1. Simulation and Example

2. Simulation
Simulation is used to study the performance of a physical system by using a physical, mathematical, or computer model of the system
Steps to take when you do simulation
Algorithm
Programming techniques and coding
Testing

3. Simulation
Simulation is used to study the performance of a physical system by using a physical, mathematical, or computer model of the system
Steps to take when you do simulation
Algorithm
Programming techniques and coding
Testing

4. Example
Write a program that simulates the operation of a busy airport that only has one run way to handle takeoff and landings. You may assume that each takeoff or landing takes 15 minutes to complete. One runway request is made during each 5 minute time interval and the likelihood of a landing request is the same as for a takeoff request. Priority is given to planes requesting a landing. If a request can not be honored, it is added to a takeoff or landing queue. This program should simulate 120 minutes of activity at the airport. Each request for runway clearance should be time- stamped to the appropriate queue. The output from your program should include the final queue contents, the number of takeoffs completed, the number of landings completed and the average number of minutes spent in each queue

5. Part 1: Determine Main Algorithm
Clock < Total time?
Clock =0
Total time = 120
Time done=0
Start No
stop
Yes
Toss a coin
Head? No
Check takeoff arrival
Clock += 5
Yes
Check landing arrival
Yes
Current serve done?
Serve the next one No

6. Part 2: Determine main data structure
Queue:
Takeoff
Landing
That queue has every attribute/action that a normal queue has and the following special attributes/actions
Number of takeoff or landing served
Total time wait
Check new arrival in landing or takeoff queues
Update statistics

7. Part 2: Determine main data structure
Request:
processingTime
arrivalTime
(may be requestID)

8. Part 3: Make the main algorithm more specific
Check takeoff or landing arrival
Given an arrival time
Create a new request
Print status for each queue

9. Part 3: Make the algorithm more specific
Serve the next one
Landing queue is NOT empty
Yes
currentRequest = get the first element in the landing queue No
Takeoff queue is NOT empty
Current request = get the first request in the takeoff queue
Timestamp = get arrival time of current request
Yes
Wait =current clock - timestamp No
The run way is idle now
TotalWait +=wait
Number of landing/takeoff served ++

10. Implementation Classes:
Queue (Using Lab3Queue with just a little bit change in Lab1LinkedList)
Request
RunawayQueue (extends from Queue)
AirportSim that actually starts the simulation and contain the main algorithm.

11. Testing Discussion: How can we test this simulation program: Criteria:
Robustness: not crashing
Accuracy: correctly computed

12. Testing Unit test Integration test
Make sure Queue is working, RunawayQueue is working and Request is working correctly
Integration test
Test if AirportSim is working correctly
Leave a trace of execution by displaying the method name as you enter it
Display values of all input parameters upon entry to a method

13. Testing Programs
There is no guarantee that a program that is syntax and run-time error free will also be void of logic errors
The “best” situation is a logic error that occurs in a part of the program that always executes; otherwise, it may be difficult to find the error
The worst kind of logic error is one that occurs in an obscure part of the code (infrequently executed)
Chapter 2: Program Correctness and Efficiency

14. Structured Walkthroughs
Most logic errors arise during the design phase and are the result of an incorrect algorithm
Logic errors may also result from typographical errors that do not cause syntax or run-time errors
One form of testing is hand-tracing the algorithm before implementing
Structured walkthrough: designer must explain the algorithm to other team members and simulate its execution with other team members looking on
Chapter 2: Program Correctness and Efficiency

15. Levels and Types of Testing
Testing: exercising a program under controlled conditions and verifying the results
Purpose is to detect program defects after all syntax errors have been removed and the program compiles
No amount of testing can guarantee the absence of defects in sufficiently complex programs
Unit testing: checking the smallest testable piece of the software (a method or class)
Integration testing: testing the interactions among units
Chapter 2: Program Correctness and Efficiency

16. Levels and Types of Testing (continued)
System testing: testing the program in context
Acceptance testing: system testing designed to show that the program meets its functional requirements
Black-box testing: tests the item based on its interfaces and functional requirements
White-box testing: tests the software with the knowledge of its internal structure
Chapter 2: Program Correctness and Efficiency

