外文翻譯----系統(tǒng)的分析與設(shè)計(jì)

1、<p><b>　　系統(tǒng)的分析與設(shè)計(jì)</b></p><p><b>　　系統(tǒng)</b></p><p>　　在存儲(chǔ)程序的控制下，計(jì)算機(jī)把數(shù)據(jù)處理成信息。對(duì)系統(tǒng)的定義略加思考，任何一個(gè)已知的計(jì)算機(jī)應(yīng)用至少包含三個(gè)部分，硬件、軟件和數(shù)據(jù)。僅僅編寫(xiě)程序是不夠的，因?yàn)槌绦蛑皇窍到y(tǒng)中的一部分。</p><p>　　系統(tǒng)就是

2、為實(shí)現(xiàn)一個(gè)目標(biāo)而共同工作的一組部件。例如，考慮一個(gè)工資系統(tǒng)，它的目標(biāo)是為雇員付工資，應(yīng)包含哪幾個(gè)部分呢？每天雇員們把他們工作的時(shí)數(shù)記錄在計(jì)時(shí)卡上，每周末把計(jì)時(shí)卡收集起來(lái)，送給計(jì)算中心，在計(jì)算中心把計(jì)時(shí)卡上的數(shù)據(jù)讀給工資程序。當(dāng)工資程序執(zhí)行時(shí)，程序存取數(shù)據(jù)文件。最后，打印出工資單，用來(lái)分發(fā)。為使系統(tǒng)工作，人、處理過(guò)程、輸入和輸出介質(zhì)、文件、硬件和軟件都必須認(rèn)真地協(xié)調(diào)。注意，程序只是系統(tǒng)中的一部分。</p><p>

3、<b>　　系統(tǒng)分析</b></p><p>　　因?yàn)槿藗冃枰畔?，所以要開(kāi)發(fā)基于計(jì)算機(jī)的系統(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(jì)算機(jī)的專門知識(shí)。計(jì)算機(jī)技術(shù)方面的專業(yè)人員，例如程序設(shè)計(jì)員有這方面的專門知識(shí)，但是可能在用戶的專業(yè)領(lǐng)域內(nèi)缺少訓(xùn)練。麻煩的是，用戶和程序似乎常常講不同的語(yǔ)言，導(dǎo)致了兩者之間的聯(lián)系障礙。系統(tǒng)分析員是一個(gè)專業(yè)人員，他能把用戶的需求轉(zhuǎn)換

4、成計(jì)算機(jī)技術(shù)術(shù)語(yǔ)因此他是用戶和技術(shù)人員之間的橋梁。</p><p>　　像工程師建筑師一樣，系統(tǒng)分析員把扎實(shí)的技術(shù)技能與見(jiàn)識(shí)，想象力和一點(diǎn)藝術(shù)結(jié)合起來(lái)去解決問(wèn)題。 通常，分析員遵循一個(gè)意義明確的有條理的過(guò)程，至少包括以下幾個(gè)步驟:</p><p><b>　　1.問(wèn)題的定義</b></p><p><b>　　2.分析</b

5、></p><p><b>　　3.設(shè)計(jì)</b></p><p><b>　　4.實(shí)現(xiàn)</b></p><p><b>　　5.維護(hù)</b></p><p>　　在每個(gè)步驟結(jié)尾，研究的結(jié)果都要形成文件，提供給用戶和程序設(shè)計(jì)員。 基本的思想是要盡早地抓住和修正錯(cuò)誤以及

6、一些未理解之處。 也許通過(guò)一個(gè)例子說(shuō)明該過(guò)程是最好的方法。</p><p>　　設(shè)想一個(gè)小服裝店，成批的購(gòu)買貨物,所貨物擺在貨架上, 零售給顧客。 一方面,庫(kù)存太多會(huì)造成不必要的開(kāi)銷；另一方面可供挑選的商品太少會(huì)使顧客失去購(gòu)買的信心。 理想上，希望達(dá)到一個(gè)平衡，貨物既充足但又不太多。</p><p>　　麻煩的是，隨著顧客的購(gòu)買庫(kù)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)

