Lists, Stacks, and Queues (plus Priority Queues)

Transcription

1 Lists, Stacks, and Queues (plus Priority Queues) The structures lists, stacks, and queues are composed of similar elements with different operations. Likewise, with mathematics: (Z, +, 0) vs. (Z,, 1) List Stack (LIFO) Queue (FIFO) Priority Queue create empty str. test emptiness add elem to head [push] add elem to end add elem access head elem [top] access front elem access highest prio. elem access tail [pop] top of stack remove front elem remove highest prio. elem modify head modify tail J.Carette (McMaster) CS/SE 2S03 1 / 1

2 Implementation a Stack via Wrapping a List c l a s s Stack { p r i v a t e L i s t c ; Stack ( f i n a l L i s t x ) { t h i s. c = x ; } } Stack ( ) { t h i s. c=n u l l ; } p u b l i c boolean i s e m p t y ( ) { r e t u r n t h i s. c == n u l l ; } p u b l i c void push ( i n t x ) { t h i s. c = new L i s t ( x, t h i s. c ) ; } p u b l i c i n t top ( ) { r e t u r n t h i s. c. hd ; } p u b l i c void pop ( ) { t h i s. c = t h i s. c. t l ; } } There are two bugs in this code! J.Carette (McMaster) CS/SE 2S03 2 / 1

3 Functional Stack c l a s s FStack { p r i v a t e f i n a l L i s t c ; FStack ( ) { t h i s. c = n u l l ; } FStack ( f i n a l L i s t l ) { t h i s. c = l ; } s t a t i c boolean i s e m p t y ( f i n a l FStack l ) { r e t u r n l. c == n u l l ; } s t a t i c FStack push ( f i n a l i n t a, f i n a l FStack l ) { r e t u r n new FStack (new L i s t ( a, l. c ) ) ; } s t a t i c FStack pop ( f i n a l FStack l ) { r e t u r n new FStack ( l. c. t l ) ; s t a t i c i n t top ( f i n a l FStack l ) { r e t u r n l. c. hd ; } } Note final FStack l means that l is never modified but its contents may be. J.Carette (McMaster) CS/SE 2S03 3 / 1

4 Exceptional Circumstances Two exceptional circumstances in FStack executions: pop of an empty stack top of an empty stack How to Deal With them: 1 figure out and deal with usual situations. 2 (Old Style Programming) let weird stuff happen otherwise: (Newer Style Programming) deal with unusual situations explicitly. J.Carette (McMaster) CS/SE 2S03 4 / 1

5 Exceptions - Cont. Safety conscious (Using Exceptions): 1 figure out unsafe situations. 2 deal with those first. 3 once safe do something usual. s t a t i c i n t top ( f i n a l FStack l ) throws E x c e p t i o n { i f ( l == n u l l ) throw new E x c e p t i o n ( ) ; r e t u r n l. c. hd ; } J.Carette (McMaster) CS/SE 2S03 5 / 1

6 Catching Exceptions. Specifying what to happen when the function raises an exception: The construct try p catch (Exception e) q; t r y { i n t x = top (new FStack ( ) ) ; } catch E x c e p t i o n e ) { system. out. p r i n t l n ( oh oh! ) ; } The exceptions will propagate until caught. Note. in {p 1 p 2 }, if p 1 throws exception, p 2 is not executed. (like return) J.Carette (McMaster) CS/SE 2S03 6 / 1

7 Exception Handling in Caml and C. Caml: throw e is written raise e. try with C: There are no exceptions in C. some constructs like long jumps have some similarities. J.Carette (McMaster) CS/SE 2S03 7 / 1

8 Exception - Cont. We can have new classes of exceptions (you can make your own) We can catch multiple exceptions try-catch-finally: The finally block always executes when the try block exits. This ensures that the finally block is executed even if an unexpected exception occurs. Error Messages in Java: System. out. p r i n t l n ( 1 / 0 ) ; throws the exception: j a v a. l a n g. A r i t h m e t i c E x c e p t i o n : / by z e r o J.Carette (McMaster) CS/SE 2S03 8 / 1

9 Objects and Classes - Dynamic Methods A class is a type plus some functions (methods) and constructors on that type. J.Carette (McMaster) CS/SE 2S03 9 / 1

10 Objects and Classes - Dynamic Methods A class is a type plus some functions (methods) and constructors on that type. Dynamic methods: void push ( f i n a l i n t a ) { c = new L i s t ( a, c ) ; } void pop ( ) { c = c. t l ; } We call these methods with p.pop(); p.push(5); dynamic belongs to an object. static belongs to a class. J.Carette (McMaster) CS/SE 2S03 9 / 1

11 Dynamic Methods - Cont. Stack p = new Stack ( ) ; p. push ( 5 ) ; p. push ( 6 ) ; System. out. p r i n t l n ( p. top ( ) ) ; p. pop ( ) ; The printed result of the above code is 6. If System.out.println(p.top()); is used after p.pop(); then the result is 5. Empty stack is an object with c == null. A common error is to write a dynamic method: L i s t f ( ) { i f ( t h i s == n u l l )... } (this == null) always false!: the method f cannot be called when the object is null. J.Carette (McMaster) CS/SE 2S03 10 / 1

12 Static Fields When we modify a static field, it is modified for all the class (global fields!). c l a s s M { s t a t i c i n t mem;... J.Carette (McMaster) CS/SE 2S03 11 / 1