Introduction to Physics Computing

1. Introduction toPhysics Computing MT 2009 Lecture 1 J Tseng

2. MT 2009 (I) Introduction to Physics Computing (Tseng) 2 Outline Objectives of the computing course Some practical course details Overall lecture scheme Lecture: basic concepts and constructs

3. MT 2009 (I) Introduction to Physics Computing (Tseng) 3 Objectives Learn basic computing skills Computation and numerical methods Data processing and analysis Modelling and simulation Awareness of other uses of computers in physics Lectures – basic ideas How to tell computers what to do How computers work Practical course – learn by doing Actual programming Actual applications

4. MT 2009 (I) Introduction to Physics Computing (Tseng) 4 Computers in physics Non-linear equations, non-analytic solutions Data processing, pattern recognition, analysis Modelling and simulation Automated control Data acquisition Communication: e-mail, web So how does logical manipulation become all this?So how does logical manipulation become all this?

5. MT 2009 (I) Introduction to Physics Computing (Tseng) 5 The Large Hadron Collider

6. MT 2009 (I) Introduction to Physics Computing (Tseng) 6 ATLAS

7. MT 2009 (I) Introduction to Physics Computing (Tseng) 7 LHC and ATLAS in action 10 Sep 2008 – first beam Next beam: late 2009

8. MT 2009 (I) Introduction to Physics Computing (Tseng) 8 Computers everywhere Java ME (Micro Edition) A computer can really be just about anything. But electronic control -> easy to build computers -> easy to control other electronic devices. How do we then tell computers how to do things?A computer can really be just about anything. But electronic control -> easy to build computers -> easy to control other electronic devices. How do we then tell computers how to do things?

9. MT 2009 (I) Introduction to Physics Computing (Tseng) 9 Computers within computers Computer programs are also used to “emulate” other computers Sometimes old computers simply don’t exist anymore, but you still want to run the programs Operating systems (such as Windows 98) aren’t supported anymore, but your program can’t be upgraded Want to run untrusted program safely – protect against viruses Increase utilization of web servers Partition computers in a “cloud” Or, you’re just nuts See http://www-jpc.physics.ox.ac.uk for some local information and demonstration on the topic

10. MT 2009 (I) Introduction to Physics Computing (Tseng) 10 Practical course details All students are signed up to do CO00 first, using the Handbook of the Physics Computing Course This will take you through the elements of the C programming language as well as how to use the Mac systems You should know when you are to show up in lab There are 20-30 computers, so pre-allocation is intended to make sure everyone gets time to do it If you get behind, you can show up for extra sessions. Check with the demonstrators on shift – pre-allocated students get priority

11. MT 2009 (I) Introduction to Physics Computing (Tseng) 11 Practical course details (II) You can also start CO00 at home This will reduce the amount of time you actually have to sit in lab Come to lab for questions and to be marked off by a demonstrator The demonstrator is not responsible for figuring out problems with your home computer – be prepared to transfer your work to the lab machines You will need a C compiler: check the “Downloads” section of the computing practical course webpage (Cygwin/GCC) Asking for help or getting marked in lab Get a demonstrator’s attention Don’t be surprised if you may have to wait Continue working as far as you can at your computer Demonstrators should attend largely in sequence

12. MT 2009 (I) Introduction to Physics Computing (Tseng) 12 Lecture scheme 4 hours MT lectures: introduction Basic concepts and the “hello world” program Control flow and data structures Data representation and input/output Thinking about computing Learn by doing in the practical lab Lectures will try to convey more general concepts Lectures 2 and 3 are perhaps not interesting if you already know how to program – and they’re not compulsory

13. MT 2009 (I) Introduction to Physics Computing (Tseng) 13 What is a program? Basic idea: it’s a sequence of instructions

14. MT 2009 (I) Introduction to Physics Computing (Tseng) 14 Programs for people

15. MT 2009 (I) Introduction to Physics Computing (Tseng) 15 Programs for people Inputs: hair, shampoo, water Outputs: clean hair Memory: hair Processor: hands Can reduce to an arbitrary level of detail until you hit philosophical issues of reductivism and free willInputs: hair, shampoo, water Outputs: clean hair Memory: hair Processor: hands Can reduce to an arbitrary level of detail until you hit philosophical issues of reductivism and free will

16. MT 2009 (I) Introduction to Physics Computing (Tseng) 16 Programs for computers Computers are not people Computers are really, really, really stupid They have to be told everything There is minimal cultural context Sequence is important: what does a computer know, and when does it know it? “A computer does what you tell it to do, not what you want it to do”

17. MT 2009 (I) Introduction to Physics Computing (Tseng) 17 Programs for computers (2) How you might tell a robot: Still have to tell the robot how to lather and rinse!