7、生變化。店主喜歡按貨物清單購(gòu)銷，剛好在商店售完某種貨物之前，再訂購(gòu)這種貨。對(duì)一項(xiàng)商品，該任務(wù)是容易完成的，只要計(jì)算一下手頭現(xiàn)存的貨物數(shù)量就行了。遺憾的是商店要經(jīng)營(yíng)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(shí)際的，也許計(jì)算機(jī)可以幫這個(gè)忙吧！</p><p><b>　　問(wèn)題定義</b></p><p>　　在系統(tǒng)分析和設(shè)計(jì)過(guò)程中的第一步，是問(wèn)題的定義。分析員的目

8、標(biāo)是確定用戶需求什么。注意，當(dāng)這個(gè)過(guò)程開(kāi)始時(shí)，用戶擁有很重要的資料，分析員必須認(rèn)真地聽(tīng)和學(xué)。用戶幾乎都不是計(jì)算機(jī)方面的專家，他們大多數(shù)人把計(jì)算機(jī)看成魔術(shù)盒，并不關(guān)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(jì)算機(jī)硬件，但他們必須用用戶自己的專業(yè)術(shù)語(yǔ)與用戶對(duì)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(dá)分析員對(duì)問(wèn)題理解程度的清晰的書(shū)面報(bào)告是必要的。用戶應(yīng)該反復(fù)地閱讀

9、和修改這份書(shū)面報(bào)告。此時(shí)是在時(shí)間、金錢和精力被浪費(fèi)之前抓出錯(cuò)誤和疏漏的好時(shí)機(jī)。</p><p>　　通常,接著初步的問(wèn)題定義,分析員要進(jìn)行可行性研究。該研究是整個(gè)系統(tǒng)分析和設(shè)計(jì)一個(gè)簡(jiǎn)略的方案,應(yīng)力圖回答以下三個(gè)問(wèn)題:</p><p>　　1. 問(wèn)題能解決嗎?</p><p>　　2. 在用戶的環(huán)境下問(wèn)題能解決嗎?</p><p>　　3.在一

10、個(gè)合適的花費(fèi)上問(wèn)題能解決嗎?</p><p>　　如果這些問(wèn)題中的任何一個(gè)得到的是否定回答,那么該系統(tǒng)就不應(yīng)開(kāi)發(fā)。有了好的問(wèn)題定義和肯定的可行性研究, 分析員就能著手計(jì)劃和研究問(wèn)題的解了。</p><p><b>　　分析</b></p><p>　　當(dāng)分析開(kāi)始時(shí)，分析員要理解問(wèn)題，下一步要決定的是為了解決問(wèn)題必須做什么。用戶清楚必須做什么。在

11、分析階段得到了這方面知識(shí)，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(guān)鍵的功能和數(shù)據(jù)元素，隨之產(chǎn)生邏輯系統(tǒng)設(shè)計(jì)。</p><p>　　從系統(tǒng)的基本功能入手，關(guān)鍵是始終監(jiān)視貨單中每種商品的現(xiàn)存量。因?yàn)轭櫩唾?gòu)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(yīng)中任意一種短缺商品的清單，如果合理的話，定貨補(bǔ)充庫(kù)存，因此系統(tǒng)必須能

12、和經(jīng)營(yíng)管理部門對(duì)話。最后，經(jīng)過(guò)經(jīng)營(yíng)管理部門的核準(zhǔn)，系統(tǒng)應(yīng)該產(chǎn)生一個(gè)重新訂貨單，準(zhǔn)備發(fā)送給供應(yīng)商。</p><p>　　已知了系統(tǒng)的基本功能，分析員的下一個(gè)任務(wù)是分析理解這些功能之間邏輯關(guān)系的知識(shí)。啟動(dòng)這項(xiàng)工作的一個(gè)好方法是描述功能之間的數(shù)據(jù)如何流動(dòng)。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。四種符號(hào)：數(shù)據(jù)的源和目的地用方框表示，輸入的數(shù)據(jù)從源進(jìn)入系統(tǒng)，而輸出的數(shù)據(jù)流到目的地。數(shù)據(jù)一進(jìn)入

