Software Design 2e - DIM

270Structured systems analysis and systems designFigure 13.9The operations of the Transform Analysis step.While this is by no means all that is involved in the Transform Analysis step, sincea lot of work remains to be done to produce a proper Structure Chart that describesthe form of the eventual system, it is sufficient for our immediate purposes, since itexplains the essential nature of the transformation it contains. We now need to considerthe implications of this for the design process.A major distinction between the DFD and the Structure Chart is that the former isnon-sequential and describes the structure of the problem, while the latter describes asolution in terms of a hierarchy of program units. That apart, both describe a systemin terms of operations (which, of course, can conveniently be packaged into subprograms),and in terms of the flow (or transfer) of information around the system. Itis therefore reasonable to begin the transformation process by making a one-to-onemapping of bubbles to subprograms, even though this may later require refinement.In order to do this, it is necessary to be able to identify exactly where in the eventualsubprogram invocation hierarchy each operation will need to be positioned. Notsurprisingly, the operations most closely concerned with physical input/output aremapped onto the procedures at the base of the Structure Chart, while the most abstractoperations (as represented by the central transform and the bubbles immediatelyaround it) are mapped onto the top level of the chart.This explains why it is sometimes necessary to create a central transform in orderto be able to perform the transformation process. A system in which this extra stepproves to be necessary corresponds to a situation where the chief role of the main bodyof the final program is to organize the sequencing of the calls to the subprograms. Sincesuch a program main body performs no specific problem-oriented operations, it isunlikely to be identified as a bubble in creating the DFD. This is why it needs to beadded to the DFD in the form of an ‘empty bubble’, in order to ‘complete’ the DFD sothat the Transform Analysis operations can be performed in a consistent manner.Once the first-cut Structure Chart has been produced in this way, quite a lotof work remains to be done in revising its form into one that is appropriate for