中英文翻譯系統(tǒng)的分析與設計

1、<p>　　Systems Analysis and Design</p><p>　　Working under control of a stored program, a computer processes data into information. Think about that definition for a minute. Any given computer application

2、 involves at least three components: hardware, software, and data. Merely writing a program isn't enough; because the program is but one component in a system.</p><p>　　A system is a group of components

3、that work together to accomplish an objective. For example, consider a payroll system. Its objective is paying employees. What components are involved? Each day，employees record their hours worked on time cards. At the e

4、nd of each week, the time cards are collected and delivered to the computer center, where they are read into a payroll program. As it runs, the program accesses data files. Finally, the paychecks are printed and distribu

5、ted. For the system to wor</p><p>　　Computer-based systems are developed because people need information. Those people, called users, generally know what is required, but may lack the expertise to obtain it.

6、 Technical professionals, such as programmers, have the expertise, but may lack training in the user's field. To complicate matters, users and programmers often seem to speak different languages, leading to communica

7、tion problems. A systems analyst is a professional who translates user needs into technical terms, thus serving a</p><p>　　Like an engineer or an architect, a systems analyst solves problems by combining sol

8、id technical skills with insight, imagination, and a touch of art. Generally, the analyst follows a well-defined, methodical process that includes at least the following steps;</p><p>　　Problem definition<

9、;/p><p><b>　　Analysis</b></p><p><b>　　Design</b></p><p>　　Implementation </p><p>　　Maintenance</p><p>　　At the end of each step, resul

10、ts are documented and shared with both the user and the programmers. The idea is to catch and correct errors and misunderstandings as early as possible. Perhaps the best way to illustrate the process is through example.&

11、lt;/p><p>　　Picture a small clothing store that purchases merchandise at wholesale, displays this stock, and sells it to customers at retail. On the one hand, too much stock represents an unnecessary expense. O

12、n the other hand, a poor selection discourages shoppers. Ideally, a balance can be achieved: enough, but not too much.</p><p>　　Complicating matters is the fact that inventory is constantly changing, with cu

13、stomer purchases depleting stock, and returns and reorders adding to it. [1] The owner would like to track inventory levels and reorder and given item just before the store runs out. For a single item, the task is easy-j

14、ust count the stock-on-hand. Unfortunately, the store has hundreds of different items, and keeping track of each one is impractical. Perhaps a computer might help.</p><p>　　2-1 Problem Definition</p>

15、;<p>　　The first step in the systems analysis and design process is problem definition. The analyst's objective is determining what the user (in this case, the store's owner) needs. Note that, as the proce

16、ss begins, the user possesses the critical information, and the analyst must listen and learn. Few users are technical experts. Most see the computer as a "magic box, "and are not concerned with how it works. A

17、t this stage, the analyst has no business even thinking about programs, files, and computer </p><p>　　The idea is to ensure that both the user and the analyst are thinking about the same thing-Thus, a clear,

18、 written statement expressing the analyst's understanding of the problem is essential. The user should review and correct this written statement. The time to catch misunderstandings and oversights is now, before time

19、, money and effort are wasted.</p><p>　　Often, following a preliminary problem definition, the analyst performs a feasibility study. The study a brief capsule version of the entire systems analysis and desig

20、n process, attempts to answer three questions:</p><p>　　Can the problem be solved?</p><p>　　Can it be salved in the user's environment?</p><p>　　Can it be solved at a reasonable

21、 cost?</p><p>　　If the answer to any one of these questions is no, the system should not be developed. Given a good problem definition and a positive feasibility study, the analyst can turn to planning and d

22、eveloping a problem solution.</p><p>　　2- 2 Analysis</p><p>　　As analysis begins, the analyst understands the problem. The next step is determining what must be done to solve it. The user kno

23、ws what must be done 1 during analysis; this knowledge is extracted and formally documented. Most users think in terms of the functions to be performed and the data elements to be manipulated. The objective is to identif

24、y and link these key functions and data elements, yielding a logical system design.</p><p>　　Start with the system's basic functions. The key is keeping track of the stock-on-hand for each product in inv

25、entory. Inventory changes because customers purchase, exchange, and return products, so the system will have to process customer transactions. The store's owner wants to selectively look at the inventory level for an

