Quarter 1 Practice Exam

Transcription

1 University of Chicago Laboratory Schools Advanced Placement Computer Science Quarter 1 Practice Exam Baker Franke 2005 APCS - 12/10/08 :: 1 of 8

2 1.) (10 percent) Write a segment of code that will produce the following pattern of X s in a console window. A solution that does not use nested for-loops will not receive full credit. X XX XXX XXXX XXXXX XXXXXX XXXXXXX for(int i=0; i<7; i++){ for(int j=0; j<7; j++){ if(i<=j) System.out.print( X ); System.out.println(); 2.) (20 percent) Write a method called arraydiff which accepts two arrays of ints as parameters, compares the two arrays and returns an array of booleans that indicates which indices in the two arrays hold different values. The example below shows two arrays arr1 and arr2 and the array of booleans that should be returned (where T = true, and F =false). arr1 -> [3][4][5][2][8][6][4][5][9][1][7] arr2 -> [3][4][3][2][7][6][4][5][5][1][7] returns-> [F][F][T][F][T][F][F][F][T][F][F] In the example above the array of booleans was created by repeatedly comparing the elements in arr1 and arr2 at some index i. If the values in arr1 and arr2 at index i are different, then index i in a boolean array of the same length as arr1 should be set to true, otherwise false. If the two arrays passed to the method are not equal in length, the method should return null. Write the method below. Be sure to write a proper method header. public boolean[] arraydiff(int[] a, int[] b){ if(a.length!= b.length) return null; boolean[] result = new boolean[a.length]; for(int i=0; i<a.length; i++){ result[i] = (a[i]!= b[i]); return result; Baker Franke 2005 APCS - 12/10/08 :: 2 of 8

3 3. (30 points) Write a simple class that represents a bank account called BankAccount. A bank account has an account number and some amount of money in it. At this bank you can carry a negative balance under certain conditions read the method descriptions to find out the conditions. A bank account has the following methods: getbalance returns the current account balance. deposit deposits some amount of money into the bank account. If the account currently has a negative balance, the amount deposited must be enough to make the balance positive or zero, otherwise the deposit is not accepted. withdraw takes some amount of money out of the account but only if the account currently has a positive balance and the result of withdrawing won t make the account have less than $ Write the entire class choosing appropriate data types, return types, private variables and method parameters. The BankAccount constructor should initialize the bank account with a balance of 0. There should be NO CALLS to System.out.println in this code. public class BankAccount{ private int accountnum; private double balance; public BankAccount(int acctnum){ balance = 0; accountnum = acctnum; public double getbalance(){ return balance; public boolean deposit(double amt){ if(amt+balance >=0){ balance += amt; return true; return false; public boolean withdraw(double amt){ if(balance>0 && balance-amt >= -1000){ balance -= amt; return true; return false; Baker Franke 2005 APCS - 12/10/08 :: 3 of 8

4 4. A robot starts on 3,2 facing East. There will be a row of beepers in front of the robot of indeterminate length. The problem will take you through several steps of building recursive methods. First consider the class in which you will be writing these methods: public class CountBot extends Robot{ public CountBot(int s, int a, Direction d, int b){ super(s,a,d,b); private int countpile(){ /* Assume this method works. It will count a pile of beepers on the corner this robot is currently standing it. It will return the pile to its original state and return the number of beepers on the pile. */ public int getlength(){ ///problem 1 public void outandback(){ //problem 2 public int getlengthandback(){ //problem 3 public int addandgetlength(int sum){ //problem 4 1. Write the recursive method called getlength which returns an integer that is the number of consecutive beeper piles that the robot is facing (In the example above the method would return 4). This method should only use one robot. Here is the algorithm: The robot should move forward If it s not next to a beeper it should return 0. If it s next to a beeper it should return 1 + another call to getlength() public int getlength(){ if(!nexttoabeeper()) return 0; Baker Franke 2005 APCS - 12/10/08 :: 4 of 8

