Software Engineering for Students A Programming Approach

112 Chapter 9 ■ Data flow design
The output from the system is:
■ plates of spaghetti bolognese
We can draw a diagram (Figure 9.1) to describe what is to happen in the kitchen.
In essence, data flows into the system, is transformed by actions (functions) and data
then flows out of the system.
The diagram is called a data flow diagram. Each line with an arrow on it represents
a stream of data flowing through the system. In this example there are three – spaghetti,
meat and plates of spaghetti bolognese. Each bubble represents a transformation, an
activity or a process that converts an input flow into an output flow. In this example
there is only one transformation – prepare food. Note that the diagram shows data flows
(which are dynamic), and does not show files (which are static objects).
We can now explore the detail that lies within the single bubble. We redraw it as
Figure 9.2 so as to show more clearly the steps that are involved.
spaghetti
meat
prepare
food
Figure 9.1 Data flow diagram for making spaghetti bolognese
spaghetti
boil
meat
fry
boiled
spaghetti
fried
meat
serve
plates of
spaghetti
bolognese
Figure 9.2 More detailed data flow diagram for making spaghetti bolognese
plates of
spaghetti
bolognese