26、y product in short supply and, if appropriate, order replacement stock, so the system must be able to communicate with management. Finally, following management authorization</p><p><b>　　Fig 1</b>

27、;</p><p>　　Given the system's basic functions, the analyst's next task is gaining a sense of their logical relationship. A good way to start is by describing how data flow between the functions. As t

28、he name implies, data flow diagrams are particularly useful for graphically describing these data flows. Four symbols are used (Fig. 1). Data sources and destinations are represented by squares; input data enter the syst

29、em from a source, and output data flow to a destination. Once in the system, the data are ma</p><p><b>　　Fig 2</b></p><p>　　Figure 2 shows a preliminary data flow diagram for the inv

30、entory system. Start with CUSTOMER. Transactions flow from a customer f into the system, where they are handled by Process transaction. A data store, STOCK, holds data on each item in inventory. Process transaction chang

31、es the data to reflect the new transaction. Meanwhile, MANAGEMENT accesses the system through Communicate, evaluating the data in STOCK and, if necessary, requesting a reorder. Once a reorder is authorized. Generate reor

32、de</p><p>　　The data flow diagram describes the logical system. The next step is tracing the data flows. Start with the destination SUPPLIER. Reorders flow to suppliers; for example, the store might want 25

33、pairs of jeans. To fill the order, the supplier needs the product description and the reorder quantity. Where do these data elements come from? Since they are output by Generate reorder, they must either be Input to or g

34、enerated by this process. Data flow into Generate reorder for STOCK; thus, product de</p><p>　　Other data elements, such as the item purchased and the purchase quantity are generated by CUSTOMER. Still other

35、s, for example selling price and reorder point, are generated by or needed by MANAGEMENT. The current stock-on-hand for a given item is an example of a data element generated by an algorithm in one of the procedures. Ste

36、p by step, methodically, the analyst identifies the data elements to be input to .stored by, manipulated by, generated by, or output by the system.</p><p>　　To keep track of the data elements, the analyst mi

37、ght list each one in a data dictionary. A simple data dictionary can be set up on index cards, but computerized data dictionaries have become increasingly popular. The data dictionary, a collection of data describing and

38、 defining the data, is useful throughout the systems analysis and design process, and is often used to build a database during the implementation stage.</p><p>　　The idea of analysis is to define the system&

39、#39;s major functions and data elements methodically. Remember that the objective is translating user needs into technical terms. Since the system starts with the user, the first step is defining the user's needs. Us

40、ers think in terms of functions and data. They do not visualize programs, or files, or hardware .and during this initial, crucial analysis stage it is essential that the analyst think like a user, not like a programmer.&

41、lt;/p><p>　　Data flow diagrams and data dictionaries are useful tools. They provide a format for recording key information about the proposed system. Also, they jog the analyst's memory) for example, if the

42、 analyst doesn't have sufficient information to complete a data dictionary entry, he or she has probably missed something. Perhaps most importantly, the data flow diagram and the data dictionary document the analyst&

43、#39;s understanding of the system requirements. By reviewing these documents, the user can cor</p><p>　　2-3 Design</p><p>　　As we enter the design stage, we know what the system must do, and t

44、hus can begin thinking about how to do it. The objective is to develop a strategy for solving the problem. At this stage, we are not interested in writing code or in defining precise data structures; instead, we want to

45、identify, at a black box level, necessary programs, files, procedures, and other components.</p><p>　　The data flow diagram defines the system's necessary functions; how might they be implemented? One po

46、ssibility is writing one program for each process. Another is combining two or more processes in a single program; there are dozens of alternative solutions. Let's focus on one option and document it.</p><

47、p>　　A system flowchart uses symbols to represent programs, procedures, hardware devices, and the other components of a physical system (Fig. 3). Our flowchart (.Fig. 4) shows that transaction data enter the system thr

48、ough a terminal, are processed by a data collection program, and then are stored on an inventory file. Eventually, the inventory file is processed by a Report and reorder program. Through it, management manipulates the d

49、ata and authorizes reorders.</p><p>　　Fig. 4 On a system flowchart, symbols represent programs, procedures, hardware devices, and the other components of a physical system.</p><p><b>　　Fig

