Software Engineering for Students A Programming Approach

116 Chapter 9 ■ Data flow design
central and most important bubble is the check bubble. We take this to be the main,
most important component. Imagine that we can touch and move the objects within
the diagram. Suppose that the bubbles are joined by pieces of string. Grasp the central
component and lift it into the air. Let the other bubbles dangle beneath. Next change
the bubbles into rectangles. We now have a structure chart that looks like Figure 9.7.
Each box is a software component. The components communicate by calls from
higher components to lower components. The data that flowed along lines in the data
flow diagram is now passed as parameters to and from the various components.
The check component calls the convert component which in turn calls the read
data component to obtain data. Then it calls the display message component to
display the output on the screen.
As we have illustrated, the steps for converting a data flow diagram into a structure
chart are:
1. identify the most important bubble (transformation)
2. convert this into the top-level component in the structure chart
3. allow the other components to dangle beneath the top-level component, creating
a hierarchical tree
4. convert the bubbles into rectangles and the data flows into lines representing
method calls.
The patient monitoring example illustrates how to use the data flow design method.
But the example chosen is simple and we have deliberately avoided drawing attention
to complications. Data flow diagrams typically have tens of bubbles and can be quite
complex. Often there are several input and output data flows. In more complex diagrams,
it can be difficult to identify a single center of the diagram.
Notice that although the method leads us to a structure for a piece of software that
is expressed in terms of well-defined, independent components, we still have to design
the detail of each and every component; we have to carry out the low-level or detailed
design. This emphasizes that data flow design is a method for high-level or architectural
design.
convert
read
data
check
Figure 9.7 Structure chart for patient monitoring software
display
message