Introduction to Computational Linguistics

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9. Objects and Methods 24 This does the following: it defines a data type number. The object of this type can be manipulated by three so–called methods: get, which gives you the value, succ which adds 1 to the value, and assign which allows you to assign any number you want to it. But it must be an integer; you may want to find out how OCaML knows that x must be integer. For this is what it will say: (66) class number : object val mutable x : int method assign int −> method get : int method succ : end Notice that it uses the type . This is the type that has no object associated with it. You may think of it as the type of actions. For succ asks OCaML to add the number 1 to the number. Once we have defined the class, we can have as many objects of that class as we want. Look at the following dialog. (67) # let m = new number;; val m : number = # m#get;; - : int = 0 The first line binds the identifier m to an object of class number. OCaML repeats this. The third line asks for the value of m. Notice the syntax: the value of m is not simply m itself. The latter is an object (it can have several parameters in it). So, if you want to see the number you have just defined, you need to type m#get. The reason why this works is that we have defined the method get for that class. If you define another object, say pi, then its value is pi#get. And the result of this method is the number associated with m. Notice that the third line of the class definition (??) asserts that the object contains some object x which can be changed (it is declared mutable) and which is set to 0 upon the creation of the object. If you define another object q and issue, for example q#succ and then ask for
10. Characters, Strings and Regular Expressions 25 the value you get 1: (68) # let q = new number;; val q : number = # q#succ;; - : unit = () # q#get;; - : int = 1 Notice the following. If you have defined an object which is a set, issuing method get = x will not help you much in seeing what the current value is. You will have to say, for example, method get = PStringSet.elements x if the object is basically a set of pairs of strings as defined above. 10 Characters, Strings and Regular Expressions We are now ready to do some actual theory and implementation of linguistics. We shall deal first with strings and then with finite state automata. Before we can talk about strings, some words are necessary on characters. Characters are drawn from a special set, which is also referred to as the alphabet. In principle the alphabet can be anything, but in actual implementations the alphabet is always fixed. OCaML, for example, is based on the character table of ISO Latin 1 (also referred to as ISO 8859-1). It is included on the web page for you to look at. You may use characters from this set only. In theory, any character can be used. However, there arises a problem of communication with OCaML. There are a number of characters that do not show up on the screen, like carriage return. Other characters are used as delimiters (such as the quotes). It is for this reason that one has to use certain naming conventions. They are given on Page 90 – 91 of the manual. If you write \ followed by 3 digits, this accesses the ASCII character named by that sequence. OCaML has a module called Char that has a few useful functions. The function code, for example, converts a character into its 3–digit code. Its inverse is the function chr. Type Char.code ’L’;; and OCaML gives you 76. So, the string \076 refers to the character L. You can try out that function and see that it actually does support the full character set of ISO Latin 1. Another issue is of course your editor: in order to put in that character, you have to learn how your editor lets you do this. (In vi, you type either ¡Ctrl¡V and then the numeric code
Page 1 and 2: Introduction to Computational Lingu
Page 3 and 4: 2. Practical Remarks Concerning OCa
Page 5 and 6: 3. Welcome To The Typed Universe 5
Page 11 and 12: 4. Function Definitions 11 4 Functi
Page 13 and 14: 5. Modules 13 can find out why the
Page 15 and 16: 5. Modules 15 let length l = length
Page 17 and 18: 6. Sets and Functors 17 write it do
Page 19 and 20: 7. Hash Tables 19 to actually see t
Page 21 and 22: 8. Combinators 21 can be used on an
Page 23: 9. Objects and Methods 23 of type
Page 27 and 28: 10. Characters, Strings and Regular
Page 29 and 30: 10. Characters, Strings and Regular
Page 31 and 32: 11. Interlude: Regular Expressions
Page 37 and 38: 12. Finite State Automata 37 Suppos
Page 39 and 40: 12. Finite State Automata 39 There
Page 41 and 42: 12. Finite State Automata 41 The pr
Page 43 and 44: 12. Finite State Automata 43 Proof.
Page 45 and 46: 13. Complexity and Minimal Automata
Page 51 and 52: 14. Digression: Time Complexity 51
Page 53 and 54: 14. Digression: Time Complexity 53
Page 55 and 56: 15. Finite State Transducers 55 Con
Page 57 and 58: 15. Finite State Transducers 57 And
Page 59 and 60: 16. Finite State Morphology 59 Then
Page 61 and 62: 17. Using Finite State Transducers
Page 67 and 68: 18. Context Free Grammars 67 18 Con
Page 69 and 70: 18. Context Free Grammars 69 symbol
Page 71 and 72: 18. Context Free Grammars 71 which
Page 73 and 74: 19. Parsing and Recognition 73 Z
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19. Parsing and Recognition 75 numb
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20. Greibach Normal Form 77 The red
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20. Greibach Normal Form 79 Definit
Page 81 and 82:
20. Greibach Normal Form 81 Proposi
Page 83 and 84:
20. Greibach Normal Form 83 nonterm
Page 85 and 86:
21. Pushdown Automata 85 pushdown.
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21. Pushdown Automata 87 machine is
Page 89 and 90:
22. Shift-Reduce-Parsing 89 differe
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23. Some Metatheorems 91 If the loo
Page 93 and 94:
23. Some Metatheorems 93 This proof
Page 95 and 96:
23. Some Metatheorems 95 a decompos
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Introduction to Computational Linguistics

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