50、 3</b></p><p>　　Look at the system flowchart. It identifies several hardware components, including a computer, a disk drive, a data entry terminal, a printer, and a display terminal. Two programs are n

51、eeded; Process transaction and Report and reorder. In addition to the hardware and the programs, we1l need data structures for the inventory file and for data flaws between the I/O devices and the software. Note that thi

52、s system flowchart illustrates one possible solution; a good analyst will develop several feasi</p><p><b>　　Fig 4</b></p><p>　　The flowchart maps the system, highlighting its major p

53、hysical components. Since the data link the components, the next task is defining the data structures. Consider, for example, the inventory file. It contains all the data elements from the data store STOCK. The data elem

54、ents are listed in the data dictionary. Using them, the file's data structure can be planned,</p><p>　　How should the file be organized? That depends on how it will be accessed. For example, in some appl

55、ications, data are processed at regular, predictable intervals. Typically, the data are collected over time and processed together, as a batch. If batch processing is acceptable, a sequential file organization is probabl

56、y best.</p><p>　　It is not always possible to wait until a batch of transactions is collected, however. For example, consider an air defense early warning system. If an unidentified aircraft is spotted it mu

57、st be identified immediately the idea of waiting until 5 _ 00 p.m. because "that's when the air defense program is run" is absurd. Instead, because of the need for quick response, each transaction must be p

58、rocessed as it occurs. Generally such transaction processing systems call for direct access file.</p><p>　　Our inventory system has two programs. One processes transactions. A direct access inventory file se

59、ems a reasonable choice. The other allows management to study inventory data occasionally; batch processing would certainly do. Should the inventory file be organized sequentially or directly? Faced with such a choice a

60、good analyst considers both options. One possible system might accept transactions and process them as they occur. As an alternative, sales slips might be collected throughout the </p><p>　　2- 4 Implementa

61、tion</p><p>　　Once the system's major components have been identified .we can begin to develop them. Our system includes two programs, several pieces of equipment, and a number of data structures. During

62、 implementation, each program is planned and written using the techniques described in Chapter 7. Files are created, and their contents checked. New hardware is purchased, installed, and tested. Additionally, operating p

63、rocedures are written and evaluated. Once all the component parts are ready, the system is </p><p>　　2- 5 Maintenance</p><p>　　Maintenance begins after the system is released. As people use it

64、, they will suggest minor improvements and enhancements. Occasionally, bugs slip through debug and testing, and removing them is another maintenance task. Finally, conditions change, and a program must be updated; for ex

65、ample, if the government passes a low changing the procedure for collecting income taxes, the payroll program must be modified. Maintenance continues for the life of a system, and its cost can easily match or exceed</

66、p><p><b>　　Summary</b></p><p>　　A system is a collection of hardware, software, data, and procedural components that work together to accomplish an objective. A program is but one compo

67、nent in a system.</p><p>　　Systems are planned and designed by systems analysts who generally follow a well-defined, methodical process. The first step in the process is problem definition, whew the analyst

68、attempts to discover exactly what the user needs. Often, following a preliminary problem definition, a feasibility study is conducted to determine if the problem can be solved.</p><p>　　Given a clear problem

69、 definition, analysis begins. During this stage, the analyst develops a logical model of the system. Key functions are linked through a data flow diagram. Using the diagram as a tool, the data flows are traced, and the s

70、ystem's data elements are identified and recorded in a data dictionary. After the logical system is reviewed with the user, design begins.</p><p>　　During design, the analyst develops a model of the phys

71、ical system. A system flowchart can be used to map the system, defining each physical component as a symbol. A good systems analyst considers a number of alternative solutions to the problem before settling on one. Imple

72、mentation follows design. Programs are planned and written; hardware is ordered and installed procedures are written; files and databases are initialized; and, finally, the pieces are assembled and tested. Following rele

73、ase o</p><p><b>　　系統(tǒng)的分析與設計</b></p><p>　　在存儲程序的控制下，計算機把數(shù)據(jù)處理成信息。對系統(tǒng)的定義略加思考，任何一個已知的計算機應用至少包含三個部分，硬件、軟件和數(shù)據(jù)。僅僅編寫一個程序是不夠的，因為程序只是系統(tǒng)中的一部分。</p><p>　　系統(tǒng)就是為實現(xiàn)一個目標而共同工作的一組部件。例如，考慮一個

