Life cycle, maintenance, stepwise refinement, state-transition diagrams and structure charts

Last time

Before we start: last time, in Program flowcharts, trace tables and dry runs, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Plan before you code

Professional programs are not written in one sitting at a keyboard. They go through stages: find out what is needed, design it, build it, test it, and keep it working. And in the design stage there are tools for drawing the plan: stepwise refinement, state-transition diagrams and structure charts. Each is a favourite in Paper 2.

The life cycle stages

Analysis: find out what the program must do, from the users, and write the requirements. Design: plan the algorithms, modules and data. Coding: write the program. Testing: run it with test data to find errors and check it meets the requirements. Maintenance: fix faults found after release, and adapt or improve the program as needs change.

Life cycle models

In the waterfall model, each stage is finished before the next starts. It is simple and well documented, but changes late on are costly. In the iterative model, a basic version is built and refined in repeated cycles, using feedback. It copes with changing requirements, but is harder to plan. Rapid application development builds prototypes quickly, and the user feedback shapes the next one.