Joseph Valacich, Joey George and Jeff Hoffer,
Essentials of System Analysis and Design , 4 th edition,
Prentice Hall, 2009.

• Definition and construction of functional models
• Documenting functional models---Data flow diagram (DFD)
• Structuring and organizing DFDs
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• Functional: what happens
• Dynamic: when it happens
• Static: what it happens to
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• Describes computations and transformation of data
– How are outputs derived from inputs
– Specifies results of computations, not how or when they are computed input x program y = f ( x ) output y
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• Does not describe timing
– Has no regard for order of operations
– No control information
– Not a flow chart
• Uses diagrams
– Use cases
– Data flow diagram (DFD, not a UML diagram)
– Activity diagrams
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1. Identify input and output values
2. Build data flow diagrams (DFD) showing functional dependencies.
3. Describe the functions.
4. Identify the constraints.
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• At the highest level of abstraction, these are the system inputs and outputs.
• Each lower level has inputs and outputs defined by some higher level.
input x program output y
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• Start with each output and determine what function computes it.
• (Or trace forward from inputs.)
• Expand each non-trivial process in a higherlevel diagram by constructing a lower-level diagram.
f
1 f
11 f
13 f
12
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• Write a description of each function
(using any of the specification techniques discussed earlier).
Function, f : Integer

Integer, such that f(x) = -1 if x < 0 f(x) = 0 if x == 0 f(x) = 1 otherwise
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• Constraints are functional dependencies between objects that are not related by an input-output dependency.
– Between two objects at the same time
– Between instances of the same object at different times
– Between different objects at different times.
• State the time the condition must hold.
• E.g. “no account balance may ever become negative.”
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• Need a signature (syntactic or interface)
• Need transformations (semantics or meaning)
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• Need a signature (syntactic or interface)
– Name
– Arguments (number, order, types)
– Values returned (number, order, types)
• Need transformations (semantics or meaning)
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• Need a signature (syntactic or interface)
• Need transformations (semantics or meaning)
– Equations and formulae
– Tables of values
– Pre and post conditions
– Decision tables
– Pseudo code
– Natural language
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• Trivial
– Access: read or write attributes. May not be necessary to show all of these.
• Non-trivial
– Queries: no side effects
– Actions: “instantaneous” (atomic)
– Activities: duration over time
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 Definition and construction of functional models
• Documenting functional models---Data flow diagram (DFD)
• Structuring and organizing DFDs
15

• Uses diagrams
– Data Flow Diagram (DFD)
– UML Activity Diagram
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• Semi-formal notation for specifying
– Functions of an information system and
– How data flow from functions to functions
• Describes systems as
– Collections of functions that manipulate data
– Data can
• Be stored in data repository ,
• Flow in data flows , and Customer
Items
• Be transferred to or from the environment .
Price List
Find Cost
Cost
Customer
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• Order of operations (functions)
• Control information
• Flow charts
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• Four components
– Processes
– Data flows
– Data stores
– Sources/sinks
(or )
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• Transform data values
• Drawn as ellipses with fixed number of in-arrows and out-arrows dividend divisor quotient
Divide
Integer remainder
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• Connect processes
• Represent intermediate data
• Values are not changed by data flow
• Arrow with name or type of data integer a copy address city split zip city street address zip aggregation
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• Stores persistent data for later use
• Has exactly two operations:
– Store
– Retrieve
• Drawn as: data store
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• Producers or consumers of data
• Also called terminators and actors
Customer
Producer (source)
Printer
Consumer (sink)
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Price Lists
Price List
Item Name
Find Cost
Cost
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Teaching
Assistant
Names
Scores
Enter scores
Grade Book
Names
Scores
Scores
Names
Scores
Average
Compute statistics
Average
Median
Names
Scores
Grades
Create report
Grade report
Grades
Calculate grades
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• Repositories to store information about all data items defined in the DFD
• DD may include:
– Name of data item
– Aliases (other names for item)
– Description and purpose
– Range of values
– Data flow (generated by, used by)
– Data structure definition and form
Alan Davis, Software Requirements , Prentice Hall, 1993.
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Pairs (5 minutes): Model the function of a candy machine described below.
– Customer inserts coins
– Machine validates coins
– Customer selects a candy
– Machine validates candy selection
– Machine validates transaction
– Machine dispenses change
– Machine dispenses product
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 Definition and construction of functional models
 Documenting functional models---Data flow diagram (DFD)
• Structuring and organizing DFDs
29

• Apply levels of abstraction (leveling)
• The initial DFD is: input program output
• This can be expanded into lower levels.
• Eventually, the process must stop.
– When atomic processes can be described.
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• Describes the system at the highest, most abstract level.
• Determines the scope of the system
• Provides a general view of the system environment identifying external interfaces.
• Comprised of:
– One bubble (the name of the system)
– System terminators (sources and sinks)
– External inputs and outputs
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Employee info
Timecard info
Pay info
Process
Payroll
Acct info
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Rules of Decomposition
I1
I2
1. A
O
Level n
O
I1
I2
1.1 J dx dx
1.2 K
1.3 L dy dy dz
Data storage Level n+1
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Rules of Decomposition (Cont.)
I
1. B
O
Level n
I dx
I1
I2
1.1 J dy
1.2 K
1.3 L
O1
O2 dy
O
Level n+1
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Inputs and outputs to level n+1 match level n
Timecard info
4: Format paycheck
Pay info
(formatted)
Pay info
Employee info 3: Calculate withholding
1: Validate timecard
Gross pay
Valid timecard info
2: Calculate gross pay
Acct. Info
5: Format accounting
Acct. Info
(formatted)

Emp. info 3.1: Compute withholding rate
Gross pay
Rate
Pay info
3.2: Compute net pay
Acct. info
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• Show all possible computation paths for values.
• Do not show what paths are executed in what order (that’s the job of the dynamic model).
• There may be many Level N+1 diagrams for each Level N diagram. Level N+1 expands a single node of a Level N diagram.
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1. Describe the system at the highest, most abstract level (context diagram)
2. Decompose the system and represent it as a
DFD with multiple bubbles.
2.1 Document the processes
2.2 Fill out the data dictionary
3. Decompose a process (following step 2) until the problem is understood
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• At the highest level of abstraction, these are the system inputs and outputs.
• Each lower level has inputs and outputs defined by some higher level.
40

• Start with each output and determine what function computes it.
• (Or trace forward from inputs.)
• Expand each non-trivial function
(process) by constructing a lower-level diagram.
41

1. Is each requirements function represented by a transform in the DFDs?
2. Is each system input and output represented in the DFDs?
3. Is each I/O from a higher-level DFD reproduced correctly on the lower-level
DFDs?
4. Is each transform in the lowest-level DFDs primitive?
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5. Are all labels of information flows in the data dictionary?
6. Do all data dictionary entries appear in the
DFDs?
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• Draw the DFD for the gas pump controller
(see handout).
• Draw a Level 0, Level1, and 2 Level 2 diagrams.
• (15 minutes)
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