“Systematic program design” refers to a mix of two concepts: design recipes and iterative refinement.
They apply to both complete programs and individual functions.
The basic steps of a function design recipe:
- From Problem Analysis to Data Definitions
Identify the information that must be represented and how it is represented in the chosen programming language. Formulate data definitions and illustrate them with examples. - Signature, Purpose Statement, Header
State what kind of data the desired function consumes and produces. Formulate a concise answer to the question what the function computes. Define a stub that lives up to the signature. - Functional Examples
Work through examples that illustrate the function’s purpose. - Function Template
Translate the data definitions into an outline of the function. - Function Definition
Fill in the gaps in the function template. Exploit the purpose statement and the examples. - Testing
Articulate the examples as tests and ensure that the function passes all. Doing so discovers mistakes. Tests also supplement examples in that they help others read and understand the definition when the need arises—and it will arise for any serious program.
Examples play a central role at almost every stage.
When programmers are given a task, they create a first design, turn it into code, evaluate it with actual users, and iteratively refine the design until the program’s behavior closely matches the desired product.
A typical “quick programming in X” book or course fails to teach principles that transfer to the next fashion language. Worse, the language itself often distracts from the acquisition of transferable skills, at the level of both expressing solutions and dealing with programming mistakes.
Learning to design programs also means acquiring two kinds of universally useful skills. Program design certainly teaches the same analytical skills as mathematics, especially (pre)algebra and geometry.
Program design teaches analytical reading and writing skills. Even the smallest design tasks are formulated as word problems. Without solid reading and comprehension skills, it is impossible to design programs that solve a reasonably complex problem. Conversely, program design methods force a creator to articulate his or her thoughts in proper and precise language.
The purpose of this book is to introduce readers without prior experience to the systematic design of programs. In tandem, it presents a symbolic view of computation, a method that explains how the application of a program to data works.
Independent readers ought to work through the entire book, from the first page to the last. We say “work” because we really mean that a reader ought to solve all exercises or at least know how to solve them.