13、系統(tǒng)，就被若干進(jìn)程加工或改變，用圓角的矩形表示這些過(guò)程。進(jìn)程可以是程序、過(guò)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)被存儲(chǔ)可以是磁盤文件、磁帶文件、數(shù)據(jù)庫(kù)、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(guò)程和數(shù)據(jù)存儲(chǔ)之間的數(shù)據(jù)流向用箭頭表示。</p><p>　　貨單管理系統(tǒng)的初步的數(shù)據(jù)流程圖。從CUSTOMER（顧客）開(kāi)始，由此事務(wù)進(jìn)入系統(tǒng)，在這所辦理的手續(xù)由PROCESS　TRANSA

14、CTION（事務(wù)處理程序）處理。STOCK保存貨單中每項(xiàng)商品的數(shù)據(jù)。為了對(duì)新的事務(wù)處理起作用，事務(wù)處理程序要改變數(shù)據(jù)。同時(shí)，MANAGEMENT（經(jīng)營(yíng)部門）可通過(guò)COMMUNICATE（通信）存取系統(tǒng)，檢查STOCK中的數(shù)據(jù)，如果需要的話，請(qǐng)求重新訂貨。訂貨單一經(jīng)核準(zhǔn)，GENERATE　REORDER（產(chǎn)生再訂貨單程序）就向SUPPLIER（供應(yīng)商）發(fā)出需要的數(shù)據(jù)，供應(yīng)商就把貨運(yùn)送到商店。注意，因?yàn)橛嗀洷憩F(xiàn)為某種待定的商品或某些商品在清

15、單中的變化，所以把訂貨作為事務(wù)處理。</p><p>　　數(shù)據(jù)流程圖描述了邏輯系統(tǒng)。下一步追溯該系統(tǒng)的數(shù)據(jù)流向了。從數(shù)據(jù)的目的地SUPPLIER開(kāi)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(xiě)訂貨單，供應(yīng)商需要商品的說(shuō)明和再訂貨的數(shù)量，那么，這些數(shù)據(jù)從什么地方來(lái)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(guò)它的處理而產(chǎn)生。數(shù)據(jù)從STOCK流入Ge

16、nerate reorder，因此商品說(shuō)明和訂貨數(shù)理必定存放在STOCK中。</p><p>　　另外一些數(shù)據(jù)，例如購(gòu)買的貨物品種和數(shù)理由CUSTOMER產(chǎn)生。還有一些數(shù)據(jù)，例如銷售價(jià)格和訂貨點(diǎn)由MANAGEMENT產(chǎn)生，或者由它提出要求。例如，某種已知貨物的現(xiàn)存量就是由某個(gè)處理過(guò)程中的某種算法產(chǎn)生出來(lái)的。分析員逐步地、有條理地區(qū)分了系統(tǒng)要求輸入、存儲(chǔ)、處理、產(chǎn)生或輸出的應(yīng)該分別是哪些數(shù)據(jù)元素。</p>

17、;<p>　　為了記錄數(shù)據(jù)元素，分析員必須把每個(gè)數(shù)據(jù)列在數(shù)據(jù)字典內(nèi)。簡(jiǎn)單的數(shù)據(jù)字典可建立在索引卡片上，但計(jì)算機(jī)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(gè)數(shù)據(jù)集合，不僅在整個(gè)系統(tǒng)的分析與設(shè)計(jì)過(guò)程中有用，而且在實(shí)現(xiàn)階段經(jīng)常被用來(lái)建立數(shù)據(jù)庫(kù)。</p><p>　　分析階段的目標(biāo)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(biāo)是把用戶的需求轉(zhuǎn)換成技術(shù)術(shù)語(yǔ)。因?yàn)橄到y(tǒng)是由用戶提出的

