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York University
Information Technology
ITEC 3010

Systems Analysis and Design I ITEC3010 – Fall 2010 – Luiz Cysneiros Lecture 9 – Design – Nov 16 The Structure Chart - Structure chart o A hierarchical diagram showing the relationships between the modules of a computer program o The objective of structured design is to create a top-down decomposition of the functions to be performed by a given program in a system – use a structure chart to show this o E.g. shows functions and sub functions (such as Calculate base amount, Calculate overtime amount etc.) o Uses rectangles to represent each such module (the basic component of a structure chart o Higher-level modules are “control” modules that control flow of execution (call lower level modules which are the “worker bee” modules that contain program logic) The Structure Chart Characteristics - Structure chart is based on the idea of modular programming (and top-down programming) o Breaking a complex problem down into smaller modules o Modules are well formed with high internal cohesiveness and minimum data coupling o Vertical lines connecting the modules indicate calling structure o Little arrows next to the lines show the data passed between modules (inputs and outputs)  Data couples: individual items that are passes between modules  Program call: the transfer of control from a module to a subordinate module to perform a requested service (can be implemented as e.g. a function or procedure call) Structure Chart Symbols - Rectangles represent modules o Can represent a function (e.g. C), a procedure (e.g. Pascal), a paragraph (e.g. COBOL) , subroutine (e.g. FORTRAN) etc. o Rectangle with double bars represents a module used several places (optional) - Little arrows with open circles represent data couples - Little arrows with closed circles represent control couple flag (e.g. end of file flag) - Curved arrows immediately below a boss module represents iteration (looping) - Darkened diamond represents a conditional call to lower modules Notes on Structure Chart - Each module does a very specific function - Module at top of the tree is the boss module o Function is to call modules on level below, pass information to them - Middle level modules control the modules below o Call them and pass data - At the very bottom, the leaves contain the actual algorithms to carry out the program functions - Call of modules is left to right across the tree Developing a Structure Chart - Transaction analysis o The development of a structure chart based on a DFD that describes the processing for several types of transactions o Uses as input the system flow chart and the event table to develop the top level of the tree in a structure chart - Transform analysis o The development of a structure chart based on a DFD that describes the input-process-output data flow o Used to develop the subtrees in a structure chart – one for each event in the program Transaction Analysis - First step is to identify the major programs o Can do it by looking at the system flow chart and identifying the major programs - For a subsystem or program we want to make a structure chart for, we can look at the event-partitioned DFD to identify the major processes o These major processes will become the modules in the resulting high level structure chart (see slide after that) - Based on the idea that computer program “transforms” input data into output data - Structure charts developed with transform analysis usually have 3 main subtrees o Input subtree to get data o A calculate subtree to perform logic o An output subtree to write the results - Can create it rearranging elements from o DFD fragment for an event (e.g. “create new order”) o The detailed DFD for that event - E.g. see next two slides for “create new order” DFD fragment, and its corresponding detailed DFD Steps for Creating the Structure Chart from the DFD - Identify the primary information flow - Find the process that represents the most fundamental change from an input stream to an output stream – the central transform o Afferent data flow: the incoming data flow in a sequential set of process bubbles o Efferent data flow: the outgoing data flow from a sequential set of process bubbles o Central transform: the central processing bubbles in a transform analysis type of data flow - In our example, the record (build) order process is the central process - Redraw the data flow diagram (DFD) with o The inputs to the left o The central transform process in the middle o The outputs to the right o The parent process (top level – e.g. “Create new order”) above the central transform process (e.g. “Build order”) - Generate the first-draft structure chart including data couples (directly from our rearranged DFD on the previous slide) - Add other modules as necessary to o Get input data via the user-interface screens o Read and write to the data stores o Write output data or reports - These are lower level modules (utility modules) - Also add data couples - Using structured English or decision table documentation, add any other required intermediate relationships (e.g. looping and decision symbols) - Make the final refinements to the structure chart based on quality control concepts (to be discussed) Combining the Top-Level Structure Chart with the Chart Developed by Transaction Analysis - Basically “glue” the diagram we made (high-level) using transaction analysis on top of the more detailed diagram (for lower-processing) we just made using transform analysis Evaluating the Quality of a Structure Chart - Module coupling o The manner in which modules relate to each other - Desirable to have loosely coupled modules o Have modules as independent as possible o Module does not need to know who invoked it o Best coupling is through simple data coupling  The module is called and a specific set of data items is passed  Module function performs its function and returns the output - Cohesion o Refers to the degree to which all of the code within a module contributes to implementing one well-defined task - Desirable to have modules with high cohesion implementing a single function o Modules that implement a single task tend to have relatively low coupling because all of their internal code acts on the same data item(s) o Modules with poor cohesion tend to have high coupling because loosely related tasks are typically performed on different data items across modules o A flag passed down the structure chart is an indicator of poor cohesion Module Algorithm Design: Pseudocode - Next requirement o Describe the internal logic within each module o Three methods  Flow charts  Structured English  Pseudocode (variation of structured English similar to programming language that will be used – e.g. COBOL like or C like pseudocode) Designing the Application Architecture: The Object-Oriented Approach - Object-oriented design models provide the bridge between the object-oriented analysis models and the object-oriented programs - Object-oriented programs o Basic concept: the program consists of a set of program objects that cooperate to accomplish a result o Objects work together by sending messages o Example: in a graphical user interface (GUI) windows and menus are objects. If you click on a window object a message is sent to display itself (e.g. window, a menu bar etc.) Differences between Object Oriented and Traditional Approaches - In traditional computer environments there is some sort of central control o E.g. a mainframe computer may be connected to thousands of terminals and controls them centrally o No terminal does work unless directed to by the mainfr
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