Software Engineering for Students A Programming Approach

130 Chapter 10 ■ Data structure design
10.5 ● Structure clashes
In a minority of problems, the two or more data structures involved cannot be
mapped onto a single program structure. The method terms this a structure clash.
It happens if we try to use the method to design a program to solve the following
problem.
Design a program that inputs records consisting of 80 character lines of words and
spaces. The output is to be lines of 47 characters, with just one space between words.
This problem looks innocuous enough, but it is more complex than it looks. (Have
a go if you don’t agree!) A problem arises in trying to fit words from the input file neatly
into lines in the output file. Figures 10.11 and 10.12 show the data structure diagrams
for the input and output files. Superficially they look the same, but a line in the
input file does not correspond to a line in the output file. The two structures are fundamentally
irreconcilable and we cannot derive a single program structure. This situation
is called a structure clash.
Although it is difficult to derive a single program structure from the data structure
diagrams, we can instead visualize two programs:
■ program 1, the breaker, that reads the input file, recognizes words and produces
a file that consists just of words.
■ program 2, the builder, that takes the file of words created by program 1 and
builds it into lines of the required width.
We now have two programs together with a file that acts as an intermediary between
the programs.
Input
file
Figure 10.11 Data structure diagram for input file
*
Line
Output
file
*
Line
Figure 10.12 Data structure diagram for output file