17. Preparations for Testing
A test plan should be developed early in the design phase
Aspects of a test plan include deciding how the software will be tested, when the tests will occur, who will do the testing, and what test data will be used
If the test plan is developed early, testing can take place concurrently with the design and coding
A good programmer practices defensive programming and includes code to detect unexpected or invalid data
Chapter 2: Program Correctness and Efficiency

18. Testing Tips for Program Systems
Most of the time, you will test program systems that contain collections of classes, each with several methods
If a method implements an interface, its specification should document input parameters and expected results
Carefully document each method parameter and class attribute using comments as you write the code
Leave a trace of execution by displaying the method name as you enter it
Display values of all input parameters upon entry to a method
Chapter 2: Program Correctness and Efficiency

19. Testing Tips for Program Systems (continued)
Display the values of any class attributes that are accessed by this method
Display the values of all method outputs after returning from a method
Plan for testing as you write each module rather than after the fact
Chapter 2: Program Correctness and Efficiency

20. Developing the Test Data
Test data should be specified during the analysis and design phases for the different levels of testing: unit, integration, and system
In black-box testing, we are concerned with the relationship between the unit inputs and outputs
There should be test data to check for all expected inputs as well as unanticipated data
In white-box testing, we are concerned with exercising alternative paths through the code
Test data should ensure that all if statement conditions will evaluate to both true and false
Chapter 2: Program Correctness and Efficiency

21. Testing Boundary Conditions
When hand-tracing through an algorithm or performing white-box testing, you must exercise all paths
Check special cases called boundary conditions
Chapter 2: Program Correctness and Efficiency

22. Why do Testing? Normally testing is done by
The programmer
Other members of the software team who did not code the module being tested
Final users of the software product
Do not rely on programmers for testing as they are often blind to their own oversights
Companies also have quality assurance organizations that verify that the testing process is performed correctly
In extreme programming, programmers work in pairs where one writes the code and the other writes the tests
Chapter 2: Program Correctness and Efficiency

23. Stubs
It may be difficult to test a method or class that interacts with other methods or classes
The replacement of a method that has not yet been implemented or tested is called a stub
A stub has the same header as the method it replaces, but its body only displays a message indicating that the stub was called
Chapter 2: Program Correctness and Efficiency

24. Drivers
A driver program declares any necessary object instances and variables, assigns values to any of the method’s inputs, calls the method, and displays the values of any outputs returned by the method
You can put a main method in a class to serve as the test driver for that class’s methods
Chapter 2: Program Correctness and Efficiency

25. Using a Test Framework
A test framework is a software product that facilitates writing test cases, organizing the test cases into test suites, running the test suites, and reporting the results
A test framework often used for Java products is JUnit, an open-source product that can be used in a stand-alone mode and is available from junit.org
Chapter 2: Program Correctness and Efficiency

26. Debugging a Program
Debugging is the major activity performed by programmers during the testing phase
Testing determines if there is an error, debugging determines the cause of it
Debugging is like detective work
Inspect carefully the information displayed by your program
Insert additional diagnostic output statements in the method to determine more information
Chapter 2: Program Correctness and Efficiency

27. Using a Debugger Debuggers often are included with IDEs
A debugger can execute your program incrementally rather than all at once
Single-step execution executes in increments as small as one program statement
Breakpoints are used to traverse large portions of code before stopping
The actual mechanics of using a debugger depend on the IDE that you are using
Chapter 2: Program Correctness and Efficiency

28. Using a Debugger (continued)
Chapter 2: Program Correctness and Efficiency

29. Reasoning about Programs: Assertions and Loop Invariants
Assertions: logical statements about a program that are claimed to be true; generally written as a comment
Preconditions and postconditions are assertions
A loop invariant is an assertion
Helps prove that a loop meets it specification
True before loop begins, at the beginning of each repetition of the loop body, and just after loop exit
Chapter 2: Program Correctness and Efficiency

30. Efficiency of Algorithms
Difficult to get a precise measure of the performance of an algorithm or program
Can characterize a program by how the execution time or memory requirements increase as a function of increasing input size
Big-O notation
A simple way to determine the big-O of an algorithm or program is to look at the loops and to see whether the loops are nested
Chapter 2: Program Correctness and Efficiency

31. Efficiency of Algorithms (continued)
Chapter 2: Program Correctness and Efficiency