18、，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(shè)計(jì)程序、文件、和硬件，并且在這個(gè)初始的帶有決定性的分析階段，分析員必須像用戶而不像程序設(shè)計(jì)員一樣去思考問(wèn)題。這是該階段的基本要求。</p><p>　　數(shù)據(jù)流程圖和數(shù)據(jù)字典是有用的工具。這些工具為記錄用戶系統(tǒng)的關(guān)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(méi)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

19、能丟失一些內(nèi)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(guò)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(duì)下一步――設(shè)計(jì)階段，奠定了一個(gè)良好的基礎(chǔ)。</p><p><b>　　設(shè)計(jì)</b></p><p>　　當(dāng)進(jìn)入設(shè)計(jì)階段的時(shí)候，已知道必須做什么事。因此可以開(kāi)始考慮系統(tǒng)如何完成這些事。目標(biāo)是研究出解決問(wèn)題的策略。在該階

20、段上，對(duì)寫(xiě)代碼或確定精確的數(shù)據(jù)結(jié)構(gòu)并不感興趣，而是希望從整體上確定整個(gè)系統(tǒng)所需要的程序、文件、過(guò)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(shí)現(xiàn)這些功能呢？一種可能性是為每個(gè)處理過(guò)程寫(xiě)一個(gè)程序；另一種可能性是把兩個(gè)或更多的處理過(guò)程合并在一個(gè)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(xì)的資料。</p><p>　　系統(tǒng)流程

21、圖用符號(hào)表示程序、過(guò)程、硬設(shè)備及物理系統(tǒng)中的其它部件。流程圖表明，通過(guò)終端進(jìn)入系統(tǒng)的事務(wù)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(gè)Report and reorder（報(bào)告和再訂貨）程序處理。經(jīng)營(yíng)部門用該程序處理數(shù)據(jù)和核對(duì)訂貨單。</p><p>　　請(qǐng)看系統(tǒng)流程圖，它指明了若干硬部件，其中包括一臺(tái)計(jì)算機(jī)、一個(gè)磁盤驅(qū)動(dòng)器、一個(gè)數(shù)據(jù)輸入終端、一臺(tái)打印機(jī)和一個(gè)顯示終端等；還需要兩個(gè)程序，事

22、務(wù)處理程序和報(bào)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(shè)備與軟件之間數(shù)據(jù)流的數(shù)據(jù)結(jié)構(gòu)。注意，本系統(tǒng)的流程圖只說(shuō)明一種可能的方案。一個(gè)好的系統(tǒng)分析員在他選定一個(gè)方案之前要研究出若干個(gè)可行的方案，以備替換。</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?yàn)閿?shù)據(jù)鏈接了部件，所以接下來(lái)的工作就是確定數(shù)據(jù)結(jié)構(gòu)。以貨物清單文件為例，它包含來(lái)自STOCK的所有數(shù)據(jù)元素，這些數(shù)據(jù)元素被

23、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(jié)構(gòu)。</p><p>　　如何組織文件呢？這取決于如何訪問(wèn)它。例如，在某些應(yīng)用中數(shù)據(jù)是在有規(guī)律的預(yù)定時(shí)間內(nèi)被處理，一般是整段時(shí)間收集數(shù)據(jù)，成批結(jié)一起處理。如果可采用批處理的話，那么一個(gè)順序文件結(jié)構(gòu)可能是最好的了。</p><p>　　然而，不可能總是等到一批事務(wù)都被收集好以后再去處理。例如，設(shè)想一個(gè)防空預(yù)警系統(tǒng)，如果一個(gè)不明的飛行物被標(biāo)定了

24、位置，就必須立即分辨它。因?yàn)榉揽粘绦蛳挛缥鍟r(shí)才運(yùn)行，要等到那時(shí)才處理的思想是荒謬的。該種情況要求快速響應(yīng)，每個(gè)事務(wù)必須在它發(fā)生時(shí)得到處理。一般來(lái)說(shuō)，這些事務(wù)處理系統(tǒng)要求直接存取文件。</p><p>　　我們的貨單系統(tǒng)有兩個(gè)程序，一個(gè)事務(wù)處理，直接存取貨單文件似乎是個(gè)合適的方案；另一個(gè)為經(jīng)營(yíng)部門偶爾分析貨物清單數(shù)據(jù)用，當(dāng)然批處理就能完成了。貨物清單文件應(yīng)按順序組織還是直接組織呢？面對(duì)著這種選擇，一個(gè)好的系統(tǒng)分析員

