5. Applicative Programming. 1. Juli 2011

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1 1. Juli 2011 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 1 of 41

2 Outline Recapitulation Computer architecture extended: Registers and caches Header files Global variables and constants Namespaces Loops revisited Enums Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 2 of 41

3 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { r e t u r n b ; b = b%a ; What is missing in this code? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 3 of 41

4 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { r e t u r n b ; b = b%a ; What is missing in this code? What does the return b do? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 3 of 41

5 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { r e t u r n b ; b = b%a ; What is missing in this code? What does the return b do? What happens if we replae a==0 with a=0? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 3 of 41

6 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { i n t a ; a=b ; r e t u r n b ; b = b%a ; r e t u r n b ; What value does a have in each single code line? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 41

7 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { i n t a ; a=b ; r e t u r n b ; b = b%a ; r e t u r n b ; What value does a have in each single code line? What happens with the following snippet? i n t a=10; i n t b=20; i n t r =foo ( a, b ) Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 41

8 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { i n t a ; a=b ; r e t u r n b ; b = b%a ; r e t u r n b ; What value does a have in each single code line? What happens with the following snippet? i n t a=10; i n t b=20; i n t r =foo ( a, b ) Can foo() modify the value of the variable a in our main application? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 41

9 Recapitulation i n t foo ( i n t a, i n t b ) { i f ( a==0) { i n t a ; a=b ; r e t u r n b ; b = b%a ; r e t u r n b ; What value does a have in each single code line? What happens with the following snippet? i n t a=10; i n t b=20; i n t r =foo ( a, b ) Can foo() modify the value of the variable a in our main application? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 4 of 41

5.1. Computer Architecture Excursus: Registers and Caches John von Neumann 1903 1957 Manhattan Project (Los Alamos) June 30, 1945 (but Turing et. al.

10 5.1. Computer Architecture Excursus: Registers and Caches John von Neumann Manhattan Project (Los Alamos) June 30, 1945 (but Turing et. al. published similar ideas) Computer Consists of Four Components There is a Von-Neumann bottleneck John von Neumann Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 5 of 41

However, number of registers still is limited.

11 The Neumann Bottleneck One or two documents in the office (two registers in the ALU) ain t sufficient. Introduce more registers (Itanium e.g. has 128 of them). However, number of registers still is limited. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 6 of 41

The faster the processor, the more annoying the slow search in the memory is.

12 The Neumann Bottleneck Running into the basement is time consuming, and The bigger the basement (memory), the slower the search becomes. The faster the processor, the more annoying the slow search in the memory is. Can we study this effect? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 7 of 41

13 The Memory Gap Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 8 of 41

14 Idea of a Cache Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 9 of 41

15 Cache Levels Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 10 of 41

16 Caches Computers have a hierarchy of caches and lots of registers. The time to finish one operation depends significantly on where the data is located right now. It is important for many algorithms to exhibit spatial locality and temporal locality. Brain teaser 1: How would you implement a matrix-vector product? Brain teaser 2: What is the fundamental challenge if we transpose a matrix? Brain teaser 3: What is the best way to run through a Cartesian grid? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 11 of 41

17 5.2. Header Files C/C++ distinguishes declaration and definition. Motivation for this is still missing and requires a more general definition than before. Declaration: Tell the compiler what names are available. Definition: Define where the name is stored. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 12 of 41

18 Functions and Their Declaration void foo ( i n t a ) { / / do something i n t e l l i g e n t... foo ( 4 4 ) ; This definition is both a declaration and a definition. A function call is replaced by a reset of the program counter (and some additional things). Sometimes, we d like to split up files into several files. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 13 of 41

19 Functions and Their Declaration / / F i l e A void foo ( i n t a ) { / / do something i n t e l l i g e n t / / F i l e B void bar ( i n t a ) { foo ( a + 2 ) ; We can invent some clever C/C++-concatenate operation (C/C++ actually has such a thing) and make file B compile. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 14 of 41

