Software Engineering for Students A Programming Approach

10.5 Structure clashes 133
Let us review the situation so far. We drew the data structure diagrams, but then saw
the clash between the structures. We resolved the situation by identifying two separate
programs that together perform the required task. Next we examine the two file structures
and identify a component that is common to both. (In the example program this
is a word of the text.) This common element is the substance of the intermediate file
and is the key to dealing with a structure clash.
What do we do next? We have three options open to us.
First, we might decide that we can live with the situation – two programs with an
intermediate file. Perhaps the overhead of additional input-output operations on the
intermediate file is tolerable. (On the other hand, the effect on performance might be
unacceptable.)
The second option requires special operating system or programming language facilities.
For example, Unix provides the facility to construct software as collections of programs,
called filters, that pass data to and from each other as serial streams called pipes.
There is minimal performance penalty in doing this and the bonus is high modularity.
For the above problem, we write each of the two programs and then run them with
a pipe in between, using the Unix command:
breaker < InputFile | builder > OutputFile
or the DOS command:
InputFile | breaker | builder > OutputFile
in which the symbol | means that the output from the filter (program) breaker is used
as input to the program (filter) builder.
The third and final option is to take the two programs and convert them back into
a single program, eliminating the intermediate file. To do this, we take either one and
transform it into a subroutine of the other. This process is known as inversion. We will
not pursue this interesting technique within this book.
On the face of it, structure clashes and program inversion seem to be very complicated,
so why bother? Arguably structure clashes are not an invention of the data structure
design method, but a characteristic inherent in certain problems. Whichever
method that was used to design this program, the same essential characteristic of the
problem has to be overcome. The method has therefore enabled us to gain a fundamental
insight into problem solving.
In summary, the data structure design method accommodates structure clashes like
this. Try to identify an element of data that is common to both the input file and the
output file. In the example problem it is a word of text. Split the required program into
two programs – one that converts the input file into an intermediate file that consists
of the common data items (words in our example) and a second that converts the intermediate
file into the required output. Now each of the two programs can be designed
according to the normal data structure design method, since there is no structure clash