Structure Chart Structure charts are derived for DFDs It is the detailed representation of system than DFD It breaks down the system into its lowest functional modules and describes functions and their sub functions in much more detailed than DFD The structure chart is an important technique which helps the analyst design the program for the new system Structure chart hierarchical displays all the components of code that must be included in a program at high level that implies sequence in what order the components are invoked selection under what condition the module is invoked and iteration how often a component is repeated The components are usually read from top to bottom left to right and they are numbered in hierarchical scheme in which the lower levels have an additional level of numbering Structured charts have been used to create transaction based mainframe applications which have many lines of code They help analyst create programs that are easy to understand and maintain Structure charts can be helpful in building many types of systems because of reusability feature The symbols used in construction of structure charts Module A structure chart consists of modules that work together to form a program The modules are represented by a rectangle and are connected by lines which represent the passing of control
A control module is a high level component which contains the main logic for performing other modules and the component that it calls and controls are subordinate modules Sometimes this modules are used in many places throughout the system This are called as library modules and are represented by a rectangle along with vertical lines on both sides Condition It is represented by a diamond at the base of the module It represents that the control can select either sub module based on the condition Loop The curved arrow represents loop in the module The loop indicates that the execution of some or all sub modules is repeated Couples This are shown by arrows This are drawn on the structure chart to show that the information is passed between modules The arrow head indicates which way the information is sent Data Couples A data couple is represented by a directed arrow with empty circle at the end This could be passed up or down Control Couple This is represent by a directed error with a filled circle It should only be passed up not down Designing structure chart The structure charts comes from requirements and use case descriptions A DFD represents the process but not the sequence selection or iteration Each DFD process represents a module in the structure chart DFD levels will roughly correspond to structure chart levels Eg context level DFD Top Module There are 3basics kinds of processes
Afferent process which provides inputs to the system Central process performs critical functions in the operation of the system Efferent process Deals with system output Each process of a DFD tends to represent one module on the structure chart And if DFDs used are levelled then each DFD level tends to correspond to a different level of the structure chart hierarchy We have two basic structures for combining the modules Transaction structure This is used when each module performs one of a group of individual transactions It contains of a control module that calls the subordinate modules each of which handles a particular transaction It contains few afferent processes Many efferent processes It is concerned with coordinating the production of outputs It is often used at higher level of a structured chart Transform structure It has a control module which calls several subordinate modules in some sequence This modules together form a process that transforms some input into an output hence they are related Each module accepts an input from the module preceding it works on the input and passes on to the next module for processing further It is often used in lower levels of structure chart Design Guidelines High quality structure charts result in programs that are modular reusable and easy to implement Measures of good design include Cohesion It refers to how well the program code within each structure chart module relate to each other A module should only perform one task making it highly cohesive
Cohesive modules are easy to understand and build because their code performs one function and are built to perform that function very efficiently The more task the module performs the more complex the logic in the code to implement the task correctly Having and in the titles and many downwards control flags are signs of poor cohesion Coupling Coupling involves how closely modules are interrelated and the second guideline for good structure chart design states that modules must be loosely coupled This makes the module independent from each other which keeps the code changes in one module The fewer the arrows on the diagram the easier it will be to make future alterations Appropriate levels of fan in and fan out Create high fan in Fan in describes the number of control modules that communicate with a subordinate A module with high fan in has different control modules that call it High fan in indicates that a module is reused in many places on the structure chart This indicates that the module has a well written generic code Structures with high fan in promote the reusability of modules and make it easier for programmers to record when changes are made or mistakes are uncovered because a change can be made in one place Avoid high fan out A large number of subordinates associated with single control should be avoided A general thump rule upto 7 modules With respect to the above mentioned design guidelines I have designed structure chart for two processes of the future system NoAsthma
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