Software Engineering-Structured Programming

The foundations of component-level design were formed in the early 1960s and were solidified with the work of Edsgar Dijkstra and his colleagues . In the late 1960s, Dijkstra and others proposed the use of a set of constrained logical constructs from which any program could be formed. The constructs emphasized "maintenance of functional domain." That is, each construct had a predictable logical structure, was entered at the top and exited at the bottom, enabling a reader to follow procedural flow more easily.
The constructs are sequence, condition, and repetition. Sequence implements processing steps that are essential in the specification of any algorithm. Condition provides the facility for selected processing based on some logical occurrence, and repetition allows for looping. These three constructs are fundamental to structured programming—an important component-level design technique.

The structured constructs were proposed to limit the procedural design of software to a small number of predictable operations. Complexity metrics  indicate that the use of the structured constructs reduces program complexity and thereby enhances readability, testability, and maintainability. The use of a limited number of logical constructs also contributes to a human understanding process that psychologists call chunking. To understand this process, consider the way in which you are reading this page. You do not read individual letters but rather recognize patterns or chunks of letters that form words or phrases. The structured constructs are logical chunks that allow a reader to recognize procedural elements of a module, rather than reading the design or code line by line. Understanding is enhanced when readily recognizable logical patterns are encountered.

Any program, regardless of application area or technical complexity, can be designed and implemented using only the three structured constructs. It should be noted, however, that dogmatic use of only these constructs can sometimes cause practical difficulties.

Graphical Design Notation

"A picture is worth a thousand words," but it's rather important to know which picture and which 1000 words. There is no question that graphical tools, such as the flowchart or box diagram, provide useful pictorial patterns that readily depict procedural detail. However, if graphical tools are misused, the wrong picture may lead to the wrong software.

A flowchart is quite simple pictorially. A box is used to indicate a processing step. A diamond represents a logical condition, and arrows show the flow of control. Figure  illustrates three structured constructs. The sequence is represented as two processing boxes connected by an line (arrow) of control. Condition, also called ifthen- else, is depicted as a decision diamond that if true, causes then-part processing to occur, and if false, invokes else-part processing. Repetition is represented using two slightly different forms. The do while tests a condition and executes a loop task repetitively as long as the condition holds true. A repeat until executes the loop task first, then tests a condition and repeats the task until the condition fails. The selection (or select-case) construct shown in the figure is actually an extension of the if-then-else. A parameter is tested by successive decisions until a true condition occurs and a case part processing path is executed.

The structured constructs may be nested within one another as shown in figure. Referring to the figure, repeat-until forms the then part of if-then-else (shown enclosed by the outer dashed boundary). Another if-then-else forms the else part of the larger condition. Finally, the condition itself becomes a second block in a sequence. By nesting constructs in this manner, a complex logical schema may be developed. It should be noted that any one of the blocks in figure could reference another module, thereby accomplishing procedural layering implied by program structure.
In general, the dogmatic use of only the structured constructs can introduce inefficiency when an escape from a set of nested loops or nested conditions is required. More important, additional complication of all logical tests along the path of escape can cloud software control flow, increase the possibility of error, and have a negative impact on readability and maintainability. What can we do?

The designer is left with two options: (1) The procedural representation is redesigned so that the "escape branch" is not required at a nested location in the flow of control or (2) the structured constructs are violated in a controlled manner; that is, a constrained branch out of the nested flow is designed. Option 1 is obviously the ideal approach, but option 2 can be accommodated without violating of the spirit of structured programming.

Another graphical design tool, the box diagram, evolved from a desire to develop a procedural design representation that would not allow violation of the structured constructs. Developed by Nassi and Shneiderman  and extended by Chapin , the diagrams (also called Nassi-Shneiderman charts, N-S charts, or Chapin charts) have the following characteristics: (1) functional domain (that is, the scope of repetition or if-then-else) is well defined and clearly visible as a pictorial representation, (2) arbitrary transfer of control is impossible, (3) the scope of local and/or global data can be easily determined, (4) recursion is easy to represent.

The graphical representation of structured constructs using the box diagram is illustrated in figure. The fundamental element of the diagram is a box. To represent sequence, two boxes are connected bottom to top. To represent if-then-else, a condition box is followed by a then-part and else-part box. Repetition is depicted with a bounding pattern that encloses the process (do-while part or repeat-until part) to be repeated. Finally, selection is represented using the graphical form shown at the bottom of the figure.

Like flowcharts, a box diagram is layered on multiple pages as processing elements of a module are refined. A "call" to a subordinate module can be represented within a box by specifying the module name enclosed by an oval.
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