74、工資系統(tǒng)，它的目標是為雇員付工資，應包含哪幾部分呢？每天雇員們把他們工作的時數(shù)記錄在計時卡片上，每周末把計時卡片收集起來，送給計算中心，在計算中心把計時卡片上的數(shù)據(jù)讀給工資程序．當工資程序執(zhí)行時，程序存取數(shù)據(jù)文件。最后，打印出工資單，用來分發(fā)。為使系統(tǒng)工作，人、處理過程、輸入和輸出介質、文件、硬件和軟件都必須認真地協(xié)調。注意，程序只是系統(tǒng)中的一部分。</p><p>　　因為人們需要信息，所以要開發(fā)基于計算機的系

75、統(tǒng)。被稱為用戶的那些人常常知道需求什么信息，但是他們可能缺乏得到這些信息的計算機的專門知識。計算機技術方面的專業(yè)人員，例如程序設計員有這方面的專門知識，但是可能在用戶的專業(yè)領域內缺少訓練。麻煩的是。用戶和程序員似乎常常講不同的語言，導致了兩者之間的聯(lián)系障礙。系統(tǒng)分析員是一個專業(yè)人員，他能把用戶的需求轉換成計算機技術術語。因此他是用戶和技術專業(yè)人員之間的橋梁。</p><p>　　象工程師或建筑師一樣，系統(tǒng)分析員把

76、他扎實的技術技能與知識、想象力和一點藝術結合起來去解決問題．通常，分析員遵循一個意義明確的有條理的過程，至少應包括以下幾個步驟：</p><p><b>　　1.問題的定義</b></p><p><b>　　2．分析</b></p><p><b>　　3．設計</b></p><

77、;p><b>　　4．實現(xiàn)</b></p><p><b>　　5.維護</b></p><p>　　在每一步驟的結尾，研究的結果都要形成文件，提供給用戶和程序設計員?；镜乃枷胧且M早地抓住和修正錯誤以及一些未理解之處。也許通過一個例子來說明該過程是最好的方法。</p><p>　　設想一個小服裝店，成批地購買貨物

78、，把貨物擺在貨架上，零售給顧客。一方面庫存太多會造成不必要的開銷；另一方面可供挑選的商品太少會使顧客失去購買的信心。理想上，希望達到一個平衡，貨物即充足但又不太多。</p><p>　　麻煩的是，隨著顧客的購買庫存減少了，還有退貨以及追加訂貨等情況，所以貨物的清單經(jīng)常地發(fā)生變化．店主喜歡按貨物清單購銷，剛好在商店售完某種貨物之前，再訂購這種貨。對一項商品，該任務是容易完成的，只要計算一下手頭現(xiàn)存的貨物數(shù)量就行了。

79、遺憾的是商店要經(jīng)管幾百種不同的貨物，始終跟蹤每一種貨物的銷售情況是不實際的，也許計算機可以幫這個忙吧！</p><p><b>　　2.1問題定義</b></p><p>　　在系統(tǒng)分析和設計過程中的第一步，是問題的定義。分析員的目標是確定用戶需求什么（本例中就是店主需求什么）。注意，當這個過程開始時，用戶擁有很重要的資料，分析員必須認真地聽和學。用戶幾乎都不是計算機

80、方面的專家，他們大多數(shù)人把計算機看成魔術盒，并不關心它是如何工作的。在該階段上，分析員不必考慮程序、文件或計算機硬件，但他們必須用自己的專業(yè)術語與用戶對話。</p><p>　　目的是確保用戶和分析員兩者都思考同一件事。因此，一份能表達分析員對問題理解程度的清晰的書面報告是必要的。用戶應該反復地閱讀和修改這份書面報告。此時是在時間、金錢和精力被浪費之前抓出錯誤和疏漏的好時機。</p><p&g

81、t;　　通常，接著初步的問題定義，分析員要進行可行性研究。該研究是整個系統(tǒng)分析和設計的一個簡略的方案，應力圖回答以下三個問題：</p><p><b>　　1.問題能解決嗎？</b></p><p>　　2．在用戶的環(huán)境下問題能解決嗎？</p><p>　　3.在一個合適的花費上問題能解決嗎？</p><p>　　如果這

