Software Design 2e - DIM

149Black box notationsFigure 7.17 An example of an incorrect Structure Diagram. (a) The sequence and iterationforms are wrongly mixed on a single level. (b) How this can be resolved by adding a further levelof abstraction in the description.behavioural viewpoints, when they are used to describe the actions of a program or anentity. In particular, they provide a much more abstract form of sequencing descriptionthan can normally be provided through pseudocode (see Section 7.3.4).Figure 7.18 shows an example of a data structure described in this manner. In thiscase, the data structure happens to be a physical one, as the diagram is used to describethe structure of a textbook such as this one. A more conventional programming datastructure is shown in Figure 7.19, where a simple file of text is described using thisform.In Figure 7.20 an example of this form is used to describe the dynamic form of aprogram. In this example, the task of the program is to print out a page of a monthlybank statement. Clearly this is closely related to the appearance of the statement on thepage.Uses for the Structure DiagramAs already indicated, this form is ubiquitous. In examining the JSP design method(described in Chapter 14), it will be used to describe the structures of the input andoutput data streams. In both the JSD (Chapter 15) and SSADM (Longworth, 1992)design methods it is used to describe the ‘evolution’ of entities over time. The title ofthe form might vary, but the basic notation does not.Since this form will occur so often later in the book, these roles will not be outlinedany further at this point, and fuller descriptions will be left to the appropriatechapters.