20 ... However / / F i l e A void foo ( i n t a ) { / / do something i n t e l l i g e n t / / F i l e B void bar ( i n t a ) { foo ( a + 2 ) ; / / F i l e C void t a r ( i n t a ) { foo ( a + 4 ) ; We can invent some clever C/C++-concatenate operation (C/C++ actually has such a thing) and make file B and file C compile. However, the linking process fails as the symbol foo() is defined twice. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 15 of 41

21 Function Definitions / / F i l e A void foo ( i n t a ) { / / do something i n t e l l i g e n t / / F i l e B void foo ( i n t a ) ; void bar ( i n t a ) { foo ( a + 2 ) ; / / F i l e C void foo ( i n t a ) ; void t a r ( i n t a ) { foo ( a + 4 ) ; First, this code compiles fine. Second, this code links fine, if we pass the linker all three files. Third, we now know why! The compiler does not resolve the program counter modifications, but the linker does. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 16 of 41

22 Function Definitions Writing the function s declaration over and over again is cumbersome and error prone. Let s put it into a file of its own. Files containing solely declarations, are called header files and typically have the extension.h or.hpp. / / F i l e my. h void foo ( i n t a ) ; / / F i l e my. cpp # i n c l u d e my. h void foo ( i n t a ) { / / do something i n t e l l i g e n t / / F i l e B # i n c l u d e my. h void bar ( i n t a ) { foo ( a + 2 ) ; / / F i l e C # i n c l u d e my. h void t a r ( i n t a ) { foo ( a + 4 ) ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 17 of 41

23 Header Files / / F i l e my. h void foo ( i n t a ) ; / / F i l e my. cpp # i n c l u d e my. h void foo ( i n t a ) { / / do something i n t e l l i g e n t A good header file never contains definitions. A good header files has lots of comments as other editors of files see it frequently. A header file is typically accompanied by one implementation file (best practice). Every C/C++ and every UNIX installation comes along with tons of header files. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 18 of 41

24 Linker Usage C comes along with lots of useful standard libraries. These libraries provide tons of useful operations. See math.h for example or iostream which provides the terminal output function << (this is also just a function with the name <<). We add definitions due to include statements. We add the compiled files either by passing the object files to the linker or passing the library files (a,so) due to the -l argument with -L giving the compiler the library search path. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 19 of 41

25 5.3. The Precompiler and Global Variables Circulant Declarations / / F i l e a. h void foo ( i n t a ) ; void foo ( i n t a ) ; / / m u l t i p l e d e f i n i t i o n s. Has to be removed / / F i l e b. cpp # i n c l u d e my. h void bar ( i n t a ) ; / / F i l e b. cpp # i n c l u d e my. h # i n c l u d e my. h / / m u l t i p l e d e f i n i t i o n s. Has to be removed void bar ( i n t a ) ; Sometimes, it is pain to remove multiple definitions. In particular, if a headers include other headers (transitivity of includes). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 20 of 41

26 Include Guards / / F i l e a. h # i f n d e f INCLUDE A H # define INCLUDE A H void foo ( i n t a ) ; # e n d i f / / F i l e b. h # i n c l u d e my. h void bar ( i n t a ) ; / / F i l e b. cpp # i n c l u d e b. h void bar ( i n t a ) {\ l d o t s / / F i l e c. h # i n c l u d e my. h # i n c l u d e b. h void t a r ( ) ; / / F i l e c. cpp # i n c l u d e c. h void t a r ( ) {\ l d o t s Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 21 of 41

27 The C/C++ Precompiler / / This i s my header # i f n d e f IDENTIFIER FOR MY HEADER H # define IDENTIFIER FOR MY HEADER H / / A l l the declarations here # e n d i f / / This i s my implementation # i n c l u d e myheader. h / / a l l the d e f i n i t i o n s here The C/C++ is a powerful precompiler (does something similar like cut- n-paste). Its symbol mechanism allows us to avoid multiple declarations. Embed every header file into ifndef-define-endif statements. Precompiler can do a lot of additional things we won t discuss here. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 22 of 41

