1. Week 7 - Wednesday
CS 113

2. Last time What did we talk about last time? Euler paths and cycles
Minimum spanning trees
Shortest paths

3. Questions?

4. Project 2

5. Software Engineering

6. Software development
Often goes through phases similar to the following:
Understand the problem
Plan a solution to the problem
Implement the solution in a programming language
Test the solution
Maintain the solution and do bug fixes
Factor of 10 rule!

7. A Tragedy of Software Engineering

8. Therac-25 Radiation treatment machine for cancer patients Caused:
Used June Jan 1987
Caused:
At least six serious overdoses
Several deaths
Many permanently maimed
Radiation treatment is normally around 200 rads
500 rads can be lethal
Therac-25 delivered dosages of over 20,000 rads because of a software error
11 units in US and Canada had treated hundreds of patients over the course of several years
Earlier Therac-20 and Therac-6 models had been in service, without incident, for many years

9. Problems not caught for two years
The doctors involved had never seen radiation overdoses of this magnitude
Manufacturer was both tragically ill-prepared and probably willfully dishonest
Government regulatory agencies slow and out of the loop
The fundamental problem stemmed from the machine having two different modes
Low power which projected a beam directly into the patient
High power which projected a beam onto a beam spreader plate which changed the kind of radiation and shaped it

10. Race condition //thread 1 value = 3 //thread 2 value = 7
One of the problems behind the Therac-25 disaster was a race condition
A race condition is when the output of a program is dependent on the scheduling of certain threads
Threads are parts of the same program that can run independently at the same time
Threads can share data
We won't talk about how to do multi-threaded programming in Python
But try to imagine which number will be stored into value if the following two threads are both running
//thread 1
value = 3
//thread 2
value = 7

11. Race conditions in Therac-25
The problem emerged with the Therac-25 only after operators became skilled at typing commands
It took time for the keyboard loop to read keystrokes and time for the magnets to adjust the radiation beam
Operators changed settings faster than the second magnet alignment loop
The device could activate in high power mode with no beam spreader plate

12. Overflow error
Even with the race condition, a safety check should have prevented the accident
There was a loop which checked a counter
The value 0 meant okay
The counter was only 1 byte
Every 256 cycles, it would roll around to 0
When that happened, the operator could proceed without a safety check

13. Subsequent investigation
The Therac-25 software was programmed by a single programmer, who had long since left the company
No one at the company could say anything about his education or qualifications
FDA investigators said
Program was poorly documented
No specification document
No formal test plan

14. In the programmer's defense
Therac-6 and 20 had no computer control, only add-on computer monitoring
Legacy software from the earlier devices had been adapted
Hardware safety devices had been removed, probably to save cost
Programmer had probably been under time pressure, and had been learning as he went along
Programmer probably had no idea his code would go into a device in which software bugs could kill people

15. Who is to blame?
Techniques for synchronization and data sharing already existed which would have prevented several of these deaths
What if the programmer had a formal education that involved these techniques or had bothered to study the literature?
What if management had understood software engineering and quality assurance principles?
What if hospitals or regulatory agencies had strong safety requirements for safety-critical software?

16. Quotes
A significant amount of software for life-critical systems comes from small firms, especially in the medical device industry; firms that fit the profile of those resistant to or uninformed of the principles of either system safety or software engineering. Frank Houston FDA investigator into the Therac-25

17. What is different today?
Does Elizabethtown require a course on testing methods, software engineering principles, or formal specification writing?
Do other major universities?
Does the government understand how to properly regulate safety-critical systems?
Do consumers have stringent requirements for the quality of the software they buy?

18. Testing

19. Unit testing
Unit tests are tests for a single piece of code in isolation from the rest of your program
A unit test could test
A single method or function
A whole class
None of the programs we've written in CS113 is large enough to go beyond unit testing

20. Test case selection Positive test cases Boundary condition test cases
Cases that should work, and we can easily check the answer
Boundary condition test cases
Special cases, like deleting the first or last item in a list
Sometimes called "corner cases"
Negative test cases
Cases that we know should fail

21. Regression testing
When you find a bug, keep the test case that demonstrated the bug
Include it as part of your test suite
Always run all your tests when you make a change to your code
Sometimes a "fix" can break something that was working
Especially if you didn't fix the bug correctly the first time

22. Black box testing One philosophy of testing is making black box tests
A black box test takes some input A and knows that the output is supposed to be B
It assumes nothing about the internals of the program
To write black box tests, you come up with a set of input you think covers lots of cases and you run it and see if it works
In the real world, black box testing can easily be done by a team that did not work on the original development

23. White box testing
White box testing is the opposite of black box testing
Sometimes white box testing is called "clear box testing"
In white box testing, you can use your knowledge of how the code works to generate tests
Are there lots of if statements?
Write tests that go through all possible branches
There are white box testing tools that can help you generate tests to exercise all branches
Which is better, white box or black box testing?

