chapter 2.1 program design and documentation. summary system/program specifications system/program...

Chapter 2.1

Program Design

and Documentation

Summary• System/Program specifications

• System/Program design– Top-down & Bottom-up design– Object Oriented design

• Tools for system/program documentation– Decision tables– Control-flow & Nassi-Shneiderman diagrams– Data-flow diagrams– State diagrams– Unified Modeling Language

System/Program specifications

• What should the system/program do ?

• Formally:– Set of all possible inputs {I}– Set of all possible outputs {O}– Function mapping all elements of {I} into

elements of {O}

• Possible for simple and small programs

• Unfeasible for large scale systems

The effort of Specifying

Interactionswith otherprograms

Number of users

Only the author

Many,

None Many

1

9

3

3

System/Program specifications

• Specifications for large scale systems ?

1 Make reasonable set of specifications

2 REPEAT

2.1 Build prototype

2.2 Evaluate prototype

2.3 Update specifications

UNTIL satisfaction of (potential) users

3 Write down final specifications

The Cost of Software

for large systems

Design (18%)

Coding (7%)

Debugging(25%)

Maintenance(50%)

During design and coding efforts should be madeto reduce the cost of debugging and maintenance

Design Methods

• Top-down design: From global to detail– Decompose the problem in simpler subproblems– Further decompose subproblems– UNTIL all subproblems have trivial solutions

• Bottom-up design: From detail to global– Solve some small problems that might be useful

for solving the big problem– Use the partial solutions for assembling a global

solution

Example : Repairing a flat tire

• Specifications : – Given : A car with a flat tire– Wanted : Instructions for fixing it

• Strategy choice :– Wait with a smile until somebody fixes it …– Call a repair service> Try to repair yourself

WHAT IS A PROGRAM ?

PROGRAM

=

Description of data

+

Actions to perform upon these data

Repairing a flat tire

“Data” and Actions

• “Data”– Car, Defective wheel, Spare wheel, Tools.

• Actions– All that need to be done with the Car, the

Defective wheel, the Spare wheel and the Tools in order to solve the problem.


Top level design of actions

• Inspect the spare wheel

• If available and in good condition,

– Then, repair yourself

– Else, you will need to call help

Repairing a flat tireRefinement of “repair

yourself”

• Fetch the tools

• Fetch the spare wheel

• Exchange defective and spare wheels

• Store defective wheel

• Store tools


Refinement of “tools”

• Tools :– Jack : device to lift a car– Wrench : device to loose or fasten bolts

Repairing a flat tireRefinement of “Exchange defective and spare

wheels”• REPEAT loose one bolt with wrench UNTIL all bolts loose;• Lift car with Jack;• REPEAT remove one bolt UNTIL all bolts removed;• Remove defective wheel;• Put spare wheel in place;• REPEAT replace one bolt UNTIL all bolts in place;• Lower car with Jack;• REPEAT fasten one bolt with wrench UNTIL all bolts fastened;

Object oriented Design

• Problem : – If a data item is slightly changed, the entire

program needs to be checked.

• Solution :– Group data with description of all actions

that can be performed upon such data– Access data exclusively through the actions

predefined for these data

• Object = data + actions to access it.


Object-oriented style

• Instance of the object :• MyJack

• Possible actions :• Fetch

• Inspect

• Store

• Lift a car

• Lower a car

Object Jack =


Decision TableSpare Wheel

Tools

Available & OK

No good tools available

OK Not OK

Repairyourself

Get Help

Get Help

Try toborrowtools


Top-level Controlflowchart

Spare WheelOK ?

Get Help Repairyourself

No Yes

Repairing a flat tireRefinement of “repair

yourself”

Spare WheelOK ?


No Yes Get Tools

Get Spare Wheel

Change Wheel

Store Bad Wheel

Store Tools


Refinement of “Get Tools”

Tools present & OK ?


Pick up the Tools

Try borrowing Tools

NoYes

Got Tools or Tired ?

Yes

Yes

No

No

Help Needed !!!

Repairing a flat tireAdapted refinement of “Get

Tools”


Pick up the Tools

Try borrowing Tools

NoYes

Got Tools or Tired ?

Yes

No


Improved top-level design

Spare Wheel OK ?

No Yes

Tools OK ?

Fetch Tools

Yes

No


Selection Statement

No

selector = a ? Tasks to be doneif selector = A

Yes

No

selector = b ? Tasks to be doneif selector = B

Yes

No

selector = c ? Tasks to be doneif selector = C

Yes

Tasks to be doneif no criteria OK

Selection Statement

selectorTasks to be done

if selector = A

selector = a

Tasks to be doneif no criteria OK

selector # aselector # bselector # c

Tasks to be doneif selector = B

Tasks to be doneif selector = C

selector = b selector = c

selector

Iteration Statement

Termination Test

Initialization

Loopbody, part 1

Loopbody, part 2

Continue

Finish

Iteration Statement

Initialisation

Loopbody Part 1

Loopbody Part 2

Termination Condition


Spare Wheel ?

Not OK OK

Get

Help

Fetch Tools

Tools ?

Get

Help

Repair

Yourself

Not OK OK


Yes No

Get

your

own

tools

Try toborrowtools

Until You got tools OR you are tired

Get spare wheel

Change wheel

Store bad wheel

Return tools

Data-flow Diagrams

Bad Wheel Tools Spare Wheel

FETCHED

STORED

MOUNTED

STORED

REMOVED

State Diagrams

Flat tire problemDriving

Flat tire

Need help Goodspare wheel

Repairing

Try borrowtools

Puncture

No goodSpare wheel

Successfulhelp

Nosuccess

Spare wheelOK

Done

Notools Tools

OK

Success

State Diagrams

Electronic lockFirst digitentered

Waiting1st digit

Testing key

Waiting2nd digit

Waiting3rd digit

Second digitentered

Third digitentered

Wrong Key Opening door

Good Key

Spontaneoustransition

Unified Modeling Language

• Tool for supporting / documenting object oriented designs• Applicable to the

– Organizational level– System level– Program level

• Set of different diagrams, including– Class diagrams (= objects and their static relations)– Interaction diagrams– Activity diagrams (= extended control flow diagrams)– State diagrams– Deployment diagrams

chapter 2.1 program design and documentation. summary system/program specifications system/program...

Documents

data slide

documentation slide

design methods

maintenance slide

level design of actions

flat tire specifications

global solution slide

update specifications