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Low-level design
1
Announcements
Midterm next Tuesday 12:30-1:45
Room MEB 253 netid a*-jke*
Room DCL 1310 (this) netid jmc*-z*
Rules like in previous years
Conflict: email Steve by the end of the day
Milestone 3 meetings are March 15-19
But practice UML part BEFORE midterm
WA3 due today
CS428 12-2
A good design
Satisfies requirements
Easy to implement
Easy to understand
Easy to change
Easy to check for correctness
Is beautiful
CS428 12-3
Keep It Simple (S)
Everything should be as simple as
possible, but no simpler. (Einstein)
A design is finished not when there is
nothing to add, but when there is nothing
to take away. (Saint-Exupery)
Measuring a program by the number of
lines of code in it is like measuring an
airplane by how much it weighs. (Gates)
CS428 12-4
Keep it Simple
Eliminate duplication
Eliminate unnecessary features
Hide information
No surprises (follow standards)
CS428 12-5
Davis Design Principles
Don’t reinvent the wheel.
Exhibit uniformity and integration.
Minimize the intellectual distance between
the software and the problem as it exists
in the real world.
Structure the design to accommodate
change.
CS428 12-6
Davis
The design should be structured to
degrade gently, even when aberrant data,
events, or operating conditions are
encountered.
The design should be assessed for quality
as it is being created, not after the fact.
The design should be reviewed to
minimize conceptual errors.
CS428 12-7
“Design”: Verb
Bounce from high-level to low-level
New requirements come after design is
created
Design is created incrementally
As requirements are known
As better design ideas are invented
As design flaws are discovered
CS428 12-8
Low-level Design
Many different techniques
Flow charts
Decision tables
Pseudo-code
State machines
CS428 12-9
Decision tables
Used to specify program with complex
conditions
Makes it easy to see if any cases are
missing
Can be implemented with IF statements
CS428 12-10
Tax code example
The recovery period is 27.5 years for
residential real property, 31.5 years for non-
residential real property placed in service
before May 13, 1993, 39 years for non-
residential real property placed in service after
May 12, 1993, 50 years for railroad gradings
and tunnel bores, except that nonresidential
real property on an Indian reservation has a
recovery period of 22 years.
Decision table for recovery
period
Conditions 1 2 3 4 5
Real property T T T T T
Residential T F F F F
Placed before May 13, 1993 T F
Railroad grading or bore F F T
On Indian reservation F F F T
Actions
27.5 years x
31.5 years x
39 years x
50 years x
22 years x
CS428 12-12
Pseudo-code
Also known as “Program Design Language”
Advantages:
Expressive and compact
Can use any editor
(Sometimes) can compile it
Disadvantages:
Must know the language
CS428 12-13
Pseudo-code
float computerRecoveryPeriod(property) {
if (isReal(property)) {
if (isResidential(property)) return 27.5;
if (onReservation(property)) return 22;
if (isRailroad(property)) return 50;
if (property.date > “May 13, 1993”)
return 31.5;
else return 39;
}
}
CS428 12-14
Pseudo-code
Works well with refinement
Write pseudo-code
As comments
With stubs
Gradually implement it all
Execute and test as you go
CS428 12-15
State Machines (FSM)
Lots of theory
How to minimize number of states
How to merge state machines
How to tell whether two state machines are
equal
Can generate code directly from state
machines
But usually do not
CS428 12-16
State diagram for stop light
30
2 R/G R/Y 4
R/R R/R
4 2
Y/R G/R
Events are time
25 delays, in seconds.
CS428 12-17
Pseudocode for stop light
Action = Record {integer wait, east, north}
Action: Actions[1 .. 6]
repeat forever
for I = 1 to 6 do
setEast(actions[i].east)
setNorth(actions[i].north)
wait(actions[i].wait)
end for CS428 12-18
State diagram for sockets
uninitialized
listen() connect()
passive active disconnect()
newconn1() newconn1() isconnecting()
qlen!=0
isconnected() CONNECTING
qlen!=0 isconnected()
isconnected()
accept() CONNECTED
CS428 12-19
Implementing socket
Socket is an object
Actions are methods of the socket
State is stored in variable of the object
Each method uses IF statement to make
sure socket is in the right state
When IF statements get too complicated,
use “State Pattern”
CS428 12-20
State Pattern
From “Design Patterns”
ConnectingState
Socket SocketState
listen()
ConnectedState
connect()
newconn1() ...
... PassiveState
CS428 12-21
Detailed design
Lots of different techniques
Most only works in a few places
Pseudocode works most places, but often
there is a better technique
Often best to use code and skip detailed
design
CS428 12-22
Good design
Good design is product of many small
steps
“Do the right thing”
Each step adds a feature
Do one more thing right
“Do the thing right”
Each step makes design a little simpler
Eliminates one more unnecessary complexity
CS428 12-23
Next: Midterm
Tuesday, March 9, 12:30-1:45
Two rooms
Check Wiki and newsgroup for details
Look at previous exams and Charlie’s Qs
Covered somewhat different material
Material includes all lectures and readings
until today, including guest lecture
CS428 12-24
