Software Design for Engineers and Scientists

Modularization 279

module. If we were looking for a tag to describe modularization based

on the criterion of information hiding, we might say ‘object based’

simply because the structure that drops out is one that emphasizes

objects rather than actions.

Here is a modularization problem originally proposed by Parnas in his

discussion of information hiding. It is about the right size to justify

care in modularization, without being unwieldy for illustrative purposes.

The specification is as follows:

Input: Text file consisting of several lines of several words each.

Output: Text file consisting of all circular shifts of all lines of the

input, in alphabetical order.

Definition: A circular shift is done by removing the first word from a

line and appending it to the end of the line.

Example:

Input (All circular shifts) Output

the quick brown fox the quick brown fox brown fox the quick

jumps over quick brown fox the dog the lazy

the lazy dog brown fox the quick fox the quick brown

fox the quick brown jumps over

jumps over lazy dog the

over jumps over jumps

the lazy dog quick brown fox the

lazy dog the the lazy dog

dog the lazy the quick brown fox

(Note that giving an example clears some potential ambiguity: the

alphabetization goes on to subsequent words after the first.)

Modularization A is based on a top-down decomposition.

1. Input

Read lines into memory

Set up pointer to start of each line

2. Circular shift

Add pointer to start of every word

3. Alphabetizing

Sort pointers (Sort function looks at words being pointed to and

checks alpha order)

4. Output

Go through pointer list sequentially.

For any entry, print out until CR

Go back to previous CR then print out from there to pointer

position.

5. Master control

Sequencing, error messages, etc.

This decomposition into modules has implicitly defined the data

structures that will be used. It has also introduced design decisions

which have global effects. For example, the decision has been made to

12.4 A modularization

example