82、些問題中的任何一個得到的是否定回答，那么該系統(tǒng)就不應開發(fā)。有了好的問題定義和肯定的可行性研究，分析員就能著手計劃和研究問題的解了。</p><p><b>　　2.2分析</b></p><p>　　當分析開始時，分析員要理解問題，下一步要決定的是為了解決問題必須做什么。用戶清楚必須做什么。在分析階段礙到了這方面知識，并正式地形成文件。大多數(shù)用戶是按著所要完成的功能和

83、要處理的數(shù)據(jù)元素去思考的，目的是要區(qū)分并聯(lián)接這些關鍵的功能和數(shù)據(jù)元素，隨之產(chǎn)生邏輯系統(tǒng)設計。</p><p>　　從系統(tǒng)的基本功能入手，關鍵是始終監(jiān)視貨單中每種商品的現(xiàn)存量。因為顧客購買、換貨和退貨，所以貨單要改變，因此系統(tǒng)必須處理顧客辦理的手續(xù)。店主希望有選擇地查看供應中任意一種短缺商品的清單，如果合理的話，定貨補充庫存，因此系統(tǒng)必須能和經(jīng)營管理部門對話。最后，經(jīng)過經(jīng)營管理部門的核準，系統(tǒng)應該產(chǎn)生一個重新訂貨單

84、，準備發(fā)送給供應商。</p><p><b>　　圖 1</b></p><p>　　已知了系統(tǒng)的基本功能，分析員的下一個任務是分析理解這些功能之間邏輯關系的知識。啟動這項工作的一個好方法是描述功能之間的數(shù)據(jù)如何流動．顧名思義，為了用圖解方法描述這些數(shù)據(jù)流，那么數(shù)據(jù)流程圖就是特別有用的．圖1中用到四種符號數(shù)據(jù)的像和目的地用方框表示，輸入的數(shù)據(jù)從源進入系統(tǒng)，而輸出的數(shù)據(jù)

85、流到目的地。數(shù)據(jù)一進入系統(tǒng)，就被若干進程加工或改變，用圓角的矩形表示這些過程。進程可以是程序、過程以及能夠改變或傳送數(shù)據(jù)的任何事件．為了后續(xù)處理，數(shù)據(jù)被保留在數(shù)據(jù)存儲器內，這可用末端開口的矩形符號表示。數(shù)據(jù)存儲可以是磁盤文件、磁帶文件、數(shù)據(jù)庫、一些筆記或者甚至是人的記憶。最后，數(shù)據(jù)源、數(shù)據(jù)目的地、處理過程和數(shù)據(jù)存儲之間的數(shù)據(jù)流向用箭頭表示。</p><p>　　圖2展示了貨單管理系統(tǒng)的初步的數(shù)據(jù)流程圖。從CUST

86、OMER（顧客）開始，由此事務進入系統(tǒng)，在這所辦理的手續(xù)由PROCESS TRANSATION（事務處理程序）處理。STOCK保存貨單中每項商品的數(shù)據(jù)．為了對新的事務處理起作用，事務處理程序要改變數(shù)據(jù)。同時，MANAGEMENT（經(jīng)營部門）可通過COMMUNICATE（通信）存取系統(tǒng)，檢查STOCK中的數(shù)據(jù)，如果需要的話，請求重新訂貨．訂貨單一經(jīng)核準。GENERATEREORDER（產(chǎn)生訂貨單程序）就向SUPPLIER（供應商）發(fā)出需要

87、的數(shù)據(jù)，供應商就把貨運送到商店。注意，因為訂貨表現(xiàn)為某種待定的商品或某些商品在清單中的變化，所以把訂貨作為事務處理。 </p><p><b>　　圖 2</b></p><p>　　數(shù)據(jù)流程圖描述了邏輯系統(tǒng)。下一步該追溯該系統(tǒng)的數(shù)據(jù)流向了。從數(shù)據(jù)的目的地SUPPLIEF開始。例如，重訂貨單送給了供貨商們，商店可能想要25條工作褲，為了填寫訂貨單，供應商需要商品的

