Chapter 8 Structuring System Logical Requirements.
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Transcript of Chapter 8 Structuring System Logical Requirements.
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Chapter 8 Structuring System Logical Requirements
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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.
• 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.
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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 Can represent sequence, conditional statements
(If-Then or Case) and repetition (Do-Until, Do-While)
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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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Decision Table
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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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Reduced Decision Table
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Decision Table - Benefits
Model complicated logic of a process Tabular, compact Can check that logic is complete, consistent and
not redundant
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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
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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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