25、應(yīng)考慮兩種方案。一種可能的系統(tǒng)是當(dāng)事務(wù)一發(fā)生就接收并處理該事務(wù)；另一種可供選擇的方案是可整天的收集銷售單，商店關(guān)門以后，成批地處理。在第一個(gè)系統(tǒng)中，這兩個(gè)程序應(yīng)處理直接存取文件；在第二個(gè)系統(tǒng)中，這兩個(gè)程序應(yīng)鏈接到順序文件上。處理可直接訪問(wèn)的數(shù)據(jù)的程序和處理可按順序一個(gè)接一個(gè)訪問(wèn)的數(shù)據(jù)的程序是不同的。數(shù)據(jù)驅(qū)動(dòng)了系統(tǒng)，所以數(shù)據(jù)結(jié)構(gòu)的選擇決定了程序的結(jié)構(gòu)。注意，程序在系統(tǒng)的意義上被定義和規(guī)劃。</p><p><

26、b>　　實(shí)現(xiàn)</b></p><p>　　系統(tǒng)的主要部分一分析清楚了，就能開(kāi)始開(kāi)發(fā)這幾部分。本系統(tǒng)包括兩個(gè)程序、若干臺(tái)設(shè)備和大量的數(shù)據(jù)結(jié)構(gòu)。在實(shí)現(xiàn)階段，每個(gè)程序都用在所描述的技術(shù)來(lái)編排和書(shū)寫(xiě)；要建立文件并檢查文件的內(nèi)容；購(gòu)買、安裝和測(cè)試新的硬件；另外還要寫(xiě)出操作步驟，并對(duì)系統(tǒng)做出評(píng)價(jià)。所有的部分都準(zhǔn)備好了，就調(diào)試系統(tǒng)。若用戶滿意了，系統(tǒng)就交付使用。</p><p><

27、;b>　　維護(hù)</b></p><p>　　系統(tǒng)交付以后，維護(hù)就開(kāi)始了。當(dāng)人們使用系統(tǒng)時(shí)，他們將提出一些小的改善和提高。有時(shí)在調(diào)試和檢測(cè)階段一些缺陷被忽略了，那么修改程序，例如，政府通過(guò)了一個(gè)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(hù)延續(xù)在系統(tǒng)的整個(gè)生命期內(nèi)。維護(hù)的費(fèi)用和初期的研制費(fèi)用差不多，甚至更高。良好的計(jì)劃、實(shí)用的文件資料和合適的程序結(jié)構(gòu)都有助于減少維護(hù)費(fèi)用。</p&g

28、t;<p>　　Systems Analysis and Design</p><p><b>　　Systems</b></p><p>　　Working under control of a stored program, a computer processes data into information. Think about that de

29、finition for a minute. Any given computer application involves at least three components: hardware, software, and data. Merely writing a program isn’t enough, because the program is but one component is in a system.</

30、p><p>　　A system is a group of components that work together to accomplish an objective. For example, consider a payroll system. Its objective is paying employees. What components are involved? Each day, employ

31、ees record their hours worked on time cards. At the end 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 f

32、iles. Finally, the paychecks are printed and distributed. For the system to wo</p><p>　　Systems Analysis</p><p>　　Computer-based systems are developed because people need information. Those peop

33、le, called users, generally know what is required, but may lack the expertise to obtain it. Technical professionals, such as programmers, have the expertise, but may lack training in the user’s field. To complicate matte

34、rs, users and programmers often seem to speak different languages, leading to communication problems. A systems analyst is a professional who translates user needs into technical terms, thus serving a</p><p>

35、;　　Like an engineer or an architect, a systems analyst solves problems by combining solid technical skills with insight, imagination, and a touch of art. Generally, the analyst follows a well-defined, methodical process

