Records A Tuple (Mathematically): is a function f : { 0, 1, , n 1 - PowerPoint PPT Presentation

Records A Tuple (Mathematically): is a function f : { 0, 1, · · · , n − 1 } → Set Example: ( 5, − 3.1, 6.0, ′ c ′ , ′′ foo ′′ , 13, 14, ′ d ′ ) J.Carette (McMaster) CS 2S03 1 / 19

Records A Tuple (Mathematically): is a function f : { 0, 1, · · · , n − 1 } → Set Example: ( 5, − 3.1, 6.0, ′ c ′ , ′′ foo ′′ , 13, 14, ′ d ′ ) bad style !: Instead of implicit labels use explicit labels! Tuples (Programming Languages): usually of named fields tuple of named fields ≡ records Example: labels: { latitude, longitude, altitude } record: { latitude = 48.715, longitude = 2.208, altitude = 156 } J.Carette (McMaster) CS 2S03 1 / 19

Defining a Record (Java) class Point { final double latitude; final double longitude; final double altitude ;} J.Carette (McMaster) CS 2S03 2 / 19

Records - Declaration Point x; x r null J.Carette (McMaster) CS 2S03 3 / 19

Records - Declaration Point x; x r null new Point (); x 0.0 0.0 0.0 The value of this expression is the reference r ′ = { latitude = 0.0, longitude = 0.0, altitude = 0.0 } J.Carette (McMaster) CS 2S03 3 / 19

Records - Allocation x = new Point (); x 0.0 0.0 0.0 the environment is e ⊕ [ x = r ] the memory is [ r = r ′ , r ′ = { latitude = 0.0, longitude = 0.0, altitude = 0.0 } ] You can also write: Point x = new Point (); J.Carette (McMaster) CS 2S03 4 / 19

Records - Allocation - Cont. The following statement creates the environment [ x = r ′ ] and the memory state [ r ′ = { latitude = 0.0, longitude = 0.0, altitude = 0.0 } ] . final Point x = new Point (); x 0.0 0.0 0.0 J.Carette (McMaster) CS 2S03 5 / 19

Records - Accessing Fields x.latitude x.thing : error at compile time Consider an expression x with x mutable of type Point . Then value of x is m ( e ( x ) (a record) and the value of x . latitude is that of the field latitude of m ( m ( e ( x )) . x e" m" m" 0.0 0.0 0.0 J.Carette (McMaster) CS 2S03 6 / 19

Assignment of Fields class Point { final double latitude; final double longitude; final double altitude ;} Point x = new Point (); x x 0.0 0.0 0.0 ✔ ! ✖ ! 0.0 0.0 0.0 The compiler “knows” which fields are mutable and not and will enforce it! J.Carette (McMaster) CS 2S03 7 / 19

Constructs for Handling Records define a type: class allocate a cell: new access a field: t.l assign to a field: t.l = u; J.Carette (McMaster) CS 2S03 8 / 19

Constructors x = new Point (48.715 , 2.208 , 156.0); To use, we need to change the class and add a constructor: Point (final double x, final double y, final double z) { this.latitude = x; this.longitude = y; this.altitude = z;} a constructor must always have the same name as the type it belongs to, in the definition of a constructor, you never define a return type, you never use the return statement in the body of a constructor, in the body of a constructor, you can assign to fields of the allocated record using the keyword this to refer to this record. J.Carette (McMaster) CS 2S03 9 / 19

Constructors - Cont. 1 Constructors can be overloaded ! J.Carette (McMaster) CS 2S03 10 / 19

Constructors - Cont. 1 Constructors can be overloaded ! 2 When no constructor is defined, there is a default constructor with no arguments. You cannot use the default constructor once one has been defined in your program. ( bad style! ) J.Carette (McMaster) CS 2S03 10 / 19

Constructors - Cont. 1 Constructors can be overloaded ! 2 When no constructor is defined, there is a default constructor with no arguments. You cannot use the default constructor once one has been defined in your program. ( bad style! ) 3 When declaring a type, it is also possible to define default values for each field: class Point { double latitude = 90.0; double longitude = 0.0; double altitude = 0.0;} J.Carette (McMaster) CS 2S03 10 / 19

