner
;; Example: (area-of-ring 5 3) should produce 50.24
;; Definition: [refines the header]
(define (area-of-ring outer inner)
(- (area-of-disk outer)
(area-of-disk inner)))
;; Tests:
(area-of-ring 5 3)
;; expected value
50.24
```

A summary of these phases is provided in Figure 4 in this section of the textbook, but I’ll summarize here quickly:

- The
**Contract**names the function and specifies the class of the input and output data - The
**Purpose**explains the name - The
**Example**lists a sample use case - The
**Definition**is a scheme expression - Finally,
**Tests**ensure output is correct

(Funny, Elixir has real good support for inline tests with the `doctest`

feature. Makes me miss Elixir.)

- 3.1 Composing Functions
- 3.2 Variable Definitions
- 3.3 Finger Exercises on Composing Functions

- 4.1 Booleans and Relations
- 4.2 Functions that Test Conditions
- 4.3 Conditionals and Conditional Functions
- 4.4 Designing Conditional Functions

- 5.1 Finger Exercises with Symbols

- 6.1 Structures
- 6.2 Extended Exercise: Drawing Simple Pictures
- 6.3 Structure Definitions
- 6.4 Data Definitions
- 6.5 Designing Functions for Compound Data
- 6.6 Extended Exercise: Moving Circles and Rectangles
- 6.7 Extended Exercise: Hangman

- 7.1 Mixing and Distinguishing Data
- 7.2 Designing Functions for Mixed Data
- 7.3 Composing Functions, Revisited
- 7.4 Extended Exercise: Moving Shapes
- 7.5 Input Errors

- 8.1 The Scheme Vocabulary
- 8.2 The Scheme Grammar
- 8.3 The Meaning of Scheme
- 8.4 Errors
- 8.5 Boolean Expressions
- 8.6 Variable Definitions
- 8.7 Structure Definitions

9.1 Lists 9.2 Data Definitions for Lists of Arbitrary Length 9.3 Processing Lists of Arbitrary Length 9.4 Designing Functions for Self-Referential Data Definitions 9.5 More on Processing Simple Lists

- 10.1 Functions that Produce Lists
- 10.2 Lists that Contain Structures
- 10.3 Extended Exercise: Moving Pictures

- 11.1 Defining Natural Numbers
- 11.2 Processing Natural Numbers of Arbitrary Size
- 11.3 Extended Exercise: Creating Lists, Testing Functions
- 11.4 Alternative Data Definitions for Natural Numbers
- 11.5 More on the Nature of Natural Numbers

- 12.1 Designing Complex Programs
- 12.2 Recursive Auxiliary Functions
- 12.3 Generalizing Problems, Generalizing Functions
- 12.4 Extended Exercise: Rearranging Words

- 14.1 Structures in Structures
- 14.2 Extended Exercise: Binary Search Trees
- 14.3 Lists in Lists
- 14.4 Extended Exercise: Evaluating Scheme

- 15.1 Lists of Structures, Lists in Structures
- 15.2 Designing Functions for Mutually Referential Definitions
- 15.3 Extended Exercise: More on Web Pages

- 16.1 Data Analysis
- 16.2 Defining Data Classes and Refining Them
- 16.3 Refining Functions and Programs

- 17.1 Processing Two Lists Simultaneously: Case 1
- 17.2 Processing Two Lists Simultaneously: Case 2
- 17.3 Processing Two Lists Simultaneously: Case 3
- 17.4 Function Simplification
- 17.5 Designing Functions that Consume Two Complex Inputs
- 17.6 Exercises on Processing Two Complex Inputs
- 17.7 Extended Exercise: Evaluating Scheme, Part 2
- 17.8 Equality and Testing

- 18.1 Organizing Programs with local
- 18.2 Lexical Scope and Block Structure

- 19.1 Similarities in Functions
- 19.2 Similarities in Data Definitions

- 20.1 Syntax and Semantics
- 20.2 Contracts for Abstract and Polymorphic Functions

- 21.1 Abstracting from Examples
- 21.2 Finger Exercises with Abstract List Functions
- 21.3 Abstraction and a Single Point of Control
- 21.4 Extended Exercise: Moving Pictures, Again
- 21.5 Note: Designing Abstractions from Templates

- 22.1 Functions that Produce Functions
- 22.2 Designing Abstractions with Functions-as-Values
- 22.3 A First Look at Graphical User Interfaces

- 23.1 Sequences and Series
- 23.2 Arithmetic Sequences and Series
- 23.3 Geometric Sequences and Series
- 23.4 The Area Under a Function
- 23.5 The Slope of a Function

- 25.1 Modeling a Ball on a Table
- 25.2 Sorting Quickly

- 26.1 Termination
- 26.2 Structural versus Generative Recursion
- 26.3 Making Choices

- 27.1 Fractals
- 27.2 From Files to Lines, from Lists to Lists of Lists
- 27.3 Binary Search
- 27.4 Newton’s Method
- 27.5 Extended Exercise: Gaussian Elimination

- 28.1 Traversing Graphs
- 28.2 Extended Exercise: Checking (on) Queens

- 29.1 Concrete Time, Abstract Time
- 29.2 The Definition of ``on the Order of''
- 29.3 A First Look at Vectors

- 30.1 A Problem with Structural Processing
- 30.2 A Problem with Generative Recursion

- 31.1 Recognizing the Need for an Accumulator
- 31.2 Accumulator-Style Functions
- 31.3 Transforming Functions into Accumulator-Style

- 32.1 Extended Exercise: Accumulators on Trees
- 32.2 Extended Exercise: Missionaries and Cannibals
- 32.3 Extended Exercise: Board Solitaire

- 33.1 Fixed-size Number Arithmetic
- 33.2 Overflow
- 33.3 Underflow
- 33.4 DrScheme’s Numbers

- 35.1 Simple Assignments at Work
- 35.2 Sequencing Expression Evaluations
- 35.3 Assignments and Functions
- 35.4 A First Useful Example

- 36.1 The Need for Memory
- 36.2 Memory and State Variables
- 36.3 Functions that Initialize Memory
- 36.4 Functions that Change Memory

- 37.1 Initializing State
- 37.2 State Changes from User Interactions
- 37.3 State Changes from Recursion
- 37.4 Finger Exercises on State Changes
- 37.5 Extended Exercise: Exploring Places

- 38.1 The Vocabulary of Advanced Scheme
- 38.2 The Grammar of Advanced Scheme
- 38.3 The Meaning of Advanced Scheme
- 38.4 Errors in Advanced Scheme

- 39.1 Abstracting with State Variables
- 39.2 Practice with Encapsulation

- 40.1 Structures from Functions
- 40.2 Mutable Functional Structures
- 40.3 Mutable Structures
- 40.4 Mutable Vectors
- 40.5 Changing Variables, Changing Structures

- 41.1 Why Mutate Structures
- 41.2 Structural Design Recipes and Mutation, Part 1
- 41.3 Structural Design Recipes and Mutation, Part 2
- 41.4 Extended Exercise: Moving Pictures, a Last Time

- 42.1 Extensional Equality
- 42.2 Intensional Equality

- 43.1 More Practice with Vectors
- 43.2 Collections of Structures with Cycles
- 43.3 Backtracking with State

**FIN.**

Title: Scheme

Word Count: 1592 words

Reading Time: 8 minutes

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