Understanding Searching and Map ADTs: Fundamentals for Large Scale Programming

1. CSC 213 – Large Scale Programming Lecture 10:MAPS, Searching, & Map ADT

2. Today’s Goal • Consider the basics of searchable data • How do we search using a computer? • What are our goals while searching? • ADTs used for search & how would they work? • Most critically, where the $&*#%$# are my keys? • How does Map ADT work & enable searching? • Methods to add, remove, and access data? • Sequence-based Maps would be implemented how? • When & why would we use Sequence-based Maps

3. Searching • Search for unknown data in most cases • Consider the reverse: why search for what you have? • Seek data related to search terms used • Already have idea, want web pages containing terms • Get encoded proteins given set of amino acids • Given “borrowed” credit cards, get credit limits • Exacting, but boring, work doing these searches • Make this work ideal for computers & students

4. Smart Parrot We captured the ship,drank their rum, & stole treasure chests. Life is good.

5. Smart Parrot Do you have the keyso we canget its valuables? We captured the ship,drank their rum, & stole treasure chests. Life is good.

6. Smart Parrot Do you have the keyso we canget its valuables? …

7. Smart Parrot Do you have the keyso we canget its valuables? You know, I could just eat you.

8. Search Terms • Keygets valuables • We already have key • Want valueas a result of this • Mapworks similarly • Give it keyvaluereturned • Uses Entryto do this work

9. Entry ADT • Each instance of Position holds 1 element • Abstracts storage of items in a Collection • Useful for Lists: make elements available only • Searching needs more: data has multiple parts • First part is the key: data we have • Value: data we want is second item

10. Entry Interface • Need a key to get valuables • key used to search – it is what we already have • What we want is the result of search – value interface Entry<K,V> { K key();V value(); }

11. Map Method Madness, Mmmm… • Describes a searchable Collection • put(K key, V value)adds data as an Entry • remove(K key)removes Entry containing key • get(K key)returns valueassociated with key • Several Iterablemethods are also defined • Methods to use are entries(), keys(), & values() • Iterates over expected data so can use in for(-each) loops • Also defines usual Collectionmethods • isEmpty() & size()

12. Too Smart Parrot We got another ship, even more rum, & I got the key, too. It’s party time.

13. Too Smart Parrot Great! What about keys to these valuables? We got another ship, even more rum, & I got the key, too. It’s party time.

14. Too Smart Parrot Great! What about keys to these valuables? We got another ship, even more rum, & I got the key, too. It’s party time.

15. Too Smart Parrot Great! What about keys to these valuables? …

16. Too Smart Parrot Great! What about keys to these valuables? You know, I hear you taste like chicken.

17. At Most 1 Value Per Key • Entrys have unique keys in a Map • If key exists, put(key,value)replaces existing Entry • Returns prior value forkey in the Map so its not lost • If before call key not in Map, null returned

18. Smart Parrot Well, I tried the key on all the treasures like you suggested. Nothing happened!

19. Smart Parrot Nothing was thrown? I thought you’d crash! Well, I tried the key on all the treasures like you suggested. Nothing happened!

20. Smart Parrot Nothing was thrown? I thought you’d crash! Cook, start the grill!We are having bird tonight!

21. Searching Through a Map • Map is a Collection of key-valuepairs • Give it key& get value in return from ADT • Now we have ADT to work with searchable data • Many searches unsuccessful • Unsuccessful search is normal, not unusual • Expected events should NOTthrow exceptions • This is normal; return null when nothing found

22. Sequence-Based Map • Using Sequence is easiest Map implementation • Using an ADT means it allows any implementation • Uses all elements, so a List does make sense • Only needs to store Entry as the element

23. Sequence-Based Map • Sequence’s perspective of Mapthat it holds Positions elements

24. Sequence-Based Map • Outside view of Map and how it is stored Positions Entrys

25. Sequence-Based Map Performance • get & removeboth take ____ time

26. Sequence-Based Map Performance • get & remove both take O(n) time • Scans entire Sequencewhen key is not found

27. Sequence-Based Map Performance • get & remove both take O(n) time • Scans entire Sequencewhen key is not found • puttakes ______ time

28. Sequence-Based Map Performance • get & remove both take O(n) time • Scans entire Sequencewhen key is not found • puttakes O(n)time also • Go through Sequenceto see if already has key • If the key is found, replace Entry’s value • Make & add Entry if no match exists in Map • When could this be used?

29. Lessons from Polly… • Used to convert the keyinto value • valuescannot share keyand be in same Map • When searching failure is not exceptional

30. Before Next Lecture… • Week #4 assignment due Tuesday at 5PM • Continue to do reading in your textbook • Learn more about hash & what it means in CSC • How can we tell if a hash is any good? • Hash tables sound cool, but how do we make them? • Monday is when lab project phase #1 due • Will have time in lab, but then will be the weekend