88、說明和再訂貨的數(shù)量，那么，這些數(shù)據(jù)從什么地方來呢？由于數(shù)據(jù)是Generate reorder輸出的，所以既要有數(shù)據(jù)輸入給它，還要有數(shù)據(jù)經(jīng)過它的處理而產(chǎn)生。數(shù)據(jù)從STOCK流入Generate reorder,因此商品說明和訂貨數(shù)量必定存放在STOCK中。</p><p>　　另外一些數(shù)據(jù)，例如購買的貨物品種和數(shù)量由CUSTOMER產(chǎn)生，還有一些數(shù)據(jù)，例如銷售價格和訂貨點由MANAGEMENT產(chǎn)生，或者由它提出要求

89、。例如，某種已知貨物的現(xiàn)存量就是由某個處理過程中的某種算法產(chǎn)生出來的。分析員逐步地、有條理地區(qū)分了系統(tǒng)要求輸入、存儲、處理、產(chǎn)生或輸出的應該分別是哪些數(shù)據(jù)元素。</p><p>　　為了記錄數(shù)據(jù)元素，分析員必須把每個數(shù)據(jù)列在數(shù)據(jù)字典內簡單的數(shù)據(jù)字典可建立在索引卡片上，但計算機化的數(shù)據(jù)字典已經(jīng)變得日益流行了。數(shù)據(jù)字典是描述和定義數(shù)據(jù)的一個數(shù)據(jù)集合，不僅在整個系統(tǒng)的分析與設計過程中有用，而且在實現(xiàn)階段經(jīng)常被用來建立數(shù)

90、據(jù)庫。</p><p>　　分析階段的目標是定義系統(tǒng)的主要功能和有條理地確定數(shù)據(jù)元素。記住，它的目標是把用戶的需求轉換成技術術語。因為系統(tǒng)是由用戶提出的，所以首先要確定用戶的需求。用戶只是從功能和數(shù)據(jù)出發(fā)考慮系統(tǒng)，他們并不具體設計程序、文件和硬件，并且在這個初始的帶有決定性的分析階段，分析員必須象用戶而不象程序設計員一樣去思考問題。這是該階段的基本要求。</p><p>　　數(shù)據(jù)流程圖和數(shù)

91、據(jù)字典是有用的工具。這些工具為記錄用戶系統(tǒng)的關鍵信息提供了一種格式。還有，這些工具可幫助喚醒分析員的記憶。例如，如果分析員沒有足夠的信息使數(shù)據(jù)字典的條目完整化，那么分析員就可能丟失一些內容。也許，數(shù)據(jù)流程圖和數(shù)據(jù)字典為分析員理解系統(tǒng)的需求提供了最重要的文件，通過查閱這些文件，用戶能改正誤解或疏忽大意。最終，這些有用的工具對下一步一設計階段，奠定了一個良好的基礎。</p><p><b>　　2. 3設計

92、</b></p><p>　　當進入設計階段的時候，已知道必須做什么事。因此可以開始考慮系統(tǒng)如何完成這些事．目標是研究出解決問題的策略。在該階段上，對寫代碼或確定精確的數(shù)據(jù)結構并不感興趣，而是希望從整體上確走整個系統(tǒng)所需要的程序、文件、過程和其它一些部分。</p><p>　　數(shù)據(jù)流程圖確定了系統(tǒng)的必須具有的功能，如何才能實現(xiàn)這些功能呢？一種可能性是為每個處理過程寫一個程序；另

93、一種可能性是把兩個或更多的處理過程合并在一個程序里，因而有幾十種可能的方案。讓我們集中在一種選擇上并為它提供一些詳細的資料。</p><p>　　系統(tǒng)流程圖用符號表示程序、過程、硬設備及物理系統(tǒng)中的其它部件（圖4),流程圖（圖5）表明，通過終端進入系統(tǒng)的事務數(shù)據(jù)被數(shù)據(jù)采集程序處理，然后存放在貨單文件上。最后，貨物清單文件由一個Report and reorder（報告和訂貨）程序處理。經(jīng)營部門用該程序處理數(shù)據(jù)和核

