CS261 Data Structures. Iterator ADT Dynamic Array and Linked List

Transcription

1 CS261 Data Structures Iterator ADT Dynamic Array and Linked List

2 Goals Why do we need iterators? Iterator ADT Linked List and Dynamic Array Iterators

3 IteraAng through a Linked List Write a code segment to print the elements of our LinkedListDeque implementaaon.

4 Iterator Concept Problem: How do you provide a user of a container access to the elements, without exposing the inner structure? Think of two developers: one wriang the container (that s you!!!), the other using the container (that s someone using your library to build an applicaaon where they need, for example, a stack implementaaon)

5 Traversing a Container as Developer For example, within the Linked List container you (the developer) wrote a loop such as the following: struct LinkedList *list; struct Link *l;... /* IniAalize list. */ l = list->frontsentinel->; while(l!=list->backsentinel){ do something l=l->; } This is fine within the container library code itself, but we don t want users of the container library to have to know about links or worse yet, be able to manipulate links!

6 EncapsulaAon Chapter 5: Hide the implementaaon details behind a simple and easy to remember interface (ie. abstracaon mechanism) Users should not know about links, arrays, size, capacity, etc. Users should know and use the public abstracaons: push, pop, contains, remove, etc.

7 How do we abstract away loops for(i = 0; i < sizedynarr; i++) val = getdynarr(i); while( cur!= backsent) do something cur = cur -> do { something cur = cur->; }while (cur!= backsent

8 Iterator ADT SoluAon: define an interface that provides methods for wriang loops int hasnextiter(struct Iter *itr); TYPE Iter(struct Iter *itr); void removeiter(struct Iter *itr); void changeiter(struct Iter *itr, TYPE val); void additer(struct Iter *itr, TYPE val);

9 Iterator: Typical Usage TYPE cur; /* current collection val */ struct linkedlist *list; linkedlistiterator *itr; list = createlinkedlist( ) itr = createlinkedlistiter(list) while (hasnextlistiter(itr)) { cur = ListIter(itr); if (cur...) removelistiter(itr); }

10 Simplifying AssumpAons & ProperAes of the Iterator ADT FuncAon and hasnext must be interleaved Call remove a\er Cannot call remove twice in a row without a calling hasnext

11 LinkedListIterator struct linkedlistiterator { } struct linkedlist *lst; struct dlink *currentlink; struct linkedlistiterator *createlinkedlistiterator ( struct linkedlist *lst) { } struct linkedlistiterator *itr; itr = malloc(sizeof(struct linkedlistiterator)); itr->lst = lst; itr->currentlink = lst->frontsentinel;

12 A\er IniAalizaAon list Itr current List backsent frontsent Link Link front

13 Strategy HasNext Returns T (F) to the user if there is (is not) another value remaining to be enumerated Updates current Next Return the value to be enumerated Remove Removes the value (and link) that was last returned by call to Next()

14 A\er IniAalizaAon list Itr while (hasnextlistiter(itr)) { curval = ListIter(itr); } current List backsent frontsent Link Link front

15 A\er One IteraAon:hasNext, list Itr while (hasnextlistiter(itr)) { curval = ListIter(itr); } current List backsent frontsent front

16 A\er Two IteraAons list Itr while (hasnextlistiter(itr)) { curval = ListIter(itr); } current List backsent frontsent front

17 A\er Three IteraAons list Itr current while (hasnextlistiter(itr)) { } curval = ListIter(itr); We ve enumerated all elements Subsequent calls to HasNext will evaluate to false List backsent frontsent front

18 A\er Two IteraAons then Remove Remove the last value enumerated (10) Where should current be a\er the removal? list Itr current while (hasnextlistiter(itr)) { curval = ListIter(itr); if(curval == 10) removelistiter(itr); } List backsent frontsent front

19 A\er Two IteraAons then Remove list Itr Remove the last value enumerated (10) Where should current be a\er the removal? current List frontsent backsent 5 15 front

20 Your Turn Worksheet#24 Linked List Iterator