The Semantics of Records To cover records: we need to add a fifth argument to the functions Σ and Θ : the list of constructed types, each type being associated to an ordered pair composed: T �→ ( list of fields, list of constructors ) J.Carette (McMaster) CS 2S03 11 / 19

Sharing In the first case (sharing), all changes of the cell associated with x automatically change the cell associated with y, and vice versa. x ! x x ! x 48.715 ! 2.208 ! 156.0 ! 48.715 2.208 156.0 48.715 ! 2.208 ! 156.0 ! 48.715 2.208 156.0 y ! y ! y y 90.0 0.0 0.0 90.0 ! 90.0 0.0 0.0 0.0 ! 0.0 ! 90.0 ! 0.0 ! 0.0 ! x = y (Sharing!) y,lat = x.lat x ! x y.long = x.long 48.715 ! 2.208 ! 156.0 ! 48.715 2.208 156.0 y.alt = x.alt y ! y 48.715 ! 2.208 ! 156.0 ! 48.715 2.208 156.0 J.Carette (McMaster) CS 2S03 12 / 19

Equality a = new Point (1, 2, 3); b = new Point (1, 2, 3); Two types of equality: Physical. Two records of the same type are physically equivalent (a==b) only when they are identical (share the same cell). Hence, a == b is false. Structural. Two records of the same type are structural equivalent when their field’s value are equal. Hence, a and b are structural equivalent. J.Carette (McMaster) CS 2S03 13 / 19

Wrapper Types (from Base type to Object type) A wrapper is a type of record with one lone field. class Integer { int c; Integer (int x) { this.c = x; } } Why? 1 Uniformity 2 Sharing J.Carette (McMaster) CS 2S03 14 / 19

Wrapper Types - Cont. y ! y x ! x 4 ! 5 ! 4 5 Integer x = new Integer(4); int x = 4; Integer y = new Integer(x.c); int y = x; x.c = 5; x = 5; System.out.println(y.c); System.out.println(y); prints 4 prints 4 If we replace the second line with Integer y = x then it prints 5 x ! x 5 ! 5 y ! y J.Carette (McMaster) CS 2S03 15 / 19

Wrapper Types - Cont. The following function swaps values of x and y. static void swap (Integer x, Integer y) { int temp = x.c; x.c = y.c; y.c = temp ;} Remember, the following functions do nothing on x and y: static void swp (int x, int y) { int temp = x; x = y; y = temp ;} J.Carette (McMaster) CS 2S03 16 / 19

Wrapper Types - Cont. When we call the function swap(a, b) , we create the state: b ! b a ! a 7 ! 7 4 ! 4 x ! y ! y x and so swapping the contents of x and y will also swap those of a and b. If a, b are constant ( final ), and x, y too, then: a ! x ! b ! y ! a x b y 4 ! 7 ! 4 7 and swap works too. J.Carette (McMaster) CS 2S03 17 / 19

Records in Caml 1 Record Type: type point = { latitude : double; longitude : double; altitude : double ;} 2 Creating: let x = {lat = 90.0; longi = 0.0; altitude = 0.0;} or let y = ref{lat = 90.0; longi = 0.0; altitude = 0.0;} 3 Accessing: x.latitude (!y). latitude 4 Assigning: all fields are constant by default! type int -wrap = {mutable c: int} let x = {c = ref 5} x.c <- 4 J.Carette (McMaster) CS 2S03 18 / 19

Records in C 1 Record Type: struct Point { double latitude; double longitude; double altitude ;}; 2 Creating: struct Point x; or struct Point x = {5.0 ,7.0 ,100.0}; 3 Accessing: x.latitude 4 Assigning: all fields are constant by default! x.latitude = 12.0 Call by copy! (the value of x is not a reference associated with a record in memory as it is in Java and Caml!) J.Carette (McMaster) CS 2S03 19 / 19