Software Engineering for Students A Programming Approach

124 Chapter 10 ■ Data structure design
data structure, so all we have to do is to write “process” in every box of the data structure
diagram. We thereby obtain a program structure diagram. For our program this is
shown in Figure 10.5.
A program structure diagram like this is interpreted as follows:
■ the program as a whole (represented by the box at the top) consists of (lines leading
downwards) a sequence of operations (boxes alongside one another)
■ sometimes a program component is to be repeatedly executed (an “*” in the box).
The next step is to write down (in any order) a list of all the elementary operations
that the program will have to carry out. This is probably the least well-defined part of
the method – it does not tell us how to determine what these actions should be. For
the program we are working on they are:
1 display n asterisks
2 display blank line
3 display s spaces
4 increment s
5 decrement s
6 increment n
7 decrement n
8 initialize n and s
9 new line
Process
top half
Process
line
Process
picture
Process
middle
Process
bottom
half
* *
Process
line
Figure 10.5 Program structure diagram for the pattern program
For later reference, we number the operations, but the ordering is not significant.
Next each of these operations is placed in its appropriate position in the program
structure diagram. For example, operation 2 needs to be done once, for the middle
of the pattern. It is therefore associated with the box containing process middle.
Similarly, operation 1 is associated with the component process line (Figure 10.6).
This act of associating operations with positions is not automatic; instead, as indicated,
judgment has to be employed.