Set<Integer> s = new TreeSet<Integer>(); s.add( 7 ); s.add( 7 ); System.out.println( s.size() );

Transcription

1 Advanced Java Concepts Maps and Sets and Miscellany Exercises and Programs 1. This code a) contains a compiler error. b) contains a runtime error. c) displays 1 d) displays 2 2. This code a) contains a compiler error. b) contains a runtime error. c) displays 1 d) displays 2 3. What is displayed? Look up the HashSet class in the API and examine its constructors. This is useful. Set<Integer> s = new TreeSet<Integer>(); s.add( 7 ); s.add( 7 ); System.out.println( s.size() ); Set<String> s = new HashSet<String>(); s.add( "A" ); s.add( "a" ); System.out.println( s.size() ); ArrayList<String> a = new ArrayList<String>(); a.add( "A" ); a.add( "B" ); a.add( "B" ); Set<String> s = new HashSet<String>( a ); System.out.println( a.size() + ", " + s.size() ); 4. What is displayed? (The order is not important.) a) Jane Smith, Joe Smith, Joe Jones b) Jane Smith, Joe Jones c) Joe Smith, Joe Jones d) Jane Smith Map<String, String> m = new TreeMap<String, String>(); m.put( "Smith", "Jane" ); m.put( "Jones", "Joe" ); m.put( "Smith", "Joe" ); Set<String> names = m.keyset(); for ( String nm : names ) System.out.println( m.get( nm ) + " " + nm ); 5. What is displayed? Map<Integer, Integer> m = new TreeMap<Integer, Integer>(); m.put( 1, 2 ); m.put( 3, 4 ); m.put( 5, 6 ); System.out.println( m.containsvalue( 3 ) ); System.out.println( m.put( 5, 12 ) ); System.out.println( m.put( 6, 14 ) ); System.out.println( m.put( 7, 14 ) ); System.out.println( m.remove( 5 ) ); System.out.println( m.remove( 2 ) ); System.out.println( m.size() ); 1

2 6. Practice-It. Do all the problems in the Sets and Maps section of CSE 143 (CS2). The solution for removeevenlength and a brief discussion of iterators is shown below. To traverse and remove items from a set, you must use an Iterator object. Consider the following example (a problem from Practice-It and their solution). public static void removeevenlength(set<string> set) { Iterator<String> itr = set.iterator(); while (itr.hasnext()) { String s = itr.next(); if (s.length() % 2 == 0) { itr.remove(); HashSet and TreeSet implement the Iterable interface which consists of just one method: iterator which returns an Iterator object. An Iterator object has three methods: boolean hasnext() returns true if there are more elements E next() returns the next element throws an exception if there is nothing else void remove() removes from the underlying collection the last element returned by this iterator (optional operation). 7. The Cryptogram Program. A cryptogram is phrase that has been encoded. Each letter has been assigned a new value according to some table. For example, YES encoded might become AGW BALL encoded might become XHAA if Y is replaced with an A, E with a G, and S with a W if B is replaced with an X, A with H, Ls with As Each letter of the alphabet is paired with another letter of the alphabet. All non-letters (e.g. spaces, numbers, punctuation marks, etc.) are not changed. Here is an outline of the Cryptogram class. public class Cryptogram { private Map<String, String > code; // the keys are A to Z // the values are the corresponding letters public Cryptogram(){ code = new TreeMap<String, String>(); initmap(); // continued on the next page 2

3 private void initmap(){ Initialize the map so that each key has a randomly (using Math.random) assigned value with no duplicate values. Obviously the size of the map should be 26 by the time you re done. If you need to fill up a list with the letters of the alphabet (and I m not saying you have to do this), here s a neat way: List<String> letters = new ArrayList<String>(); for ( int n = 65; n <= 90; n++ ) { String s = Character.toString( (char) n ); list.add( s ); // all uppercase letters Why 65 to 90? You should know or find out why. public String encode( String s ){ Convert s to all upper case characters. Given a string this method should replace each letter with its corresponding replacement character. Any character in s that is not in the map should be unchanged. For example, if s is car 4 sale! then the method may return INP 4 UNZO! or something like that. This method must use your map. public String tostring(){ This returns a list of the letters of the alphabet and their corresponding values. Write a short test program which first calls the tostring method. Then the program prompts the user to enter a phrase and it displays the encoded version. You should be able to look at the tostring output and verify that the phrase was properly encoded. 8. Character Frequency Program. Write the getmap method of the CharacterFrequency class. The argument is a text file and it returns a map that indicates how many times each non-space character appears in the file. A link to words.txt is on mrsawyer.com. Treat all letters as lowercase letters. Do not count the number of spaces but it should count all characters, not just letters. import java.util.*; You can use a Scanner object to read a text file. Check out the appendix. public class RunFreq { public static void main( String [] args ){ Map<String, Integer> m = CharacterFrequency.getMap( "words.txt" ); Set<String> set = m.keyset(); for ( String s: set ){ Integer num = m.get( s ); System.out.printf( "%s appears %,8d times\n", s, num ); Sample outputs on the next page. 3