28 Global Variables and Constants / / This i s my header # i f n d e f IDENTIFIER FOR PI HEADER H # define IDENTIFIER FOR PI HEADER H / / A l l the declarations here const double PI ; / / T e l l s compiler t h a t t h i s v a r i a b l e may not be / / changed. # e n d i f / / This i s my implementation # i n c l u d e p i. h const double PI = 20; / / Another f i l e # i n c l u d e p i. h \ l d o t s double myvalue ; \ l d o t s myvalue = pi ; Remember difference of declaration and definition. Other files only need the definition of the variable (which is a constant here). Linker then is responsible to bring together definition (address) and references to the symbol. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 23 of 41

29 Multiple Declarations with Different Context / / This i s header A # i f n d e f IDENTIFIER FOR HEADER A H # define IDENTIFIER FOR HEADER A H / Calculates f o r c e between two p a r t i c l e s ( Lennard Jones ) / f u n c t i o n double getforce ( const double& distance ) ; # e n d i f / / This i s header B # i f n d e f IDENTIFIER FOR HEADER B H # define IDENTIFIER FOR HEADER B H / Calculates f o r c e between two p a r t i c l e s (WCA model ) / f u n c t i o n double getforce ( const double& distance ) ; # e n d i f Sometimes, functions and variables have the same name as they were developed by different people and written into different files. It is very difficult to distinguish between these names, i.e. we have to analyse the (indirect) include paths. Sometimes, we might want to use both functions within one implementation file. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 24 of 41

30 Namespaces / / This i s header A # i f n d e f IDENTIFIER FOR HEADER A H # define IDENTIFIER FOR HEADER A H namespace lennardjones { / Calculates f o r c e between two p a r t i c l e s ( Lennard Jones ) / double getforce ( const double& distance ) ; # e n d i f / / This i s header B # i f n d e f IDENTIFIER FOR HEADER B H # define IDENTIFIER FOR HEADER B H namespace wca { / Calculates f o r c e between two p a r t i c l e s (WCA model ) / double getforce ( const double& distance ) ; # e n d i f With namespaces, we can group variables, constants, and functions into logic subsets. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 25 of 41

31 Using Namespaces / / This i s the header # i f n d e f IDENTIFIER FOR HEADER A H # define IDENTIFIER FOR HEADER A H namespace lennardjones { / Calculates f o r c e between two p a r t i c l e s ( Lennard Jones ) / double getforce ( const double& distance ) ; # e n d i f / / This i s the implementation # i n c l u d e lennardjones. h double lennardjones : : getforce ( const double& distance ) { \ l d o t s Namespaces are predecessors of the identifier. Namespaces can be embedded into each other. Alternatively, you can use using namespace lennardjones ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 26 of 41

32 The Standard Library # i n c l u d e <iostream> i n t main ( ) { i n t a = 1; std : : cout << my d a r l i n g i s my no. << a << std : : endl ; r e t u r n 0; All the functions, constants,... of the standard library are embedded into the namespace std. std::endl is just a constant whose definition can be found in iostream or a header includes by iostream. Hint: Never use using namespace within a header file. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 27 of 41

33 5.4. Loops Create three files: main.cpp, fibonacci.cpp, and fibonacci.h. Define a function void printfibonacci(int max) in fibonacci.cpp. It shall be embedded into the namespace fib. Make printfibonacci(int max) print all Fibonacci numbers F n = F n 1 +F n 2 F 0 = 0 F 1 = 1 from F n {0, max. At least F 0 and F 1 shall be printed! Realise the computation with a while loop, or do it recursively. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 28 of 41

34 Do-While Loop do { / / something i n t e l l i g e n t while ( expression ) ; Rewrite your code using the do-while loop. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 29 of 41

