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Design Patterns

Design Patterns. Regular Expressions (Regular Languages) State Machines Implementation of State Machine State Pattern Observer Pattern Delegates. Regular Expressions. Definition: Regular Expressions are strings constructed by the following rules: Alphabet: Set of symbols:

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Design Patterns

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  1. Design Patterns Regular Expressions (Regular Languages) State Machines Implementation of State Machine State Pattern Observer Pattern Delegates NOEA - Computer Science

  2. Regular Expressions Definition: • 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,…. NOEA - Computer Science

  3. Regular Expressions • Names for regular expressions:It can be helpful to give names to long regular expressionsL( (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 NOEA - Computer Science

  4. 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 NOEA - Computer Science

  5. 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 )*) NOEA - Computer Science

  6. Exercises • 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). • Try to write a regular expression that defines valid email addresses. See RegExDemo.zip and jsjsgrpregexpsyntax[1].htm NOEA - Computer Science

  7. 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. NOEA - Computer Science

  8. Examples End state If the DFA stops in a state that is not an end state, then input is not valid NOEA - Computer Science

  9. NOEA - Computer Science

  10. DFA defining Integers NOEA - Computer Science

  11. 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 to set current input to next symbol in the input sequence else error:= true; endif; endwhile; if(error) report “error in input”; else report “input ok” endif NOEA - Computer Science

  12. Exercise • Construct a DFA that recognises identifiers NOEA - Computer Science

  13. 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) NOEA - Computer Science

  14. State Pattern Implements the loop that gets next state and calls any operations connected current state NOEA - Computer Science

  15. 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. NOEA - Computer Science

  16. 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! NOEA - Computer Science

  17. Signed Integer Recogniser NOEA - Computer Science

  18. 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 NOEA - Computer Science

  19. 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. NOEA - Computer Science

  20. 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 NOEA - Computer Science

  21. Exercises • Do this exercise. • Implement the DFA that recognises identifiers using the State Pattern NOEA - Computer Science

  22. Observer Pattern View source NOEA - Computer Science

  23. 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. NOEA - Computer Science

  24. 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 NOEA - Computer Science

  25. 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 NOEA - Computer Science

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