36、that includes at least the following steps:</p><p>　　Problem definition</p><p><b>　　Analysis</b></p><p><b>　　Design</b></p><p>　　Implementation&

37、lt;/p><p>　　Maintenance</p><p>　　At the end of each step, results are documented and shared with both the user and the programmers. The idea is to catch and correct errors and misunderstandings as

38、early as possible. Perhaps the best way to illustrate the process is through example.</p><p>　　Picture a small clothing store that purchases merchandise at wholesale, displays this stock, and sells it to cus

39、tomers at retail. On the one hand, too much: stock represents an unnecessary expense. On the other hand, a poor selection discourages shoppers. Ideally, a balance can be achieved: enough, but not too much.</p><

40、;p>　　Complicating matters is the fact that inventory is constantly changing, with customer purchases depleting stock, and returns and reorders adding to it. The owner would like to track inventory levels and reorder a

41、ny given item just before the store runs out. For a single item, the task is easy-just count the stock-on-hand. Unfortunately, the store has hundreds of different items, and keeping track of each one is impractical. Perh

42、aps a computer might help.</p><p>　　Problem Definition</p><p>　　The first step in the systems analysis and design process is problem definition. The analyst’s objective is determining what the u

43、ser needs. Note that, as the process 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 concer

44、ned with how it works. At this stage, the analyst has no business even thinking about programs, files, and computer hardware, but must communicate wit</p><p>　　The idea is to ensure that both the user and th

45、e analyst are thinking about the same thing. Thus, a clear, written statement expressing the analyst’s understanding of the problem is essential. The user should review and correct this written statement. The time to cat

46、ch misunderstandings and oversights is now, before time, money, and effort are wasted.</p><p>　　Often, following a preliminary problem definition, the analyst performs a feasibility study. The study, a brief

47、 capsule version of the entire systems analysis and design process, attempts to answer three questions:</p><p>　　Can the problem be solved?</p><p>　　Can it be solved in the user’s environment?&l

48、t;/p><p>　　Can it be solved at a reasonable 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 feas

49、ibility study, the analyst can turn to planning and developing a problem solution.</p><p><b>　　Analysis</b></p><p>　　As analysis begins, the analyst understands the problem. The next

50、 step is determining what must be done to solve it. The user knows what must be done; during analysis, this knowledge is extracted and formally documented. Most users think in terms of the functions to be performed and t

51、he data elements to be manipulated. The objective is to identify 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

52、 keeping track of the stock-on-hand for each product in inventory. Inventory changes because customers purchase, exchange, and return products, so the system will have to process customer transactions. The store’s owner

53、wants to selectively look at the inventory level for any product in short supply and, if appropriate, order replacement stock, so the system must be able to communicate with management. Finally, following management auth

54、orization</p><p>　　Given the system’s basic functions, the analyst’s next task is gaining a sense of their logical relation. A good way to start is by describing how data flow between the functions. As the n

55、ame implies, data flow diagrams are particularly useful for graphically describing these data flows. Four symbols are used. Data sources and destinations are represented by squares; input data enter the system form a sou

56、rce, and output data flow to a destination. Once in the system, the data are manipulated or </p><p>　　Preliminary data flow diagram for the inventory system. Start a CUSTOMER. Transactions flow from a custom

57、er into the system, where they are handled by Process transaction. A data store, the STOCK, holds data on each item in inventory. Process transaction changes the data to reflect the new transaction. Meanwhile, MANAGEMENT

58、 accesses the system through Communicate, evaluating the data in STOCK and, if necessary, requesting a recorder. Once a reorder is authorized, Generate reorder sends necessary</p><p>　　The data flow diagram

59、describes the logical system. The next step is tracing the data flow. Start with the destination SUPPLIER. Reorders flow to suppliers; for example, the store might want 25 pairs of jeans. To fill the order, the supplier

60、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 generated by this process. Data flow into Generat

61、e reorder for STOCK; thus, product and</p><p>　　Other data elements, such as purchased and the purchase quantity, are generated by CUSTOMER. Still others, for example selling price and reorder point, are gen

