CMPT 120 Lecture : Mutability

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20 Oct 2011

School

Simon Fraser University

Department

Computing Science

Course

CMPT 120

Professor

Greg Baker

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CMPT 120 Full Course Notes

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Data structures that can be modified inplace like this are mutable mystr = mystr + x , others are immutable, so are dictionaries, sets, objects (if they have methods that modify. Lists are mutable: so are numbers, boolean, tuples , objects (without modifying instances, strings are immutable methods eg. card, date, consider these statements: string = string + x list = list + [0] list. append(0) Mutability mylist[3] = xyz mylist. append(18: strings can"t do this. Lists are the only type we"ve seen that can be changed without totally rebuilding.

Related Questions

8. Write a class called WordSet representing a set of String objects which can contain an arbitrary number of strings, limited only by the memory available to the Java Virtual Machine. A set is a collection of elements, however, unlike a list, a set cannot contain duplicate elements. Therefore, as its name implies, a WordSet object cannot contain duplicate Strings that is, if one attempts to da String to a Wordset, and the wordset already contains an element which is equal to this string, the attempt will fail and the state of the Wordset will not change. All String comparisons performed in the methods defined in the Wordset class are done in a Case SENSITIVE manner. Note that in the method descriptions below, the phrase "this Wordset" means "the Wordset on which the method in question is called." Your Wordset class MUST provide the following INSTANCE methods • A constructor, which takes no parameters and initializes the new Wordset ohject so that it contains no elements • A method called add(), which takes as its only parameter a string and returns a value of type boolean. If this Wordset already contains a string which is equal to the parameter string, or the parameter String is null, then the state of this Wordset does not change and the method return false; otherwise, the parameter String is added to this Wordset. and the method returns true. • A method called remove().which takes as its only parameter a string and returns a value of type boolean. If this wordSet contains a string which is equal to the parameter String this string is removed from the WordSet and the method returns true. On the other hand, if the string parameter is null or this Wordset does not contain a string which is equal to the parameter string, then the state of this wordset does not change, and the method return false. • A method called get.size(), which takes no parameters and returns the number of strings currently stored in this Wordset. This method MUST NOT change the state of this Wordset. • A method called isMember(). which takes as its only parameter a string and retumsa value of type boolean which is true if this wordset contains the parameter String false otherwise. If the parameter String is null, this method also return false. This method MUST NOT change the state of this wordset. • A method called computebirrerence().which takes as its only parameter a Wordset and returns a new Wordset representing the difference between this wordset and the parameter Wordset: in other words, the WordSet returned by this method contains all the strings that are in this wordset, but not in the parameter Wordset. If the parameter Wordset contains all the elements of this WordSet or this WordSet contains no elements, then this method returns a new Wordset containing no elements. Finally, if the parameter Wordset is null or contains ne strings, then this method returns a new wordset which contains all the elements of this wordset. This method MUST NOT change the state of this wordset or the parameter Wordset. • A method called tostring , which takes the parameters and turns a string which is the textual representation of this wordset. This textual representation consists of the concatena tion of the following clements - The String - The text representation of each clement in the WordSet; cach pair of adjacent clements is separated by the String", but this String MUST NOT appear before the first clement or after the last element - The String") The order in which the elements of this Wordset appear in the textual representation generated by the tostring() method does not matter. If the WordSet contains to Brings, then the textual representation returned by this method is ". Note that you MUST respect proper encapsulation practices that is, the attributes and methods of your class MUST be declared using the proper visibility modifier NINT Um an array list (of string) to keep track of the string stored in a wordt ob joot Is there a method defined in the Array loist class which determines whether in clementis already wored in an array ist, and which will do a case-sensitive comparison when liked on an Arraylint of strings? See the pages 24-26 of this examination for information about potentially useful methods defined by the ArrayList class WRITE YOUR Wordset CLASS IN THE SPACE BELOW:

9. Finally, you should write a program MontrealMaps which will suggest a path to go along the metro to get from one place to another. The only required method for MontrealMaps is a main method but of course you are allowed to write others. Your program should do the following. For any library methods that you use, make sure that you are properly importing things and handling any exceptions they may throw. (10 points) • Using an appropriate library method, you should create a Subway System object. If this method fails to initialize, you should print the message Error, not enough funding. • Use a Scanner and print statements to read the current x and y position of the user. • Use a Scanner and print statements to read the x and y position that the user wants to go to. . Find the nearest Metro Station to both the start position of the user and his final destination. • If the 2 metro stations have the same name (i.e. are the same metro station) then print It is fastest to walk there. • Otherwise, call the method findPath from the previous part to find a path from these 2 metro stations. . If the method returns null, then print Sorry, there is no path on the metro, you'll have to walk • Otherwise, print the path using the metro using the method printPath defined in the MapUtilities class in the appendix. SUMMARY OF JAVA STANDARD LIBRARY METHODS POR SELECTED CLASSES • String package java.lang) Methods - public boolean gist and e rs is stingeancoject - puble boolean grease Sering another ca t ering String paint i th Stre C h eshilly before the Stripe with striert w Single weiter water N ENE fusha -publish tohy Casting de . File( p age java.io) Methods r eate - publie Yiling path : C • Scanner (package java.ut 1) Method -ple S ule - double Double Sc - le bon when : S Scanner tal de fethede h able othebean - puble String Best Mus eethe u s tepal -pulle haalean haxtLine: Check the hea ther in left PrintStream( p age java.io) Methods pul vaid pri Pribonaat - puli wid pride d oble vile. - pille vaid print Print vei. -pulle vaid printject o: Prints Object -puli vaid printString : Prints String .. - vaid primi The theme line by writing the li n e tie id print in bool bl. Prise n value the - public void platinabled Prints double ved and then the line - public void printit Prints Ent value and then tomis the - Pull vaid pri l l be t he minime publie wild prin t ing with String in the terminates the line • ArrayLiat package java.util) Methods - public boolean Arraybettype was a new ArrayList

5. Consider the following class: import java.util.Scanner; public class Money private int dollars; private int cents; public Money (int d, int c) { dollars = d; cents = c; /* Point 1 */ public String toString() { String result; if (cents > 100) int totalCents = dollars * 100 + cents; dollars = totalCents / 100; cents = total Cents % 100; /* Point 2 */ /* Point 3 */ result = "$" + dollars + ", " + cents + "C"; return result; public static void main(String[] yourGrandfather) Scanner keyboard - new Scanner (System.in); Money amount; int C; /* Point 4 */ System.out.print("Enter an amount in cents: "); C = keyboard.nextInt (); amount = new Money (c/ 100, CX 100); System.out.println("You have: " + amount.toString()); The above program compiles and runs without error. Which variables are in scope at each of the points marked by comments in the above class? In other words, if each of these comments were replaced by an actual Java statement, which variables could be used in this statement without a prefix (that is, without having to write the name of a class or other variable in front of the variable name) and without the compiler reporting an unknown variable error? List each variable that is in scope at each of the points marked by comments in the above class, and indicate whether it is an instance variable, a formal parameter, or a local variable. Note that you will be penalized for every variable that is out of scope at a given point but that you list as being in scope at that point.

CMPT 120 Lecture : Mutability

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