Structured Programming
Specific Instructional Objectives
At the end of this lesson the student will be able to:
• Identify the important features of a structured program.
• Identify the important advantages of structured programming over unstructured ones.
• Explain how software design techniques have evolved over the last 50 years.
• Differentiate between exploratory style and modern style of software development
Important features of a structured program.
A structured program uses three types of program constructs i.e. selection, sequence and iteration. Structured programs avoid unstructured control flows by restricting the use of GOTO statements. A structured program consists of a well partitioned set of modules. Structured programming uses single entry, single-exit program constructs such as if-then-else, do-while, etc. Thus, the structured programming principle emphasizes designing neat control structures for programs.
Important advantages of structured programming.
Structured programs are easier to read and understand. Structured programs are easier to maintain. They require less effort and time for development. They are amenable to easier debugging and usually fewer errors are made in the course of writing such programs.
Evolution of software design techniques over the last 50 years.
During the 1950s, most programs were being written in assembly language. These programs were limited to about a few hundreds of lines of assembly code, i.e. were very small in size. Every programmer developed programs in his own individual style - based on his intuition. This type of programming was called Exploratory Programming.
The next significant development which occurred during early 1960s in the area computer programming was the high-level language programming. Use of high-level language programming reduced development efforts and development time significantly. Languages like FORTRAN, ALGOL, and COBOL were introduced at that time.
As the size and complexity of programs kept on increasing, the exploratory programming style proved to be insufficient. Programmers found it increasingly difficult not only to write cost-effective and correct programs, but also to understand and maintain programs written by others. To cope with this problem, experienced programmers advised other programmers to pay attention to the design of the program’s control flow structure (in late 1960s). In the late 1960s, it was found that the "GOTO" statement was the main culprit which makes control structure of a program complicated and messy. At that time most of the programmers used assembly languages extensively. They considered use of "GOTO" statements in high-level languages were very natural because of their familiarity with JUMP statements which are very frequently used in assembly language programming. So they did not really accept that they can write programs without using GOTO statements, and considered the frequent use of GOTO statements inevitable. At this time, Dijkstra [1968] published his (now famous) article “GOTO Statements Considered Harmful”. Expectedly, many programmers were enraged to read this article. They published several counter articles highlighting the advantages and inevitably of GOTO statements. But, soon it was conclusively proved that only three programming constructs – sequence, selection, and iteration – were sufficient to express any programming logic. This formed the basis of the structured programming methodology.
After structured programming, the next important development was data structure-oriented design. Programmers argued that for writing a good program, it is important to pay more attention to the design of data structure, of the program rather than to the design of its control structure. Data structure-oriented design techniques actually help to derive program structure from the data structure of the program. Example of a very popular data structure-oriented design technique is Jackson's Structured Programming (JSP) methodology, developed by Michael Jackson in the1970s.
Next significant development in the late 1970s was the development of data flow-oriented design technique. Experienced programmers stated that to have a good program structure, one has to study how the data flows from input to the output of the program. Every program reads data and then processes that data to produce some output. Once the data flow structure is identified, then from there one can derive the program structure.
Object-oriented design (1980s) is the latest and very widely used technique. It has an intuitively appealing design approach in which natural objects (such as employees, pay-roll register, etc.) occurring in a problem are first identified. Relationships among objects (such as composition, reference and inheritance) are determined. Each object essentially acts as a data hiding entity.
Exploratory style vs. modern style of software development.
An important difference is that the exploratory software development style is based on error correction while the software engineering principles are primarily based on error prevention. Inherent in the software engineering principles is the realization that it is much more cost-effective to prevent errors from occurring than to correct them as and when they are detected. Even when errors occur, software engineering principles emphasize detection of errors as close to the point where the errors are committed as possible. In the exploratory style, errors are detected only during the final product testing. In contrast, the modern practice of software development is to develop the software through several well-defined stages such as requirements specification, design, coding, testing, etc., and attempts are made to detect and fix as many errors as possible in the same phase in which they occur.
In the exploratory style, coding was considered synonymous with software development. For instance, exploratory programming style believed in developing a working system as quickly as possible and then successively modifying it until it performed satisfactorily.
In the modern software development style, coding is regarded as only a small part of the overall software development activities. There are several development activities such as design and testing which typically require much more effort than coding.
A lot of attention is being paid to requirements specification. Significant effort is now being devoted to developing a clear specification of the problem before any development activity is started.
Now there is a distinct design phase where standard design techniques are employed.
Periodic reviews are being carried out during all stages of the development process. The main objective of carrying out reviews is phase containment of errors, i.e. detect and correct errors as soon as possible. Defects are usually not detected as soon as they occur, rather they are noticed much later in the life cycle. Once a defect is detected, we have to go back to the phase where it was introduced and rework those phases - possibly change the design or change the code and so on.
Today, software testing has become very systematic and standard testing techniques are available. Testing activity has also become all-encompassing in the sense that test cases are being developed right from the requirements specification stage.
There is better visibility of design and code. By visibility we mean production of good quality, consistent and standard documents during every phase. In the past, very little attention was paid to producing good quality and consistent documents. In the exploratory style, the design and test activities, even if carried out (in whatever way), were not documented satisfactorily. Today, consciously good quality documents are being developed during product development. This has made fault diagnosis and maintenance smoother.
Now, projects are first thoroughly planned. Project planning normally includes preparation of various types of estimates, resource scheduling, and development of project tracking plans. Several techniques and tools for tasks such as configuration management, cost estimation, scheduling, etc. are used for effective software project management.
Several metrics are being used to help in software project management and software quality assurance.