4 Here s my output when the file was words.txt a appears 68,582 times b appears 17,798 times c appears 34,281 times d appears 34,551 times e appears 106,755 times f appears 12,710 times g appears 27,843 times h appears 20,200 times i appears 77,409 times j appears 1,780 times k appears 9,366 times l appears 47,011 times m appears 24,741 times n appears 60,508 times o appears 54,542 times p appears 25,788 times q appears 1,632 times r appears 64,963 times s appears 86,542 times t appears 57,057 times u appears 31,154 times v appears 9,186 times w appears 8,535 times x appears 2,700 times y appears 13,472 times z appears 3,750 times Here s my output when the file was a copy of the school s home web page. " appears 300 times # appears 43 times % appears 2 times & appears 12 times ( appears 2 times ) appears 2 times - appears 55 times. appears 54 times / appears 214 times 0 appears 148 times 1 appears 8 times 2 appears 12 times 3 appears 82 times 4 appears 4 times 5 appears 3 times 6 appears 74 times 7 appears 26 times 8 appears 4 times 9 appears 7 times : appears 71 times ; appears 27 times < appears 266 times = appears 153 times > appears 266 times? appears 2 times _ appears a appears b appears c appears d appears e appears f appears g appears h appears i appears j appears k appears l appears m appears n appears o appears p appears q appears r appears s appears t appears u appears v appears w appears x appears y appears z appears 4 times 291 times 55 times 185 times 112 times 366 times 193 times 74 times 124 times 192 times 3 times 15 times 190 times 66 times 339 times 394 times 120 times 2 times 229 times 120 times 417 times 27 times 42 times 89 times 26 times 54 times 1 times 4

5 Problems related to hash codes and hashing. 9. If b is true, then a) h1 must equal h2 b) h1 might equal h2 c) h1 will not equal h2 10. If h1 equals h2, then a) b must be true b) b might be true or false c) b must be false Assume that the Clown class is well written. Clown c1 = new Clown(); int h1 = c1.hashcode(); Clown c2 = new Clown(); int h2 = c2.hashcode(); boolean b = c1.equals( c2 ); 11. For insertions and retrievals from a hash table to be run efficiently a) the hash function must generate evenly distributed index values based on item's hash codes. b) the load factor must be around 0.75 or less. c) statement A or B d) statements A and B 12. The following Integers are to be inserted (in the order shown) into the hash table below Array Index Key value The hash function is hash code % 11. The hash code of an Integer is the integer s value. Collisions will be resolved with the "hash-and-search" method. Complete the hash table above to show where each value will be stored. 13. When using a hash table and chaining to implement a map, you should calculate the index and then a) always add the new entry to the front of the chain at that bucket. b) always add the new entry to the end of the chain at that bucket. c) either the front or end if you are using a doubly linked list to implement the chain. c) add the entry anywhere in the chain because it is an unordered collection. 5

6 14. A certain hash function h(x) on a key field places records with the following key fields into a hash table Collisions are handled with a rehashing function r(x), which takes as an argument the result of applying h(x). The key values are entered in the order show above to produce the following table: What are h(x) and r(x), respectively? a) key % 20, (result + 13) % 20 b) key % 20, result % 20 c) key % 30, (result + 14) % 20 d) key % 20, (result + 7) % 20 e) key % 30, (result + 7) % Old AP Computer Science AB Free Response Questions. Complete the following problems on either paper or using an electronic digital computer. There are links on mrsawyer.com or use the link below AP Computer Science AB Problem AP Computer Science AB Problem AP Computer Science AB Problem AP Computer Science AB Problem 3 (optional it just looks interesting). 6