05 Applicative Programming

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Computer Architecture Excursus: Registers and Caches Header Files The Precompiler and Global Variables Loops Enums

5. Applicative Programming

1. Juli 2011


Einfuhrung in die ProgrammierungIntroduction to C/C++, Tobias Weinzierl page 1 of 41
http://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierlhttp://www5.in.tum.de/wiki/index.php/Dr._rer._nat._Tobias_Weinzierl


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Outline

Recapitulation

Computer architecture extended:Registers and caches

Header files

Global variables and constants

Namespaces

Loops revisited

Enums




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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?




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Recapitulation



}

What is missing in this code? What does the return bdo?




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Recapitulation



}

What is missing in this code? What does the return bdo?

What happens if we replae a==0witha=0?




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Recapitulation


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 doesa have in each single code line?




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Recapitulation




}


What happens with the following snippet?

i n t a=10;

i n t b=20;i n t r = f o o ( a , b )




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Recapitulation




}



i n t a=10;

i n t b=20;i n t r = f o o ( a , b )

Canfoo()modify the value of the variable a in our main application?




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Recapitulation




}



i n t a=10;

i n t b=20;i n t r = f o o ( a , b )

Canfoo()modify the value of the variable a in our main application?




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5.1.Computer Architecture Excursus: Registers andCaches

John von Neumann

John von Neumann

19031957 Manhattan Project (Los Alamos) June 30, 1945 (but Turing

et. al. published similar ideas)

Computer Consists of FourComponents

There is aVon-Neumann bottleneck




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The Neumann Bottleneck

One or two documents in the office (two registers in the ALU) aint sufficient.

Introduce more registers (Itanium e.g. has 128 of them).

However, number of registers still is limited.




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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?




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The Memory Gap



C t A hit t E R i t d C h H d Fil Th P il d Gl b l V i bl L E


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Idea of a Cache





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Cache Levels





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Caches

Computers have a hierarchy of caches and lots of registers.

The time to finish one operation depends significantly on where the data is locatedright now.

It is important for many algorithms to exhibit spatial localityandtemporal 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?





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5.2.Header Files

C/C++ distinguishes declaration and definition.

Motivation for this is still missing and requires a more general definition thanbefore.

Declaration: Tell the compiler what names are available.

Definition: Define where the name is stored.





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Functions and Their Declaration

v o i d foo (i n t a ) {

/ / do something i n t e l l i g e n t}

. . .f o o ( 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 additionalthings).

Sometimes, wed like to split up files into several files.





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p g p p

Functions and Their Declaration

/ / F i l e A

v o i d foo (i n t a ) {/ / do something i n t e l l i g e n t

}

/ / F i l e Bv o i d b a r (i n t a ) {

f o o ( a + 2 ) ;}

We can invent some clever C/C++-concatenate operation (C/C++ actually has such athing) and make file B compile.





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p g p p

...However

/ / F i l e A

v o i d foo (i n t a ) {/ / do something i n t e l l i g e n t

}

/ / F i l e Bv o i d b a r (i n t a ) {

f o o ( a + 2 ) ;}

/ / F i l e Cv o i d t a r (i n t a ) {

f o o ( 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.




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Function Definitions

Writing the functions declaration over and over again is cumbersome and error prone.Lets put it into a file of its own. Files containing solely declarations, are called header

filesand typically have the extension .h or .hpp.

/ / F i l e my. hv o i d foo (i n t a ) ;

/ / F i l e my. cpp# i n c l u d e my.h v o i d foo (i n t a ) {


/ / F i l e B# i n c l u d e my.h v o i d b a r (i n t a ) {

f o o ( a + 2 ) ;}

/ / F i l e C# i n c l u d e my.h v o i d t a r (i n t a ) {

f o o ( a + 4 ) ;}





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Header Files

/ / F i l e my. h

v o i d foo (i n t a ) ;

/ / F i l e my. cpp# i n c l u d e my.h v o i d foo (i n t a ) {


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.





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Linker Usage

C comes along with lots of useful standard libraries.

These libraries provide tons of useful operations.

Seemath.hfor example or

iostreamwhich provides the terminal output function


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Include Guards

/ / F i l e a . h

# i f n d e f INCLUDE A H# d e f i n e INCLUDE A Hv o i d 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 v o i d b a r (i n t a ) ;

/ / F i l e b . cpp# i n c l u d e b . h v o i d b a r (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

v o i d t a r ( ) ;

/ / F i l e c . cpp# i n c l u d e c . h v o i d t a r ( ) {\ l d o t s }




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Global Variables and Constants

/ / T h i s i s my he ad er

# i f n d e f IDENTIFIER FOR PI HEADER H#d ef in e IDENTIFIER FOR PI HEADER H/ / A l l t h e d e c l a r a t i o n s here

c o n s t d o ub l e PI ; / / T e l l s c om pi le r t h a t t h i s v a ri a b l e may n o t be / / changed .

# e n d i f

/ / T h i s i s my i m p l e m e n t a t i o n# i n c l u d e p i . h c o n s t d o ub l e P I = 2 0 ;

/ / Another f i l e# i n c l u d e p i . h \ l d o t sdouble myValue ;

\ l d o t smyValue = p i ;

Remember difference ofdeclarationand 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 tothe symbol.





