Software Development

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Cambridge International AS & A Level Computer Science (9618)

Software Development

The big idea: Good software is planned, built and checked in stages. The life cycle you pick, the way you break a problem into modules, and the way you test and maintain the result decide whether the program actually does what the client needs.

Summary — What This Chapter Covers

A program development life cycle gives the work a structure: analysis, design, coding, testing and maintenance.
Waterfall: stages in strict order, each signed off. Iterative: build and refine in repeated cycles. RAD: prototypes and heavy user involvement, fast delivery.
A structure chart decomposes a problem into modules and shows the parameters passed between them; you can derive pseudocode headers from it.
A state-transition diagram (or table) documents how a system moves between states in response to events.
Errors are syntax, logic or run-time. Testing methods include dry run, walkthrough, white-box, black-box, integration, alpha, beta, acceptance and stub testing.
A test plan lists tests with normal, abnormal, extreme and boundary data and the expected results.
Maintenance is corrective (fix faults), adaptive (meet new requirements or environments) or perfective (improve performance or usability).

1. Program Development Life Cycle

12.1

Five stages, three models

The purpose of a life cycle is to give a project a plan: each stage has a clear output, progress can be checked, and problems are found as early (and cheaply) as possible.

Stage	What happens	Typical output
Analysis	find out what the client needs; study the current system; identify the problem and requirements	requirements specification
Design	plan the solution: data structures, algorithms, modules, interface, test plan	structure charts, pseudocode, flowcharts, test plan
Coding	write the program in a programming language, module by module	program code
Testing	run the program with planned data to find and remove faults	test results, corrected code
Maintenance	keep the system working and useful after delivery	updated versions

Model	Principle	Benefits	Drawbacks
Waterfall	each stage is completed and signed off before the next starts; little going back	simple to manage; clear milestones and documentation; suits projects with fixed, well-understood requirements	the client sees a working program only at the end; changing requirements late is expensive; errors in analysis are found late
Iterative	a simple version is built, then repeatedly improved; each iteration goes through design, coding and testing	working software early; feedback after each iteration; risk found early; easier to absorb changes	needs more client involvement; total cost and end date are harder to predict; can drift without clear scope
Rapid application development (RAD)	prototypes built quickly (often with reusable components) and refined with users; modules developed in parallel to a strict time limit	very fast delivery; user feedback throughout so the result fits needs; changes are easy	needs committed users and skilled developers; less suited to large or safety-critical systems; documentation can be weak; design may be poor if speed wins

Practice Question

A school wants a timetable app. The head teacher is not sure what features are needed and wants to try early versions with staff. Recommend a life cycle and justify it.

Memorise

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Structure charts and state-transition diagrams
Errors, test methods, test data, maintenance

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Software Development

Software Development

Five stages, three models

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