35 The For-Loop i n t a=0; do { / / something i n t e l l i g e n t a++; while ( a<20); Often, we want to run through a sequence with fixed size. While loops are error-prone as we might forget the increment. Alternatively: Use the for loop. f o r ( i n t a=0; a<20; a++) { / / something i n t e l l i g e n t Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 30 of 41

36 Semantics of the For Loop f o r ( i n t a=0; a<20; a++) { / / something i n t e l l i g e n t The for statements opens a new scope. Statement int a creates a new variable (loop counter) within this scope. Statement a<20 is the termination criterion. It is evaluated before one iterate. Statement a++ is an increment. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 31 of 41

37 Scoping within a For Loop i n t a=20; f o r ( i n t a=0; a<20; a++) { / / a i s hidden w i t h i n loop body / / something i n t e l l i g e n t f o r ( i n t a=0; a<20; ) { / / something i n t e l l i g e n t a+=20; / / a ain t known outside the scope Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 32 of 41

38 A For Loop is a While Loop f o r ( i n t a=0; a<20; a++) { / / a i s hidden w i t h i n loop body / / something i n t e l l i g e n t { i n t a=0; while ( a<20) { / / something i n t e l l i g e n t a++; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 33 of 41

39 What do These Snippets Do? f o r ( i n t a=0; a<20; a++) { / / something i n t e l l i g e n t a =2; f o r ( i n t a=0; a<20; a++) { / / something i n t e l l i g e n t a =2; f o r ( i n t a=0; a<20; a =2) { / / something i n t e l l i g e n t Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 34 of 41

40 What do These Snippets Do? f o r ( i n t a=0; a<20;) { / / something i n t e l l i g e n t a+=2; i n t b=2; f o r ( i n t a=b 2; a<20;) { / / something i n t e l l i g e n t b++; f o r ( i n t a=0; a<20; a ++); { / / something i n t e l l i g e n t Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 35 of 41

41 For Best Practices f o r ( i n t a=0; a<20;) { / / something i n t e l l i g e n t a+=2; Use for for fixed ranges (compiler can optimise and parallelise). Do not invoke a function in the guard or increment section. Always open brackets after a for loop. Do not manipulate the counter within a for loop (error-prone; optimisation). Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 36 of 41

42 5.5. Enums In the next session, we will create more sophisticated data structures such as sequences of variables and complex data types that represent whole records (like tuples of time and measurement in an experiment). Before, we however have to talk about the last (primitive) datatype enumerations. enum Colour { Red, Green, Blue ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 37 of 41

43 Enums in Action enum Colour { Red, Green, Blue ; \ l d o t s Colour mycolour = Red ; \ l d o t s i f ( mycolour==green ) {... Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 38 of 41

44 Enums enum Colour { Red, Green, Blue ; Mind the semicolon terminating the enum definition. Enums are basically integers (no type safety). Enum variants belong the enclosing namespace. Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 39 of 41

45 Enums and Integers enum Colour { Red, Green, Blue ; / Maps c olour to a grey value / i n t getgreyvalue ( const Colour& colour ) { i n t a = getgreyvalue ( 1 7 ) ; Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 40 of 41

46 Enums and Namespaces namespace myspace { enum Colour { Red, / / i s mapped to 0 Green, / / i s mapped to 1 Blue / / i s mapped to 2 ; enum TUMColours { Blue, / / i s mapped to 0 Orange, / / i s mapped to 1 White, / / i s mapped to 2 Black / / i s mapped to 3 myspace : : Colour colour = myspace : : Blue ; / / what happens? Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 41 of 41

4. Functions. March 10, 2010

4. Functions. March 10, 2010 March 10, 2010 Einführung in die Programmierung Introduction to C/C++, Tobias Weinzierl page 1 of 40 Outline Recapitulation Functions Part 1 What is a Procedure? Call-by-value and Call-by-reference Functions