Chapter 8 Structuring System Logical Requirements.

Chapter 8 Structuring System Logical Requirements

Objectives:

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.

Logic Modeling

Data flow diagrams do not show the logic inside the processes.

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

• Processes must be clearly described before they can be translated into a programming language.

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

Logic Modeling Deliverables and Outcomes

Structured English Decision Tables Decision Trees State-transition diagrams Sequence diagrams Activity diagrams

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 Can represent sequence, conditional statements

(If-Then or Case) and repetition (Do-Until, Do-While)

Structured English is used here to describe input and output.

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.

Modeling Logic with Decision Tables

A matrix representation of the logic of a decision Specifies the possible conditions and the

resulting actions Best used for complicated decision logic

Decision Table

3 Parts of a Decision Table

1. Condition stubs Lists condition relevant to decision

2. Action stubs Actions that result from a given set of conditions

3. Rules Specify which actions are to be followed for a given

set of conditions

Indifferent Condition• Condition whose value does not affect which action

is taken for two or more rules

Procedure for Creating Decision Tables

Name the condition and values each condition can assume

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

Reduced Decision Table

Decision Table - Benefits

Model complicated logic of a process Tabular, compact Can check that logic is complete, consistent and

not redundant

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

Read from left to right Each node corresponds to a numbered choice on a

legend All possible actions are listed on the far right

Deciding Among Structured English, Decision Tables, and Decision Trees

Criteria Structured English

Decision Tables

Decision Trees

Determining Conditions and Actions

Second Best Third Best Best

Transforming Conditions and Actions into Sequence

Best Third Best Best

Checking Consistency and Completeness

Third Best Best Best

Deciding Between Decision Tables and Decision Trees

Criteria Decision Tables Decision Trees

Portraying complex logic

Best Worst

Portraying simple rules

Worst Best

Making decisions Worst Best

More compact Best Worst

Easier to manipulate Best Worst