18. MT 2009 (I) Introduction to Physics Computing (Tseng) 18 Hello World A simpler example in C (no robots): The program – introducing the idea of control flow (simple in this case) and outputThe program – introducing the idea of control flow (simple in this case) and output

19. MT 2009 (I) Introduction to Physics Computing (Tseng) 19 Xcode Xcode is an “integrated development environment” (IDE) Most modern development is done with IDE’s (though a lot of scientists seem to think they’re wimpy) It provides program templates (projects) – code bits that get used over and over again Provides tools to speed programming and “debugging” For practical course programming, there is a special Practical Course Project already defined for Xcode – use this to start

20. MT 2009 (I) Introduction to Physics Computing (Tseng) 20 Building and running The program may not look very readable, but it’s still too much for a computer to understand Compile – translate something “human readable” into something a computer can use This doesn’t stop us from making our programs unreadable to us Comments Spacing Link – resolve all outstanding names In previous example, printf() is from the “standard library” Compiling and linking all program units thus “builds” a single program Run – tell a computer to start a program Xcode allows you to write, build, and run in the same environment For small programs, Xcode is perhaps overkill; for large programs it’s very useful

21. MT 2009 (I) Introduction to Physics Computing (Tseng) 21 Command-line Mac OS X is built on Unix, a popular operating system for scientific computing Command line: keyboard control, like the DOS prompt underneath Windows (but a lot better!) Edit Compile/link Execute

22. MT 2009 (I) Introduction to Physics Computing (Tseng) 22 Saying more than “hello” Do a simple calculation

23. MT 2009 (I) Introduction to Physics Computing (Tseng) 23 Storage If you want to save something for later, you put it in a pigeonhole (often called a “variable”, but not in the same sense as in mathematics) Pigeonholes have a “type” int: -231 to (231-1) float: 6 digits, 10-38 to 1038 double: 15 digits, 10-308 to 10308 char: single characters (actually numerical representation) Boolean type (true/false) represented by integer 1 or 0 Have you ever noticed how your pigeonhole may not accept large packages? The reason is that it’s probably made for letters and papers.Have you ever noticed how your pigeonhole may not accept large packages? The reason is that it’s probably made for letters and papers.

24. MT 2009 (I) Introduction to Physics Computing (Tseng) 24 Storage (2) Every pigeonhole has a name, a type, and a location (like a street address) Every pigeonhole name in C must begin with a letter Upper and lower case characters are different There are “reserved words” which cannot be used as a pigeonhole name You have to warn C that you are going to store something by declaring it in the beginning This sounds restrictive, but you can also use it as a check on your programming logic

25. MT 2009 (I) Introduction to Physics Computing (Tseng) 25 Storage (3) A pigeonhole is like a “hole in the wall” Contents (for instance): Brasserie Blanc Address: 71-72 Walton Street Type: restaurant Size: 2 shops Imagine if you could think of property like this (not completely a joke) Sometimes you will need a pigeonhole’s address (&a) rather than the contents

26. MT 2009 (I) Introduction to Physics Computing (Tseng) 26 For calculations, similar to arithmetic * is multiplication and / division, rather than ? and ÷ Normal operator precedence: */ before +-: Assignment is an operator! Operators

27. MT 2009 (I) Introduction to Physics Computing (Tseng) 27 Relational operators Again, similar to what you would expect > and < >= and <= for including equality != for inequality == for equality (since = is for assignment) Evaluated after arithmetic operators These operators take number types (int or float) and return an integer 0 for false and 1 for true, like a boolean value

28. MT 2009 (I) Introduction to Physics Computing (Tseng) 28 Boolean operators AND, OR, NOT of Boolean expressions In C: &&, ||, and ! You will run into these again in digital electronics (electronics practical lab) Even lower precedence than relational operators It is good to use parentheses for clarity in spite of well-defined operator precedence

29. MT 2009 (I) Introduction to Physics Computing (Tseng) 29 Operator pitfalls Commas are operators Common pitfall: using commas instead of semicolons can result in difficult-to-find errors! Bitwise operators: &, |, ^ These are different from boolean operators! Assignment vs equality: = vs == Not an exhaustive list of operators (or pitfalls!), merely a general introduction

30. MT 2009 (I) Introduction to Physics Computing (Tseng) 30 printf() – “print formatted” Format codes dictate how data is printed: %d – int %f – float %s – strings Also plain text \n tells the computer to advance one line on the screen Pretty output

31. MT 2009 (I) Introduction to Physics Computing (Tseng) 31 Make the program more useful by allowing the user to enter own values Now can use this program with different values without having to rebuild Same format codes as printf() Note use of &: scanf() needs to know where to store the values Input

32. MT 2009 (I) Introduction to Physics Computing (Tseng) 32 Adding machine Source code for program; remember that programs are step-by-step instructions for the computerSource code for program; remember that programs are step-by-step instructions for the computer

33. MT 2009 (I) Introduction to Physics Computing (Tseng) 33 Additional tips “Debugging”: Very few programs work the first time Test for unacceptable circumstances, print or return something descriptive Print intermediate values and compare with your expectations Comments: Like a log book: will you understand in 6 months? Describe at least major sections of code

34. MT 2009 (I) Introduction to Physics Computing (Tseng) 34 Next lecture Same time and place (Mon 10am Martin Wood) Data representation Control flow