### Data Dictionary Notation

The second major component of the structured analysis model of the system is the data dictionary. The data dictionary contains formal definitions of all the data items shown in the data-flow diagrams. If you didn't create data flow diagrams in your analysis, then in general, every data item in your solution must be included in the data dictionary. If you wrote Use Cases, then every noun that isn't an Actor is a data item or attribute. It is used to provide precise detail concerning the data entities. Importantly the dictionary items are defined as they are found in the problem domain, not in the implementation or solution domain. The items in the data dictionary will become candidates for classes in the design phase.

The data dictionary has two different kinds of items: composite data and elemental data.
Higher-level (composite) elements may be defined in terms of lower-level items. Elemental data are items that cannot be reduced any further and are defined in terms of the values that it can assume or some other unambiguous base type.

The general format of a data dictionary is similar to a standard dictionary; it contains an alphabetically ordered list of entries. Each entry consists of a formal definition and verbal description. The verbal description is simply a sentence or two in English describing the data element. The formal description provides a more precise definition using a mathematical sort of notation.

COMPOSITE DATA

Composite data can be constructed in three ways: sequence, repetition, or selection of other data types.

 sequence: + A plus sign indicates one element is followed by or concatenated with another element. repetition: [ ] | Square brackets are used to enclose one or more optional elements.  A vertical line stands for "or" and is used to indicate alternatives. selection: {} {}y Curly braces indicate that the element being defined is made up of a series of repetitions of the element(s) enclosed in the brackets. The upper limit on the number of allowable repetitions can be indicated by including an integer superscript on the right bracket. Here y represents an integer and indicates that the number of repetitions cannot be greater than y.

Examples

sequence:

repetition:

Completed-order = [ item-ordered ]
* A complete customer order *

selection:

Atm-transaction = { deposit | withdrawal }
* An customer transaction at an automated teller machine *

In these examples asterisks are used to delimit the comment or verbal description of the item, but other notations can be used as well.

ELEMENTAL DATA

Elemental data is described in terms of a data type or by listing the values that the item can assume.

desired-temperature  = floating point

*Desired-temperature is the value the user sets for desired water temperature in the pool.  It is a floating point number in the range from 0.0 to 100.0, inclusive. The units are Celsius.*

age = non-negative integer
*Age is how old the customer is in years.  Age is a whole number greater than or equal to zero.*

performances-attended = counter
* Performances-attended is the number of performances this customer has attended.  Note: Since the person doesn't get entered into the mailing list until they have attended a show this value can never be zero.  *

counter = positive integer
*Counter is a
whole number greater than zero that can only be incremented by one and never decremented.*

Formal data dictionary entries can easily be created for more unusual sorts of data elements as well. The example below shows how a magazine subscription system might contain category-type information. In this case the data items are enumerated types and the definition lists all the allowed values. Do not create a separate entry for each enumerated value. If necessary, the meaning for the enumerated values can be explained in the supplementary notes.

Transaction-Type = { Renewal | New-Subscription | Cancellation }

Transaction-Type identifies the kind of transaction.
New_Subscription is someone signing up for the first time.
Renewal is a previous subscriber who is renewing.
Cancellation is a subscriber terminating their subscription.

Payment-Method = { cash | check | money-order | charge-card }

Payment-Method is the method by which a customer paid for their transaction. (The values should be self-explanatory).

Furthermore, details concerning input or output formats can be specified in the data dictionary. Suppose the subscription system will print a monthly report of new subscribers. In the data-flow diagram, one activity is shown that produces a data element as output, New-Subscribers-Report. In the data dictionary, the format of the report could be defined as follows:

New-Subscribers-Report = report-header + new-subscriber-list + report-summary

report-title = 'Monthly Report of New Subscribers'

new-subscriber-list = [ subscriber-name + subscriber-address ]

report-summary = 'The total number of new subscribers:' + Number-of-new-subscribers

Any non-terminal terms used in these definitions must be defined elsewhere in the data dictionary.  Single quote marks are used to surround and indicate literals.

### HTML example

Sample HTML formatted data dictionary for Hangman

 __________________________________________________________
 name guess count
 alias turn count
 description The number of incorrect guesses the player has made this game.
 definition 0 <= guess count <= guess limit
 supplementary
 __________________________________________________________
 name guess limit
 alias none
 description the number of guesses a user is allowed before they lose the game.
 definition guess limit = 7
 supplementary
 __________________________________________________________
 name hidden word
 alias secret word
 description the word the player is trying to guess
 definition hidden word = [ alphabetic character ]30
 supplementary
 __________________________________________________________
 name guessed word
 alias none
 description the letters that the player has correctly guessed, in their proper position in the word.
 definition guessed word = [ alphabetic character ]30
 supplementary
 __________________________________________________________
 name continue choice
 alias none
 description whether the user wants to play again or not
 definition continue choice = {yes | no }
 supplementary
 __________________________________________________________

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