© 2008 Prentice Hall, Ovidiu Noran Lecture 7a 1 Modelling Logic Requirements (Textbook Chapter 8)

© 2008 Prentice Hall, Ovidiu NoranLecture 7a 1

Lecture 7a

Modelling Logic Requirements

(Textbook Chapter 8)


Learning Objectives

Use Structured English as a tool for representing steps in logical processes in data flow diagrams.

Use decision tables to represent the logic of choice in conditional statements.

Select among Structured English and decision tables for representing processing logic.


Logic Modelling

Data flow diagrams do not show the logic inside the processes or the contents of data flows.

Logic modelling involves representing internal structure and functionality of processes depicted on a DFD.

Logic modelling can also be used to show when processes on a DFD occur.

Data modelling is used to represent the contents and structure of the DFD data flows & data stores. Data Modelling is covered in 2002ICT.


Modelling a System’s Logic


Deliverables and Outcomes

Structured English representation of process logic.

Decision Tables representation. Sequence diagram (mainly OO approach). Activity diagram (mainly OO approach).


Modelling Logic with Structured English

Structured English: modified form of English language used to specify the logic of information system processes.

No single standard. Typically relies on action verbs and noun

phrases and contains no adjectives or No specific standards.


Recall the DFDs previously presented...

The Hoosier Burger Example


… and the resulting Level-0 Diagram



The Hoosier Burger Inventory Control System


Modelling Logic with Structured English (cont.)


Repetition can also be modelled using DO-WHILE:


READ Inventory recordsWHILE NOT End-of-File DO

BEGIN IFIF Quantity-in-Stock is less Minimum-order-

quantityTHEN GENERATE new orderELSE DO nothing // optional

END IFEND DO

What is the difference to DO-UNTIL ??


Cascading IF-THEN tests or a case statement can be combined to cover other possibilities. E.g. imagine modelling an emergency reorder:


READ Quantity-in-StockSELECT CASE

CASE 1 (Quantity-in-Stock greater then Minimum-order-quantity)DO nothing

CASE 2 (Quantity-in-Stock equals Minimum-order-quantity)DO nothing

CASE 3 (Quantity-in-Stock is less than Minimum-order-quantity)GENERATE new order

CASE 4 (Stock Out)INITIATE emergency reorder routine

END CASE


Modelling Logic with Decision Tables

Research (Yourdon, 1989) has shown people become confused when trying to interpret more than three nested IF-THENs.

Thus, diagrams are more appropriate for complicated decision logic.

A Decision table is a matrix representation of the logic of a decision which specifies the possible conditions for the decision and the resulting actions.


Modelling Logic with Decision Tables (cont.)



Condition stubs: that part of a decision table that lists the conditions relevant to the decision.

Action stubs: that part of a decision table that lists the actions that result for a given set of conditions.



Rules: that part of a decision table that specifies which actions are to be followed for a given set of condition.

Indifferent condition: in a decision table, a condition whose value does not affect which actions are taken for two or more rules.



Procedure for Creating Decision TablesName the condition and the values that each

condition can assume.Name all possible actions that can occur.List all possible rules.Define the actions for each rule.Simplify the table.



“IF”

“THEN”

“IF-THEN”



Simplified Decision Table – Staff is paid base salary, regardless


Modelling Logic with Decision Tables

Hoosier Burger complete Decision Table



Hoosier Burger simplified Decision Table


Structured English or Decision Tables?

Criteria Structured English

Decision Tables

Determining Conditions and Actions

Ok Average

Transforming Conditions and Actions into Sequence

Best Average

Checking Consistency and Completeness

Average Best


Summary

In this chapter you learned how to:Use Structured English as a tool for

representing steps in logical processes in data flow diagrams.

Use decision tables to represent the logic of choice in conditional statements.

Select Structured English or decision tables for representing processing logic.

© 2008 Prentice Hall, Ovidiu Noran Lecture 7a 1 Modelling Logic Requirements (Textbook Chapter 8)

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