design patterns

Spring 2007 NOEA - Computer Science 1

Design Patterns

Regular Expressions(Regular Languages)

State MachinesImplementation of State Machine

State Pattern

Observer PatternDelegates


Regular ExpressionsDefinition:• Regular Expressions are strings constructed by the

following rules:– Alphabet: Set of symbols:

• For instance: {a,b,c} or the ASCII char-set or…

– Epsilon (ε): empty string– Selection: For instance a | b (a or b)– Iteration: For instance (a)* (0 or more ‘a’s concatenated)– Concatenation: One string followed by an other– For instance:

• ((a|b)a)* means: an a or a b followed by an a – everything repeated 0 or more times, that is: strings of the form: ε, aa, ba, aaaa, aaba, baaa, baba,….


Regular Expressions

• Names for regular expressions:It can be helpful to give names to long regular expressions

L( (0 | 1 | 2 | … | 9) (0 | 1 | 2 | … | 9)* ) = {0,1,2,3, … 10,11,…}

or we could write:digit digit*

where digit = 0 | 1 | 2 | … | 9


Extensions

– One or More repetition ‘+’L( (0 | 1 | 2 | … | 9)+) = {0, 1, 2, … 10, 11 ,12}

– Any Character ‘.’.*b.* means at least one b

– A range of Characters ‘[]’[a-zA-Z] = a | b | … | z | A | B | … | Z [abc] = a | b | c

– Any Character Not In a Given Set~(a | b | c) means not either a or b or c.

– Optional ‘?’a? means zero or one a


Examples

• Some other examples:

– Let Σ = {a,b,c}. Consider the strings that contains exactly one b.

L( (a | c)*b(a | c )* ) = {b, abc, abaca, … }

– Let Σ = {a,b,c}. Consider the strings that contains at most one b.

L((a | c)*b(a | c )* | (a | c)* ) = {ε, a, c, b, abc, abaca, … }

or alternative

L( (a | c)* (b | ε)(a | c )*)


Exercises

1. Write a regular expression that defines valid identifiers in some programming language (the only characters allowed in an identifier are letters, digits and underscore (‘_’), and it always starts with a letter).

2. Try to write a regular expression that defines valid email addresses.

See RegExDemo.zip and jsjsgrpregexpsyntax[1].htm

http://public.noea.dk/programmeringsteknologi/lektion09/Eksempler/RegExDemo.zip

http://public.noea.dk/Programmeringsteknologi/lektion09/jsjsgrpregexpsyntax%5B1%5D.htm


State Machine(Deterministic) Finite Automata

• A Deterministic Finite Automata (DFA) is a device that is able to recognise regular expressions.

• There is an one-to-one relation between a regular expression and a DFA:– Given a regular expression, there is an algorithmic

construction of corresponding DFA

• A DFA has no state from which more than one transition is possible on the same input symbol.


ExamplesEnd state

If the DFA stops in a state that is not an end state, then input is not

valid


DFA defining Integers

digit unsigned

start integer digit

sign digit eotxt

signed other end integer

other everything

other error


Scanner Loop

state:= 1; error:= false;

while(not eotxt and not error)if (exists transition from state

marked with current input symbol)state:= the state that this transition leads toset current input to next symbol in the input

sequenceelse

error:= true;endif;

endwhile;if(error) report “error in input”; else report “input ok” endif


Exercise

3. Construct a DFA that recognises identifiers


State Pattern• Implements state machines (DFA) encapsulating

state. Provides addition of new states without changing existing code.

• Examples:– Dialog box for editing parameters to a program– XML– parsing protocols– Parser/scanner in a compiler or a browser or…– Event handling in a windows system– …..

Regular Languages

(Expressions)


State Pattern

Implements the loop that gets next state

and calls any operations connected

current state


The Classes of the Pattern

• Context: Defines the objects that we want maintain state information about (for instance DialogBox) . This class has a reference (static type: ContextState – the abstract super class) to some concrete state (that is an object of one of the sub classes – dynamic type).

• ContextState: The abstract super class defining a common interface to all the concrete states.

• ConcreteState1,...: The sub classes to ContextState. One sub class to each state in the DFA. The key to the design is in the processEvent-method, which takes an event as input and returns the next state.


OO Implementation

• State is an object• State Pattern can be applied:

– abstract class State specifies one or more abstract methods:

• transition(-) – returns state corresponding to input symbol

• action(-) – if any processing is to be done when a transition occurs (code generation, event handling etc.)

• each concrete state inherits from State and implements the abstract methods

• The parser loop uses references having the abstract class State as static type.

• Polymorphism and dynamic binding handles the rest!


Signed Integer Recogniser


OO Parser Loop public bool Scan(string input) { //input.Length>0 bool ok = false; int i = 0; char nextChar = input[i]; State currState = s1.Transition(nextChar); while (currState != s5 && currState != s4) { i++; if (i == input.Length) nextChar = '\0'; else nextChar = input[i]; currState = currState.Transition(nextChar); } if (currState == s5) ok = true; return ok;}

View the Code


Regular Languages (Expressions) and DFAs

• A regular language is a language which syntax can be define by a regular expression or a regular grammar. Regular languages is a subset of the context free languages (most programming language are context free).

• Regular languages don’t allow recursive constructions. Recursion can be transformed to iteration.

– This means that regular languages cannot have nested blocks as:• Expressions with nested parenthesis• begin-end –blocks (’{’-’}’) or similar constructs

• Regular expressions are important in validating input, parsing protocols etc.


RegEx in .NET

bool IsValidEmail(string strIn)

{

// Return true if strIn is in valid e-mail format.

return Regex.IsMatch(strIn,

@"^([\w-\.]+)@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.) |

(([\w-]+\.)+))([a-zA-Z]{2,4}|[0-9]{1,3})(\]?)$");

}

What is defined here?

Documentation


Exercises

4. Do this exercise.

5. Implement the DFA that recognises identifiers using the State Pattern


Observer Pattern

View source

BottleState

ChangeAmount()

IObserver

NotifyMe()

IObservable

NotifyAll()Add()Remove()

0..*0..1 0..*0..1

Optimist Pessimist


Observer Pattern

• En række forskellige objekter (observers) ønsker at få at vide, når et objekt (observable eller subject) skifter tilstand.

• Subject skal tillade observers at melde sig dynamisk og skal ikke vide noget om dem.

BottleState

ChangeAmount()

IObserver

NotifyMe()

IObservable


0..*0..1 0..*0..1

Optimist Pessimist


Observer Pattern

• Fx ønsker en række objekter at blive underrettet om ændringer i et andet objekt.

• Disse objekter (observers) skal implementere interfacet IObserver.

• Dvs. metoden NotifyMe, hvor deres handlinger kodes

• Subject skal holde styr på en collection af observers og kunne notificere alle objekter i collectionen (IObservable).

• Det konkrete Subject kalder NotifyAll, når der er et relevant tilstandsskift

BottleState

ChangeAmount()

IObserver

NotifyMe()

IObservable


0..*0..1 0..*0..1

Optimist Pessimist


Observer Pattern

• Observer Pattern er et OO-design, som simulerer funktionspointere i fx C og højereordensfunktioner i fx Pascal

• I C# findes sprogkonstruktionen delegate, som giver nogle af de samme muligheder. – Se eksempel

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