Session 3b: Defining data items

Size: px

Start display at page:

Download "Session 3b: Defining data items"

Maurice Gilbert
6 years ago
Views:

1 Session 3b: Defining data items Sources of data items Establishing a project data dictionary Defining an elementary item Defining a composite data item COMP 320 / 420, Spring, 2018 Mr. Weisert Q: Which data items need to be formally defined in the ESD? A: Every data item that appears in an output specification a process specification a dataflow diagram another data item definition What else is there? When do we define a data item? The most important ones (esp. those that the sponsoring end users must understand and approve) by the systems analyst as part of ESD. But additional data items are identified and defined, as needed, throughout the project. What constitutes a rigorous data-item definition? A. Composite data item: Purpose, need. List of its component data items Criteria for conditional or repeated inclusion of a component item. B. Elementary data item: Major subtype Unambiguous specification of its meaning Exception: if the data name is truly self-defining (note: very few are) Essential attributes 1. Defining elementary items Many Java programmers often don't bother to use the object paradigm on elementary data items. Why not? But there are great benefits to doing so: In fact, many experts consider it a serious error to use primitive (built-in) types in an O.O. programming language for any data that has a unit of measure. Even in Java Or at least in specification documentation. Why? COMP 320/420, fall, copyright 2006 Conrad Weisert

2 A common objection But it's obvious. Everyone knows what a renewal date is? amount quantity value result total Which data names are self-defining? temperature discount commission totalsales overtimehours Which ones could be class names? Which could be instances of a class (objects)? amount quantity value result total Possible class names Why none of the others? temperature discount commission totalsales overtimehours Common data names to be avoided for members (too generic) amount quantity value result total Exception: An elementary item class with a single member data item (Distance, Weight, Money, etc.) may call that member data item value. Why? But never call an accessor function getvalue()! Why not? COMP 320/420, fall, copyright 2006 Conrad Weisert

3 The data dictionary A repository for data definitions Can be maintained by a data-dictionary (C.A.S.E.) product on paper or a stack of cards in a spreadsheet Its scope can be a single project a company or other organization an industry group Its purposes include: To standardize data names To standardize data representations To clarify data meaning Benefits of data standards Standard data names Facilitate understanding of programs and documents Minimize duplication Standard data representations Simplify data exchange between systems Promote re-usable program modules 2. Defining composite items Examples: EmployeeRecord MailingAddress LibraryItem Notations: Physical layout diagram (obsolete -- Why?) Programming language (augmented struct ) XML schema DeMarco (used in Robertson text ch. 2.9) Easy for untrained users (project sponsors) to understand Attributes of composite data items List of component data items with rules for including them Can often be defined by a language-dependent commented structure definition (C struct). De Marco's language-independent notation for specifying a composite data item: Symbol Meaning = is composed of + followed by ( ) optional component { } iteration [ ] alternatives COMP 320/420, fall, copyright 2006 Conrad Weisert

4 Composite item definition Example using DeMarco notation: EmployeeRecord = name + dateofbirth + + { dependentinfo } datehired + monthlysalary [ ] + (previousemployer) + etc. Is the application system implementation obliged to mimic this exactly? Defining an elementary data item Specify: 1. A unique descriptive data name 2. The type of item What are the possibilities? 3. Essential attributes appropriate to the type may include external representation(s) Why not internal representation? 4. Its exact meaning in the application domain (real world) Discrete Numeric Text Logical Types of elementary item Are there any others? What does each of those signify? How does your favorite programming language specify them,? Which of the four basic subtypes does each of these elementary data items belong to unitcost zipcode datehired homephonenumber insidetemperature batterypower paymentduedate shippingweight courseduration startingtime accountnumber creditlimit creditapproval booktitle altitude COMP 320/420, fall, copyright 2006 Conrad Weisert

5 Should the type of the colored ones be specified as numeric? shippingweight courseduration startingtime accountnumber creditlimit creditapproval booktitle altitude Why? What subclasses are the colored ones instances of? unitcost zipcode datehired homephonenumber insidetemperature batterypower paymentduedate Specifying numeric classes We save work and assure consistency by defining classes for common objects Money unitcost creditlimit Date datehired paymentduedate TimeOfDay startingtime We can do this in the ESD even if we're not using object-oriented technology. Defining a numeric item So, the type of altitude should be specified as Distance, not numeric. The analyst doesn't bother specifying the unit of measure and the precision for altitude--they're inherited from Distance. The Distance generic class may also specify a default external representation. If that's acceptable for altitude, the analyst can ignore it. The analyst has to specify a couple of other attributes of a numeric data item. Numeric data item attributes unit of measure Note: Programmers are not obliged to use it as the internal representation range minimum and maximum possible values precision smallest meaningful increment default external representation When an item inherits from a class, the analyst specifies only what's special to the item, often just the range. COMP 320/420, fall, copyright 2006 Conrad Weisert

6 Numeric data item definition -- Example Name: monthlysalary (of an employee) Type: Money Range: <$1,200, $400,000> Meaning: Base monthly salary as of the first day of the current month. Kinds of Discrete data item: A. Limited (small) set of possible values Known now, unlikely to grow much [single, married, widowed, divorced, unknown] sex [male, female, unknown] state (of the U.S.) B. Larger set of possible values How is type A often implemented for input editing? color C.Unlimited (huge) set of possible values telephonenumber empoyeeid How do we specify each? Defining a text data item Many examples are names of something personname booktitle companyname They often have some structure format, punctuation, maximum length Others are descriptions of something productdescription Or free text customerreview Representing text data Unlike many other programming languages (Cobol, PL/I, Basic) C, a small language, supported no text (or string) data type. Why? Instead, C programmers could define an array of single characters (bytes). Clumsy and error prone. With OOP (C++, Java, C#) the array handling could then be hidden inside objects of a string class. That works well. As a consequence programmers who are used to one of the C-family of languages, may think that a text data item is naturally just a kind of array! (It isn't, but that shouldn't concern the systems analyst.) COMP 320/420, fall, copyright 2006 Conrad Weisert

7 Defining a composite data item A composite item is made up of a fixed arrangement of other data items. It's sometimes (traditionally) called a record (or a dataflow or a strtucture). It's defined in terms of its components. But we may also specify its purpose or usage. We hope those components don't change often. COMP 320/420, fall, copyright 2006 Conrad Weisert

Ways of documenting Session 5: detailed requirements The Data Dictionary one any The Project A data dictionary Data Dictionary may be maintained

Ways of documenting Session 5: detailed requirements The Data Dictionary one any The Project A data dictionary Data Dictionary may be maintained Session 5: The Data Dictionary relationship to systems analysis methodologies relationship to project management data definition vs. data representation taxonomy of data types COMP 477 /377, Spring, 2017