Chapter 8Structuring Requirements

Logic Modeling

Agenda

Overview of Requirements Structuring

Logic & Event-Response Modeling– Structured English– Decision Tables– Decision Trees

Exercises

Learning Objectives

Use Structured English, decision tables, decision trees for logic modeling

Select among each logic modeling techniques

Structuring System Requirements Models for

– Data movement (DFD, process model)– Data structure and rules (ERD, business

rules)– Processing logic and rules (this chapter)

Need all these types of models to get a more complete picture for code generation

Logic & Event-Response Modeling Rules which govern the manipulation of

data within system processes (typically primitive processes)

Rules which govern what actions are taken under what conditions/events

Common notations are:– Structured English– Decision trees – Decision tables

Modeling Logic with Structured English Modified form of English used to

specify the logic of information processes

Uses a subset of English– Action verbs– Noun phrases– No adjectives or adverbs

No specific standards

Structured English is used here to refer to input.

Structured English is used here to describe arithmetic operations.

Structured English is used here to describe repetition.

Structured English is used here to describe decisions.

Structured English is used here to describe invoking other processes.

Decision TablesWhat are they?

When would they be useful (what situations or types of decisions?)

How might a decision table be represented within programming code?

Decision Table Simplification

C1 Y N Y N Y N Y N Y N Y NC2 Y Y N N Y Y N N Y Y N NC3 A A A A B B B B C C C CAct 1 X X XAct 2 X X XAct 3 X XAct 4 X X X X

Modeling Logic with Decision Trees A graphical representation of a decision

situation Decision situation points are connected

together by arcs and terminate in ovals Main components

– Decision points represented by nodes– Actions represented by ovals– Particular choices from a decision point

represented by arcs

Decision tree representation of salary decision

Decision Tree for Prior Problem

a. Graphical tool--can be used to represent the same information as shown on the decision table

- Nodes

- Branches

b. How can decision trees be different than decision tables?

c. Construct a decision tree for decision table problem

Which Tool Do I Use?

Criteria Structured

English

Decision

Tables

Decision Trees

Determining conditions & actions

Transforming conditions & actions into a sequence

Checking consistency & completeness

Policy Modeling ExampleTravel Reimbursement Policy: First we determine if it is a

local trip. If it is, we only pay mileage of 18.5 cents a mile. If the trip was a one-day trip, we pay mileage and then check the times of departure and return. In order to reimbused for breakfast, you must leave by 7am and return later than 10am; for lunch, leave by 11am and return later than 2pm; and for dinner, leave by 5pm and return later than 7pm. On a trip lasting more than one day, we allow hotel, taxi, and airfare, as well as meal allowances as indicated above.

Write a decision table.Write a decision tree.Write structured EnglishWhich do you prefer? Why?

More Practice Exercises

Problems & Exercises 11, 13 (pp. 261)– Develop Structured English routine(s)– Develop decision table(s)– Develop decision tree(s)

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 and decision trees to represent logical choice in conditional statements.

Select among structured English, decision tables, and decision trees.