AP Computer Science Principles — 1.3 Program Design and Development

Overview

In AP Computer Science Principles (AP CSP), Program Design and Development is the process of creating, testing, and improving computer programs that solve real problems. It focuses on planning before coding, collaboration, iteration, and documentation — the same habits used by real software developers.

1. Planning and Design

Before writing any code, programmers should:

• Define the problem — What should the program do? Who is it for?
• Plan a solution — Break the problem into smaller parts. (This is called decomposition.)
• Use algorithms — Design clear step-by-step instructions for how each part should work.
• Choose data — Decide how you’ll store and update information (variables, lists, etc.).

2. Incremental and Iterative Development

Programs are not usually built all at once. Instead, you:

• Start small with a basic working version (prototype).
• Test early and often so you catch mistakes sooner.
• Add features gradually (this is called incremental development).
• Improve and refine in cycles (this is called iterative development).

This “build → test → improve” loop keeps projects organized and prevents big last-minute problems.

3. Collaboration and Version Control

Many AP CSP projects are team-based. Strong collaboration looks like:

• Splitting roles — for example, one person designs the interface, another writes the logic.
• Communicating often so everyone knows what changed.
• Tracking versions — keeping notes or using tools to record who changed what and why.

This matches how professional software teams work, where multiple programmers contribute to the same project.

4. Testing and Debugging

Testing checks if the program behaves the way it’s supposed to. Common strategies:

• Unit testing — Test one specific part or function at a time.
• Boundary testing — Try edge cases and weird inputs to see if it breaks.
• Debugging — When something goes wrong, you observe, reason, and fix the code.

Debugging builds persistence, attention to detail, and problem-solving — all core skills in AP CSP.

5. Documentation and Reflection

Good programmers don’t just write code. They also explain it.

• Comments in the code describe what each major part does.
• Documentation explains how the program works overall, how to use it, and any limitations.
• Reflection connects the program back to its purpose: Did it solve the problem? What was hard? What would you improve next?

Reflection is also important for the AP Create Performance Task: you must explain the purpose of your program and your design decisions.