2. Variables, Identifiers, and Expressions. March 8, 2011

Transcription

1 March 8, 2011 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 1 of 34

2 Outline A Movie Structure of Simple C Codes Comments & Documentation Variables & Identifiers Built-in Datatypes Excursus: Floating Point Precision Simple Expressions: Assignments and Basic Arithmetics Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 2 of 34

3 The Golden Age of (Super)Computing Chris Johnson, head of the Scientific Computing & Imaging Institute in Salt Lake City, and member of the PITAC committee. At TUM, there s similar institutes (organised as consortia) such as the Munich Centre of Advanced Computing. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 3 of 34

4 2.1. Structure # i n c l u d e <iostream> i n t main ( ) { std : : cout << Hello yourname ; r e t u r n 0; } The computer runs through the code step-by-step (left to right, top-down) In each step, it executes the commands (a code is a cooking instruction) A step is terminated by a semicolon main identifies the jump-in point std::cout plots something on the console return 0 makes the program terminate (return to the command line prompt) Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 34

5 An Addendum: C vs. C++ C is (basically) subset of C++ Some features were deprecated or augmented (see comments) Some functions were replaced, in particular the output functions i n t a=10; double b=20; std : : cout << a i s << a << and b i s << b ; / / C++ s t y l e p r i n t f ( a i s %i and b i s %f, a, b ) ; / / C s t y l e Other functions: malloc (became new), free (became delete), and structs (need a name before the bracket opens). Furthermore, some shortcuts were removed (main always needs a return type). The C++ typically are safer (type checks) and more powerful. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 5 of 34

6 2.2. Variables Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 6 of 34

7 Declaring a Variable Memory int x; x = 10; 21: 22; 23; 24; 10 x is alias 25; 26; 27; 28: Computer runs through code step-by-step Declaration = define a name/alias for a memory location Definition = find a well-suited (free) place in memory Variable then is alias for this storage point we work with names instead of addresses Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 7 of 34

8 Identifiers W. Savitch: A C++ identifier must start with either a letter or the underscore symbol, and the remaining characters must all be letters, digits, or underscore symbols. C++ identifiers are case sensitive and have no limit to their length. (p. 7) An identifier is (unique) name of a variable Must not equal a keyword Should have a name with a precise meaning (UNIX-style and Hungarian notation today are considered to be a bad smell) Examples: usernumber, UserNumber, usernumber, usernumber, user number usno, usrn, usrn, nusr, usr1, usr1no iusernumber, intusernumber Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 8 of 34

9 Built-in Datatypes Memory int x; char y; float z; 21: y // one byte 22; z // four bytes 23; 24; 25; 26; x // two bytes? 27; 28: A variable corresponds to memory location and holds a value. What type of value? char, int, float, and double, and so forth. Compiler cares for memory layout. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 9 of 34

10 What Value Does a Variable Have By Default? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 10 of 34

11 Scope of Built-in Datatypes E. W. Dijkstra: Once a person has understood the way variables are used in programming, he has understood the quintessence of programming. (Notes on Structured Programming) name size range memory footprint bool {, } 1 Byte (?) char Byte short int 32, , Bytes int 2, 147, 483, , 147, 483, Bytes (?) long int 2, 147, 483, , 147, 483, Bytes (?) float ± ± Bytes double ± ± Bytes Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 11 of 34

12 Long and Unsigned Modifier Long Integers long i n t a ; / / size? i n t b ; / / size? C/C++ standard defines at least n bits. On 64 bit architectures, int and long int typically are the same. Some 32 bit architectures support 64 bit integers due to a tailored compiler. Others don t (such as the RZG s BlueGene/P system with the IBM compiler). Recommendation: Avoid modifiers such as long and short. Unsigned Integers unsigned i n t a ; / / size? i n t b ; / / size? That sign consumes one bit of the representation. If you are sure you don t need the sign, you can squeeze out an additional bit, however, be careful with this! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 12 of 34

13 What is an Overflow? An excursus into binary number systems: b = b = b = b = 3 10 ALU internal (for an inc): b b b b b b Your turn: Increment b (it is an unsigned integer)! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 13 of 34

14 What is an Overflow? C++ does not check for overflows (unlike all the interpreted languages such as Java and C# that are slower in turn). C++ does not check for underflows (unlike all the interpreted languages such as Java and C# that are slower in turn). Now, how would you define/encode your signed integer in terms of bits and what do the operations look like? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 14 of 34