62、erated 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. Step by step, methodically, the analyst identifies the da

63、ta 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 might list each one in a data dictionary. A simple data

64、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 defining the data, is useful throughout the systems a

65、nalysis 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’s major functions and data elements methodically. Rem

66、ember 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. Users think in terms of functions and data. They do not visuali

67、ze 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.</p><p>　　Data flow diagrams and data dictionari

68、es 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 analyst doesn’t have sufficient information to complete a data

69、dictionary entry, he or she has probably missed something. Perhaps most importantly, the data flow diagram and the data dictionary document the analyst’s understanding of the system requirements. By reviewing these docum

70、ents, the user can cor</p><p><b>　　Design</b></p><p>　　As we enter the design stage, we know what the system must do, and thus can begin thinking about how to do it. The objective is

71、 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 identify, at a black box level, necessary programs, files,

72、procedures, and other components.</p><p>　　The data flow diagram defines the system’s necessary functions; how might they be implemented? One possibility is writing one program for each process. Another is c

73、ombining two or more processes in a single program; there are dozens of alternative solutions. Let’s focus on one option and document it.</p><p>　　A system flowchart uses symbols to represent programs, proce

74、dures, hardware devices, and the other components of a physical system. Our flowchart shows that transaction data enter the system through a terminal, are stored on an inventory by a data collection program, and then are

75、 stored on an inventory file. Eventually, the inventory file is processed by a Report and reorder program. Through it, management manipulates the data and authorizes reorders.</p><p>　　Look at the system flo

76、wchart. It identifies several hardware components, including a computer, a disk drive, a data entry terminal, a printer, and a display terminal. Tow programs are needed: Process transaction and Report and reorder. In add

77、ition to the hardware and the programs, we’ll need data structures for the inventory file and for data flows between the I/O devices and the software. Note that system flow chart illustrates one possible solution; a good

78、 analyst will develop several feasible</p><p>　　The flowchart maps the system, highlighting its major physical components. Since the data link the components, the next task is defining the data structures. C

79、onsider, for example, the inventory file. It contains all the data elements from the data store STOCK. The data elements are listed in the data dictionary. Using them, the file’s data structure can be planned.</p>

80、<p>　　How should the file be organized? That depends on how it will be accessed. For example, in some applications, data are processed at regular, predictable intervals. Typically, the data are collected over time

81、and processed together, as a batch. If batch processing is acceptable, a sequential file organization is probably best.</p><p>　　It is not always possible to wait until a batch of transactions is collected,

82、however. For example, consider an air defense early warning system. If an unidentified aircraft is spotted, it must be identified immediately; the idea of waiting until 5:00 p. m because “that’s when the air defense prog

83、ram is run” is absurd. Instead, because of the need for quick response, each transaction must be processed as it occurs. Generally, such transaction processing systems call for direct access file.</p><p>　　O

84、ur inventory system has tow programs. One processes transactions. A direct access inventory file seems a reasonable choice. The other allows management to study inventory data occasionally; batch processing would certain

85、ly do. Should the inventory file be organized sequentially or directly? Faced with such a choice, a good analyst considers both options. One possible system might accept transactions and process them as they occur. As an

86、 alternative, sales slips might be collected throughout the</p><p>　　Implementation</p><p>　　Once the system’s major components have been identified, we can begin to develop them. Our system inc

87、ludes two programs, several pieces of equipment, and a number of data structures. During implementation, each program is planned and written using the techniques described in Chapter 7. Files are created, and their conte

88、nts checked. New hardware is purchased, installed, and tested. Additionally, operating procedures are written and evaluated. Once all the component parts are ready, the system is </p><p><b>　　Summary&l

89、t;/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 component in a system.</p><p&g

90、t;　　System are planned and designed by system analysts who generally follow a well-defined, methodical process. The first step in the process is problem definition, when the analyst attempts to discover exactly what the

91、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 definition, analysis begins. During t

92、his 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 system’s data elements are identified a

93、nd 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 physical system. A system flowchart can be use

94、d 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. Implementation follows design. Programs are pla