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Multiple Declarations with Different Context

/ / T h i s i s he ade r A

# i f n d e f IDENTIFIER FOR HEADER A H#d ef in e IDENTIFIER FOR HEADER A H/ C al c u l a t es f o r c e between t wo p a r t i c l e s ( L ennard Jones)/

f u n c t i o n double getForc e ( c o n s t d o ub l e& d i st an ce ) ;# e n d i f

/ / T h i s i s he ade r B# i f n d e f IDENTIFIER FOR HEADER B H#d ef in e IDENTIFIER FOR HEADER B H/ C a l c u l a t e s f o r c e bet ween t wo p a r t i c l e s (WCA model )/

f u n c t i o n double getForc e ( c o n s t d o ub l e& d i st an ce ) ;# e n d i f

Sometimes, functions and variables have the same name as they were developedby 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.





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Namespaces

/ / T h i s i s he ade r A

# i f n d e f IDENTIFIER FOR HEADER A H#d ef in e IDENTIFIER FOR HEADER A Hnamespace lenn ardJo nes {

/ C al c u l a t es f o r c e between t wo p a r t i c l e s ( L ennard Jones)/

double getForc e ( c o n s t d o ub l e& d i st an ce ) ;}

# e n d i f

/ / T h i s i s he ade r B# i f n d e f IDENTIFIER FOR HEADER B H#d ef in e IDENTIFIER FOR HEADER B Hnamespace wca {

/ C a l c u l a t e s f o r c e bet ween t wo p a r t i c l e s (WCA model )

/double getForc e ( c o n s t d o ub l e& d i st an ce ) ;

}# e n d i f

With namespaces, we can group variables, constants, and functions into logic subsets.





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Using Namespaces

/ / T h i s i s t h e he ad er

# i f n d e f IDENTIFIER FOR HEADER A H#d ef in e IDENTIFIER FOR HEADER A Hnamespace lenn ardJo nes {

/ C al c u l a t es f o r c e between t wo p a r t i c l e s ( L ennard Jones)/

double getForc e ( c o n s t d o ub l e& d i st an ce ) ;}

# e n d i f

/ / T h i s i s t h e i m p l e m e n t a t i o n# i n c l u d e le n n a rd Jon e s . h double le n n a rd Jon e s : : g e tFo rce ( c o n s t d o ub l e& d i s ta nc e ) {\ 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 ;

5. Applicative ProgrammingEinfuhrung in die ProgrammierungIntroduction to C/C++, Tobias Weinzierl page 26 of 41



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The Standard Library

# i n c l u d e

i n t main ( ) {i n t a = 1 ;s t d : : c ou t


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5.4.Loops

Create three files: main.cpp,fibonacci.cpp, andfibonacci.h.

Define a function voidprintFibonacci(int max)infibonacci.cpp. It shall be embeddedinto the namespacefib.

Make printFibonacci(int max) print allFibonacci numbers

Fn= Fn1+Fn2 F0 = 0 F1 = 1

fromFn {0, max}.

At leastF0 andF1 shall be printed!

Realise the computation with a while

loop, or do it recursively.




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Do-While Loop

do {/ / something i n t e l l i g e n t

} w h i l e ( e xp re ssio n ) ;

Rewrite your code using the do-while loop.




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The For-Loop

i n t a =0 ;

do {/ / something i n t e l l i g e n ta ++;

} w h i l e ( a


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Semantics of the For Loop

f o r (i n t a = 0 ; a


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Scoping within a For Loop

i n t a=20;

f o r (i n t a = 0 ; a


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A For Loop is a While Loop

f o r (i n t a = 0 ; a


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What do These Snippets Do?

f o r (i n t a = 0 ; a


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What do These Snippets Do?

f o r (i n t a = 0 ; a


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For Best Practices

f o r (i n t a = 0 ; a


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Enums in Action


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Enums in Action

enum Co lo u r {

Red, Green , Blue} ;

\ l d o t sCo lo u r myColou r = Red ;\ l d o t si f ( myColour==Green ) {

. . .}



Enums


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Enums

enum Co lo u r {


Mind the semicolon terminating the enum definition.

Enums are basically integers (no type safety).

Enum variants belong the enclosing namespace.



Enums and Integers


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Enums and Integers

enum Co lo u r {


/ Maps c o l o u r t o a g re y v al u e /

i n t g e tGre yVa lu e ( co n st C ol ou r& c o l o u r ) {. . .

}

. . .i n t a = g e tG r ey V al u e ( 1 7 ) ;




Enums and Namespaces


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Enums and Namespaces

namespace mySpace {

enum Co lo u r {Red, / / i s mapped to 0 Green , / / i s mapped t o 1Blue / / i s mapped t o 2

} ;enum TUMColours {

Blue , / / i s mapped to 0 Orange , / / i s mapped t o 1

White , / / i s mapped t o 2 Bla ck / / i s mapped t o 3 }

}

mySpace : : Colour co lo ur = mySpace : : Blue ; / / wha t h ap pe ns?



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