15 Codes for Signed Integers 1-2: b b b b b b b b -1+1: b b b b b b b Still, the first bit is the sign, but an overflow is just running through zero. There s more negative values than positive ones. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 15 of 34

16 Syntax vs. Semantics Syntax = rules for the constructing of sentences in the languages Semantics = meaning of sentences, words, or fragments. The syntax is (more or less) prescribed by C, but the semantics is what you had in mind, i.e. the reader has to reconstruct it from your program. So, use meaningful names, and, if that is not sufficient, add documentation, documentation, and documentation. If code is not documented, it does not exist! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 16 of 34

17 2.3. Comments # i n c l u d e <iostream> / This i s the famous main operation. Here the a p p l i c a t i o n s t a r t s. / i n t main ( ) { / / hey dude, here we go / w r i t e yourname to the console / std : : cout << Hello yourname ; / / Return to the console r e t u r n 0; / The comments are not exectuded by the machine / } C/C++ is an instruction for a computer not for a human Often, we (or others) do not understand from a code what is happening Insert comments which annotate the code They are thrown away by the compiler Syntax alternatives: C/C++/C++ (JavaDoc) Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 17 of 34

18 Facts About Comments Sad But True C code is difficult to understand it ain t literate programming (Knuth s T E X experience) Documentation (design drafts, Ph.D. theses, papers, webpages) never is up-to-date (quick-fix disease) After a few days, the author is not familiar with the code anymore Every developer has a different style of writing and a different style of thinking I ll comment when it is running that is a lie! document everything in the code, otherwise its lost or won t be read! Tools Best Practices Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 18 of 34

19 Documentation Tools Screenshot from Doxygen Sad But True document everything in the code, otherwise its lost or won t be read! Tools Idea: Extract documentation from source code Format: Webpages, PDF, T E X Content: Diagrams, documentation text, mathematical formulas, images Tools: JavaDoc, Doxygen (both for C/C++) Best Practices Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 19 of 34

20 Documentation Best Practices Sad But True document everything in the code, otherwise its lost or won t be read! Tools Best Practices Document everything in the code (first place) before you write or alter the code. If you have to work with a third-party code, document it in the first place and send it back to the authors. They ll love you! Add a description before each complicated part (operations, methods,... ). Make yourself familiar with the tools and add formulas and images to your documentation. Document what, why, rationale, alternatives, and bugs fixed. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 20 of 34

21 Screenshot Peano-Sources - ispassive ispassive Peano - Source Directory and Architecture grid AbstractVertex<> ispassive Search Help grid Directories adapter checkpoint configuration integration-tests multicore records runners statistics tests Classes AbstractCell<> AbstractLeafEvent<> AbstractNewEvent<> AbstractPersistentRefined. AbstractRefinedEvent<> AbstractRootEvent AbstractStackBasedRefined. AbstractVertex<> Block<> BlockContainer<> Event<> EventHelper GridCell GridVertex LeafEvent<> NewEvent<> PassiveLeafEvent<> PassiveNewEvent<> PassivePersistentRefinedE. PassiveRefinedEvent<> PersistentRefinedEvent<> RefinedEvent<> RootEvent<> StaticLeafEvent<> Description Source Call Graph peano::grid::abstractvertex::ispassive ( Const Public Method ) Author: Tobias Weinzierl Is Vertex Passive. Syntax / parameters Return value Description Is Vertex Passive. If a vertex is passive, the event handler is not called for this vertex. A vertex is set passive by the NewEvent::createVertex() operation if the vertex and the whole surrounding support are outside of the domain. Be careful to make any vertex outside the computational domain a passive vertex. If you want to plot boundary cells, e.g., you have to ensure that all the vertices are plotted before you plot your cells. Yet, the touch operations are not invoked for passive vertices. In the parallel mode, each vertex is set passive that does not hold the actual node as adjacent element. Thus, vertices outside the computational partition are set passive although they might be inside the computational domain. This switch is implemented within derivepersistentvertexdatafromcoarsegrid(). 1 von :07 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 21 of 34

22 2.4. Floats # i n c l u d e <iostream> i n t main ( ) { / / hey dude, here we go double a ; double b ; double c ; a = 0.145e 07; b = 23.24e09 ; c = a+b ; std : : cout << c ; } Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 22 of 34

23 A Gedankenexperiment Fixed-point notation A number in a computer has to have a finite number of bits. This means a finite number of digits. Let s assume we have four digits in the form xx.yy. a = 01.50; b = 00.50; c = a / b ; d = c / 3; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 23 of 34

