25. Jackson Structured Programming (JSP) Diagrams

Earlier in this mini-website, in the procedural languages section, we mentioned 'Top-down' design and 'stepwise-refinement' as excellent approaches to design and code software applications.

Jackson Structured Programming is a way of carrying out a top-down design by means of a few simple but powerful diagrams, each type of diagram has a specific action to convey..

These diagrams are

1. Operation - a rectangle and a text description

This represents the software doing something specific, such as 'Calculate' or 'Initialise'.

2. Sequence - a set of rectangles joined by lines.

In a sequence, read the diagram first from the top, then from left to right. This diagram shows that operation A consists of a sequence of operations - B, C and D are called in that order.

3. Iteration - repeating something over and over again until a condition is met

Iteration is represented by a star symbol in the top right corner of the operation box, as shown in the 'B' box.

In this diagram, 'A' calls B to complete its task. B repeats its local operation until the condition defined by the text next to it is met.

4. Selection - a choice has to be made

Selection is represented as a circle in the top right corner of the box.

Operation 'A' will call only one of the selection boxes, depending on some selection criteria.

Summary

JSP diagrams are of four types: Operation, Sequence, Iteration and Selection. The next page will show how these diagrams are used together to describe a software application.