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
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 particular 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  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 structureoriented design techniques actually help to derive program structure from the data structure of the program. Example of...
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