24 Floating-point notation A dynamic scheme (with a header, e.g.) cannot be fast (although Maple e.g. supports this if we need arbitary number of significant/valid digits). Define number into significant digits and exponent. Make the representation unique, i.e. ab.cd 10 4 = a.bcd This is a normalisation. In a binary system, base is 2 and first digit before comma always is 0 (or 1 respectively). So, we need one bit for the sign, s bits for the significant digits, one bit for the sign of the exponent, and e bits for the exponent. Type Sign Exponent Significand Total bits Single Double This is a at least standard! And there s two additional/special values: nan and inf. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 24 of 34

25 Normalisation & round-off errors # i n c l u d e <iostream> i n t main ( ) { / / hey dude, here we go double a ; double b ; double c ; a = 0.145e 07; b = 23.24e09 ; c = a+b ; std : : cout << c ; } Use your pocket calculator and assume there s eight significant bits! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 25 of 34

26 Normalisation & round-off errors C 1 = C 2 = N i i=1 N/2 (i + N i) i=1 Which variant is the better one? What means stability in this context? Why is the condition number important within this context? We have to study all our algorithms in detail! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 26 of 34

27 Some remarks on performance A Flop is a floating point operation. A MFlop is a...? How many Flops does the SuperMUC provide? If one of these GPGPU guys tells you something about performance, ask him about which precision he is using! Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 27 of 34

28 2.5. Simple Expressions The Assignment Statement int x; x = 10; Memory 21: 22; 23; 24; 10 x is alias 25; 26; 27; 28: The assignment operator takes expression from the right-hand side, evaluates it, and stores the result in the variable on the lefthand side. From a mathematical point of view, choosing = as assignment operator is a poor choice. Pascal, e.g., uses the := operator, which, from the author s point of view, is a better symbol. A declaration can directly be combined with an assignemnt. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 28 of 34

29 Arithmetic Expressions (for Numbers) c=10+2; assign variable c the value 12. a=10; b=a; a=2; afterwards, a holds 2 and b holds 10, i.e. a b. c=10*2; assign variable c the value 20. b=3; a=(b+2)*10; assign variable b the value 3. a becomes 50. a=2; a=a*2; this ain t a fixpoint formula, i.e. a holds the value 4 afterwards. a=3; a++; increment operator makes a hold 4. (unary operator) a=3; a--; decrement operator. a=2; a+=4; shortcut for a=a+4. a=2; a-=4; shortcut for a=a-4. a=2; a*=4; shortcut for a=a*4. a=a/2; divide value of a by 2. a=4; a/=2; shortcut for a=a/2. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 29 of 34

30 Further Expressions For booleans bool a=true; bool a=false; bool a=0; bool a=1; bool a=2; a=!a; a=!(a a); For characters char x= a ; char x=40; Comparsions bool x=a>4; bool x=a>b; bool x=a==b; bool x=a!=b; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 30 of 34

31 Fancy Initialisation : In C++, you can initialise all variables with brackets! int a = 10; int a(10); Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 31 of 34

32 Initialisation Pitfalls I Multiple Declarations int a; int b; int a,b; Shortcut int a,b =2 ; What does this mean? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 32 of 34

33 Initialisation Pitfalls I Multiple Declarations int a; int b; int a,b; Shortcut int a,b =2 ; What does this mean? either declare one variable per line or use C++ initialisation with brackets. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 32 of 34

34 Initialisation Pitfalls II Implicit Type Conversion double a; double a=0.3; double a=10.0 / 4.0; double a=10.0 / 4; double a=10 / 4; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 33 of 34

35 Initialisation Pitfalls II Implicit Type Conversion double a; double a=0.3; double a=10.0 / 4.0; double a=10.0 / 4; double a=10 / 4; If you use floating point arithmetics always write.0 for natural numbers. If you are interested in the underlying techniques, study the field of type inference. If you wanna have a more dynamic feeling, use a language with a dynamic type system. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 33 of 34

36 Initialisation Pitfalls III Assignment Operators int a = 3 bool b = (a==3); bool c = (a=3); int d = 0 bool e = (d==0); bool f = (d=0); Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 34 of 34

37 Initialisation Pitfalls IV Freaky Collaborators int a = 3; int b = ++a; (that is want we want) int c = 3; int d = c++; (that is something different) int e = (d+=a)++; (also very funny) int f = ((d=4)+a)--; C/C++ offer many strange constructs. This is good luck for posers, freaks, and people giving lectures. Others should avoid them. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 35 of 34