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需求分析

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CHAP8SOFTWARE ENGINEERING ISSUES
8.1 Software engineering concepts (1)What is software engineering
. to obtain software that is reliable and works efficiently on real machine.
. to create techniques that reduce high software cost and complexity while increasing reliability and modifiability.
. to provide software products that are developed on time and within cost estimates.
(2)The software development process:
. Requirements analysis is used to determine what the software system must do.
. System specification is used to formulate a precise description of the desired system insoftware development terms.
. System design is used to address the solution which describe the most important datastructures and algorithms.
. Implementation is used to describe the actual coding and testing which include thedetails concering the algorithm and datastructure used.
. System testing is used to make sure that software correctly perform their actions.
需求分析系统规格系统设计制作testing系统测试Maintenance维护Fig 8.1 Software life model)
(3)Softrware maintenance and evolution:
. System that are used overalong period of time inevitably must change in order tomeet changing requirements.
. Four categories of software maintenance:
Corrective maintenance : fix maintenance : maintenance : meet maintenance : add new requirements
8.2 System specifications (1) It is important that specifications issues such as the from system and how the softwareshould interact with the users.The selection ofone type of user interface might optimize thespeed of data entry but require more time for the users.The also form the basis for thesystem testing phase.
(2)Types of
Input specifications constraints on the input to the systremOutput specification are intended to define precisely the results to be produced by the system.
Qnpuality specifications are concerned with global , such as of use , and reliability.
(3) Figure8.3 in decisiontable form.
(4) One of the most aspects of software writing isthe handling of error conditions.
(5) Effective handling if error conditions is onemark ofawell written piece of software.
Error Violatesnumber specification Statement
11 ALPHA LDA BETA
21 ALPHALDA BETA
42 ALPHAXX LDA BETA
62 ALPA LDA BETA
73 ALPHA XXX BETA 125BETA BYTEXA3BFig 8.4 Sample input errors derived from specifications
8.3 Data Flow diagrams (1)A data flow diagram ofthe movement of information betweenstorage and processing steps system.
(2) One method for refining the diagram is tobegin with the desired outputs from thesystem.
(3) As the diagram is developed , it isimportant to write down documentation forthe data objects and processing actionsthat are being fileFile 1File 8.5 Sample data flow diagrams
8.4 Modular design (1)The goal of the modular design process isasoftware design that meets the
(2) The modules should be small enough thateach could be implemented by one short time period.
(3) Systems organized into modules accordingto these divide and tendto be much easier to understand.
(4)The division of the diagram into units may be based onavariety of factors.
the sequence in which functions are performedthe type of function being performedthe desirability of minimizing the coupling between modules.
(5)Abetter solution to reduce coupling and increase cohesion is referred to as isolation of design factors : called information hiding or data abstraction.
(6) Modularization of the data flow diagramfrom figure 8.8 according to the principlesjust described , ,
(7) If an object is used by only one module,itis natural to place that object within themodule that uses it. If an object isrequired by more than on module,the datacan be shared either via parameter passingor through the use of global variables.
(8) In general,it seems desirable toavoid the use of global there isaclear reason forpreferring to use them
8.5 System testing strategies (1)Levels of testing:unit testing
system testing:alpha and acceptance testing.
(2) Bottom-up testing is the most frequentlyused strategy for unit testing and with , design errors that between modules are notdiscovered until the later stages of testing.
(3) Top-down testing detects design errorsthat involve module interfaces at an earlierstage than bottom-up testing.
of the bottom-up and top-down approaches is used in practice.If usesboth stubs and test drivers for the unittesting process,the resulting modules canbe integrated together in any

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