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Chapter 2c

System Analysis

- Logic Modeling

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Learning 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.

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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.

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

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Logic Modeling Deliverables and Outcomes Structured English Decision Tables Decision Trees State-transition diagrams Sequence diagrams Activity diagrams

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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

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Structured English is used here to describe input and output.

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Structured English is used here to describe arithmetic operations.

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Structured English is used here to describe repetition.

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Structured English is used here to describe decisions.

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Structured English is used here to describe invoking other processes.

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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

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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

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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

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Decision Table

Note: for salaried employees the action stub chosen will always be the same…therefore hours worked is an indifferent condition

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Reduced Decision Table

Because of indifferent condition, the complete decision table can be reduced to one with fewer rules

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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

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Modeling Logic with Decision Trees (cont.) Read from left to right Each node corresponds to a

numbered choice on a legend All possible actions are listed on the

far right

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Decision tree representation of salary decision

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Alternative decision tree representation of salary decision

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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

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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

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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.