Data Structures and Abstract Data Types

Transcription

1 Data Structures and Abstract Data Types What are we ing today: Introduction to Abstract Data Types (ADTs) specification of a data structure without giving details of its implementations ne via short form, or interface, of a class / clauses crucial! Development of data structures from these class invariants useful Generic data structures (classes) COMP1120: DA Lecture 3: Data Structures and Types

2 Abstraction Example COMP1120: DA Lecture 3: Data Structures and Types

3 Requirements for a Telephone Answering Machine Simulator (Answers incoming calls after set delay.) (Plays welcome message.) Records caller s message. Replays caller s message on request. (Records new welcome message on request.) Our simple phone system will be based on characters rather than sounds. A message will therefore be a sequence of characters. COMP1120: DA Lecture 3: Data Structures and Types

4 Data Structures for Storing Messages an array building up B o r n i n t h e i an array building wn i e h t n i n r o B a linked list CHAR_ELT CHAR_ELT CHAR_ELT CHAR_ELT letter: next: B letter: o letter: r letter: n next: next: next: COMP1120: DA Lecture 3: Data Structures and Types

5 An Abstract Data Type for Messages make full letter valid_letter rewind finished app next class interface MESSAGE A Telephone Message creation make (size: INTEGER) An empty message Max size is size finished ; not valid letter feature(s) from MESSAGE app (c: CHARACTER) add c to not full COMP1120: DA Lecture 3: Data Structures and Types

6 An Implementation (Data Structure) of the ADT for Messages feature {NONE} curr : INTEGER curr. letter is data@curr len : INTEGER msg. is data@1..data@len data: ARRAY[CHARACTER] make(size:integer) is empty msg...!! data.make(1, size) len := 0; curr := 0 finished ; not valid letter feature app(c:character) is add c to not full len := len + 1; data.put(c, len) not finished letter : CHARACTER is current letter valid letter Result := data.item(curr) rewind is curr := 0 not valid letter next is get next letter not finished curr := curr + 1 valid letter valid letter : BOOLEAN is letter available? Result := 1<=curr and curr<=len finished : BOOLEAN is no more? Result := curr = len full : BOOLEAN is full? Result := len = data.upper invariant data.lower = 1; 0<=curr and curr<=len and len<=data.upper class MESSAGE COMP1120: DA Lecture 3: Data Structures and Types

7 A Smarter Answering Machine Suppose we want a phone that can record many messages, rather than just one: (Answers incoming calls after set delay.) (Plays welcome message.) Records caller s message in a queue. Replays next message on request. Rewinds to first message on request. (Records new welcome message on request.) COMP1120: DA Lecture 3: Data Structures and Types

8 class interface MESS QUEUE creation make (size: INTEGER) new queue finished ; not valid message feature(s) from MESS QUEUE record (m: MESSAGE) record m not full not finished message: MESSAGE current message A Message Queue Abstract Data Type valid message valid message: BOOLEAN message avail? next get next message not finished valid message finished : BOOLEAN no more? full : BOOLEAN full? reset not valid message of MESS QUEUE COMP1120: DA Lecture 3: Data Structures and Types

9 Generalising Messages and Message Queues MESSAGE MESS_QUEUE make app full letter valid_letter finished rewind next SEQUENCE [G] make record full message valid_mess finished reset next make restart full item valid_item finished put next COMP1120: DA Lecture 3: Data Structures and Types

10 Generic Classes We are familiar with the concept of generic containers: a set of students a list of jobs a set of units a list of prizes Here the terms set and list describe generic containers. Eiffel lets you parameterise classes to create generic containers. The classes MESSAGE and MESS QUEUE differ only in the kinds of objects they store. We can define a single generic class of sequences to both jobs. COMP1120: DA Lecture 3: Data Structures and Types

11 class interface SEQ[G] creation make (size: INTEGER) new sequence finished ; not valid item feature(s) from SEQ put (e: G) add e to not full item : G not finished current item valid item valid item : BOOLEAN item available? next get next item not finished valid item finished : BOOLEAN no more? full : BOOLEAN full? restart of SEQ[G] not valid item COMP1120: DA Lecture 3: Data Structures and Types

12 Using Generic Classes The class ARRAY is generic. You have already been using generic classes! calar: ARRAY[MONTH] A message is just a sequence of characters: message: SEQ[CHARACTER] A message queue is just a sequence of messages: message queue: SEQ[SEQ[CHARACTER]] COMP1120: DA Lecture 3: Data Structures and Types

13 The Sequence Implementation class SEQ[G] creation make feature put(e:g) is add e to not full len := len+1 data.put(e,len) not finished item : G is current item valid item Result := data.item(cur) valid item : BOOLEAN is item available? Result := (1<=cur) and (cur<=len) COMP1120: DA Lecture 3: Data Structures and Types

14 Summary assertions play a key role in specifying ADTs and implementing corresponding data structures the class invariant, as well as the precondition, might be d to convince us the body of the routine in fact establishes the postcondition we should also check the body the maintains invariant classes with similar features often recur software reuse is enhanced by using generic classes the SEQ[G] implementation: the above implementation uses features of another generic class, ARRAY[G] a direct implementation is also possible. A very relevant example is in text book pages COMP1120: DA Lecture 3: Data Structures and Types