The Software Lifecycle: Part B (Lecture 3)
Edited from Dr. Mark Green's notes

Case Study 1 (An example)

• Most of this course will be taught through the use of examples, show by doing
• During the first part of the course we will develop one example, it will be used to illustrate features of C, UNIX and some tools for software development
• The example that we will use is a computer system to maintain lists of phone numbers and addresses for people
• We will start with the simplest possible program for this purpose, and add features to show different aspects of C programming
• The first version of the program will retrieve a phone number given the name of a person

Requirements

• The requirements for this version of the program are quite simple
  1. given the name of a person retrieve their phone number
  2. the list of phone numbers must be stored somewhere on the computer
  3. the user will not be able to modify the phone list from inside of the program
  4. the program should be robust, but it doesn't need to have a fancy interface or perform any elaborate formatting.

Specification

• How do we approach the specification of a program?
• There are many ways of doing this, some are quite formal, while others use simple English statements
• The approach that we will use is based on the use of objects and operators
• Object: something that will be manipulated by the program or the user, a conceptual entity that we want to do something with
• Operators: used to manipulate the objects, used to change the state of the objects
• We can specify an object by listing its properties - the data that is stored in the object, plus any constraints on how this data can be changed
• We can specify the operators by using pre- and post-conditions;
  pre-conditions state the operator's assumptions, post-conditions state what the operator does: how it changes the objects
• We start by specifying the objects (abstract datatypes) in the application
• We need some way of recording the phone number of an individual, this is done by the phone_record object:

  Object phone_record;
      name : text;
      number : text;
  End;
  

• The complete phone book is given by the following object:

  Object phone_book;
      contents : set of phone_record;
  End;
  

• There are two operators in this application
• The first operator is used to initialize the phone book, in other words read the file containing the phone list

  Operator read_phone_list;
      Pre
          phone_book.contents is empty;
      Post
          phone_book.contents is not empty;
  End;
  

• The other operator looks up the number corresponding to a name

  Operator look_up(name) -> number;
      Pre
  	phone_book.contents has a phone record with name in it;
      Post
          the number returned is in the same phone_record as the name parameter;
  End;
  

Design (Decisions)

• We start by designing the data structures
• On disk the phone list will be stored as a fixed format text file, with one entry per line - the first 20 columns will be the name and the next 10 columns will be the phone number
• In memory the phone list will be stored as an array, with each array entry having one name and one phone number, both the name and number will be stored as character strings
• A separate module, called phone, will be used for this data structure and the routines that manipulate it
• The read_phone_file procedure will transfer the phone list from disk to main memory
• The get_phone_number procedure will retrieve a phone number given a name
• The main program will call read_phone_file and ask the user for the names to look up