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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.