94、對訂貨單。</p><p>　　請看系統(tǒng)流程圖，它指明了若干硬部件，其中包括一臺計算機、一個磁盤驅動器、一個數(shù)據(jù)輸入終端、一臺打印機和一個顯示終端等；還需要兩個程序，事務處理程序和報告與再定貨程序；除了硬件和程序以外，我們還需要貨單文件以及I/O設備與軟件之間數(shù)據(jù)流的數(shù)據(jù)結構。注意，本系統(tǒng)的流程圖只說明一種可能的方案。一個好的系統(tǒng)分析員在他選定一個方案之前要研究出若干個可行的方案，以備替換。</p>

95、<p><b>　　圖3</b></p><p><b>　　圖 4</b></p><p>　　流程圖擬訂了系統(tǒng)，突出了系統(tǒng)的主要物理部件。因為數(shù)據(jù)鏈接了部件，所以接下來的工作就是確定數(shù)據(jù)結構。以貨物清單文件為例，它包含來自STOCK的所有數(shù)據(jù)元素，這些數(shù)據(jù)元素被列在數(shù)據(jù)字典中．利用這些數(shù)據(jù)元素能安排文件的數(shù)據(jù)結構。</p&g

96、t;<p>　　如何組織文件呢？這取決于如何訪問它。例如，在某些應用中數(shù)據(jù)是在有規(guī)律的預定時間內被處理，一般的是整段時間地收集數(shù)據(jù)，成批地一起處理。如果可采用批處理的話，那么一個順序文件結構可能是最好的了。</p><p>　　然而．不可能總是等到一批事務都被收集好以后再去處理。例如，設想一個防空預警系統(tǒng)，如果一個不明的飛行物被標定了位置，就必須立即分辨它。因為防空程序下午五時才運行．要等到那時才處

97、理的思想是荒廖的。該種情況要求快速響應，每個事務必須在它發(fā)生時得到處理。一般來說，這些事務處理系統(tǒng)要求直接存取文件。</p><p>　　我們的貨單系統(tǒng)有二個程序，一個事務處理，直接存取貨單文件似乎是個合適的方案；另一個為經(jīng)營部門偶爾分析貨物清單數(shù)據(jù)用，當然批處理就能完成了。貨物清單文件應按順序組織還是直接組織呢？面對著這種選擇，一個好的系統(tǒng)分析員應考慮兩種方案。一種可能的系統(tǒng)是當事務一發(fā)生就接收并處理該事務；另

98、一種可供選擇的方案是可整天的收集銷售單，商店關門以后，成批地處理。在第一個系統(tǒng)中，這兩個程序應處理直接存取文件；在第二個系統(tǒng)中，這兩個程序應鏈接到順序文件上。處理可直接訪問的數(shù)據(jù)的程序和處理可按順序一個接一個訪問的數(shù)據(jù)的程序是不同的。數(shù)據(jù)驅動了系統(tǒng)，所以數(shù)據(jù)結構的選擇決定了程序的結構。注意，程序在系統(tǒng)的意義上被定義和規(guī)劃。</p><p><b>　　2.4實現(xiàn)</b></p>

99、<p>　　系統(tǒng)的主要部分分析清楚了，就能開始開發(fā)這幾部分。本系統(tǒng)包括兩個程序、若干臺設備和大量的數(shù)據(jù)結構。在實現(xiàn)階段，每個程序都用應用軟件技術來編排和書寫；要建立文件并檢查文件的內容：購買、安裝和測試新的硬件；另外還要寫出操作步驟，并對系統(tǒng)做出評價。所有的部分都準備好了，就調試系統(tǒng)。若用戶滿意了，系統(tǒng)就交付使用。</p><p><b>　　2.5維護</b></p>

100、;<p>　　系統(tǒng)交付以后，維護就開始了。當人們使用系統(tǒng)時，他們將提出一些小的改善和提高．有時在調試和檢測階段一些缺陷被忽略了，那么排除這些缺陷就是另一個維護任務了。最后還有，條件改變了，必須修改程序，例如，政府通過了一個法律，修改了征收所得稅的方法，則工資程序就必須修改。維護延續(xù)在系統(tǒng)的整個生命期內。維護的費用和初期的研制費用差不多，甚至更高。良好的計劃、實用的文件資料和合適的程序結構都有助于減少維護費用。</p&