24. Finally… Understand each error before you fix it
If you don't understand a bug, you might be able to "fix" it so that one test case succeeds
Nevertheless, the real problem might be unsolved
Nothing is harder than truly fixing problems
This is a very important skill for computer scientists
But we're just happy if you know what a bug is in CS113

25. Quotes Testing can only show the presence of bugs, not their absence.
Edsger Dijkstra
Turing Award Winner
Designer of shortest path and MST algorithms
Worked on Dining Philosophers Problem

26. Programming Languages

27. Unambiguous languages
English is no good
"The peasants are revolting!"
We need precise, unambiguous languages to write programs in
Syntax is the rules for constructing meaningful statements
Like the grammar for the language
E.g., English sentences are made up of a subject and a predicate
E.g., Python has to be indented under an if statement
Semantics are the meaning of the statements

28. Type systems
A feature of most modern programming languages is a type system
In a type system, each piece of data has a particular type (string, integer, floating point value, Wombat)
In a static type system, the type of a variable is known before the program runs and never changes
In a dynamic type system, the type of the variable is only known at run-time and might change
Type systems help define what code is legal (and what legal code does)

29. Python vs. Java Interpreted (code is not compiled before running)
Dynamic typing
Whitespace has meaning
Doesn't take much code to do stuff
Relatively slow (in terms of execution time)
Object oriented (somewhat)
Compiled
Static typing
Whitespace is ignored
Takes a lot of code to do stuff
Relatively fast (in terms of execution time)
Object oriented
x = 4
int x = 4;

30. Object oriented programming
Object orientated programming (OOP) is a way of organizing code into objects containing
Data
Methods to manipulate the data
OOP is used
To protect data inside of objects from being changed in unexpected ways
To safely reuse code
It's hard to see why OOP is useful until you've programmed a big project
The shapes in Visual Python are examples of objects
ball = sphere(color=color.red)
ball.pos.x = 10 #data inside the ball

31. Early developments
The very earliest programming languages used punch cards for player pianos and looms
A mechanical computer, the Analytical Engine, would have used punch cards, but it was never completed
Work was done on the Engine during the mid to late 19th century
Ada Lovelace described an algorithm for it and is considered by some to be the first programmer

32. The electronic era
Early electronic computers were built in the 1940s and 1950s
They were programmed directly in the machine language of 1s and 0s that they would understand
Machine language was succeeded by assembly language
Had human readable commands like load, store, and add
But the commands were specific to machines and very low level

33. Modern programming languages
The third generation of languages is a set of general-purpose, high-level languages still used today
Pioneered in the 1960s and 1970s
Although they are written with a computer now, programs in these languages were written on punch cards before PCs existed
Examples: C
FORTRAN
LISP

34. Fourth and fifth generations
Java and Python are actually newer additions to the third generation of general purpose languages
Fourth generation languages are for specific purposes
SQL for talking to databases
Unix shell for interacting with a Unix file system
Fifth generation languages are given constraints on a problem and use artificial intelligence to solve it
Prolog is the best known example

35. Popular programming languages
Description
Typing
C/C++
Fast and powerful systems language, prone to nasty bugs
Static
Java
Platform independent OO language, uses virtual machine
Objective-C
Apple language for writing Mac, iPhone, and iPad apps C#
Microsoft language for applications and the web, like Java
Python
Interpreted language with unusual syntax, easy to read
Dynamic
Perl
Scripting language, good at processing text
Ruby
Interpreted language, popular for making websites
PHP
Interpreted, C-like language that runs on web servers
JavaScript
Interpreted language that runs on web browsers
Visual Basic
Microsoft language, good at making GUIs
SQL
Language for talking to databases

36. Quiz

37. Upcoming

38. Next time…
Finish programming languages and compilers
Functions
Lab 7

39. Reminders Finish Project 2 Keep reading Python Chapter 7
Due this Friday before midnight!
Keep reading Python Chapter 7
Think about what you want to do for your Final Project