5 else return 1+getLength(); 2. Write the method outandback which is similar to getlength. It should recursively, walk to the end of the pile and then walk back to where it started. For this one, at you stopping case, have the robot turn around. The recursive calls should have the robot move, recurse, and move again if the robot turns around at the end then the second move will be moving it the opposite direction. public int outandback(){ if(!nexttoabeeper()){ turnaround(); else{ outandback(); 3. Write getlengthandback which combines getlength and outandback. It should return the number of beeper piles and the robot should end up where it started. This is really just a combination of the code you wrote for 1 and 2. public int getlengthandback(){ if(!nexttoabeeper()){ turnaround(); return 0; else{ int result = 1+getLength(); return result; 4. Write addandgetlength which does the same thing as getlengthandback AND it also deposits a beeper pile at the end of the row containing a number of beepers equal to the sum of all the piles. This method requires your recursion do work on the way down as well as on the way back up the recursive stack. The sum should be accumulated as the robot walks down the row by passing a value down through the method argument sum. When it gets to the end, it should Baker Franke 2005 APCS - 12/10/08 :: 5 of 8

6 deposit a beeper pile (you can use World.placeBeepers), turnaround and walk back returning the length of the row as it goes. public int addandgetlength(int sum){ if(!nexttoabeeper()){ for(int i=0; i<sum; i++){ putbeeper(); //assumes robot starts with enough beepers turnaround(); return 0; else{ int mysum = sum+countpile(); int result = 1+addAndGetLength(mySum); return result; Baker Franke 2005 APCS - 12/10/08 :: 6 of 8

7 5.) (20 points) Write a console application that repeatedly asks the user for, and reads in, a pair of strings. The program should count the number of times the strings are the same and the number of times they re different. The program should stop asking for strings when: you ve collected 100 pairs; either of the strings is quit ; or you ve collected six consecutive pairs that are the same. Before the program exits, you should print out the number of times the pair was the same and the number of times the pair was different. /** NOTE: in this solution if the user enters quit it gets counted as a pair. That maybe desirable or it may not be. **/ public class Test{ public static void main(string[] args){ String one=, two= ; int samecount=0, totalcount=0, conseccount=0; Scanner S = new Scanner(System.in); while( conseccount<6 && totalcount <100 &&!one.equals( quit ) &&!two.equals( quit )){ System.out.println( Enter two strings ); one = S.next(); two = S.next(); if(one.equals(two)){ samecount++; conseccount++; else{ conseccount=0; totalcount++; System.out.println( Num times same: +samecount); System.out.println( Num times different: + (totalcount-samecount)); Baker Franke 2005 APCS - 12/10/08 :: 7 of 8

8 5. Please know the definitions of these terms and be able to use them with examples: Inheritance Polymorphism Interface Abstract Class Abstract Method Strategy Think of classes of real things that you could describe with a Java class structure. Example: A coffee shop sells multiple items. Every item has a price and a stock number. The coffee shop sells multiple kinds of coffee and multiple kinds of snacks. Every type of coffee has a size. There is regular coffee and Espresso drinks which include latte and cappuccino. Draw a class diagram with boxes and arrows, etc. that describes the Objects at this coffee shop. Be sure to label your diagram with the list of words shown above where appropriate. Also invent your own hierarchy for some snacks that a coffee shop might offer. (NOTE: there are many correct solutions to this, but more credit will be given for designs that use more than just plain classes in ways that are robust and appropriate.) Disclaimer: You might feel that this question requires you to know a lot about coffee in order to answer fully this is true, to some degree. However, if you answered something like what s written below, you ll be in good shape. A class structure might include an interface called Item at the root of the hierarchy. It would have methods getprice() and getstocknum() which all sub-classes would have to implement. This would leave storing the price and stock number values to the subclasses at well. Alternatively, you could make Item an abstract class with protected variables that subclasses could inherit. Either way is acceptable. At the next level you could have abstract classes Coffee and Snack which would implement methods common to all respective sub-classes. For example: all coffee drinks would have a size, (small, medium, large), so the Coffee class could implement methods setsize( ) and getsize(). Coffee could have abstract methods as well, for example: setmilkamount( ) since the amount of milk means totally different things for regular coffee and espresso. You want abstract methods so that you can force things like setmilkamount( ) to be handled polymorphically when called, rather than assuming a default, inherited, behavior. Finally, you would have classes RegularCoffee and EspressoDrink extend Coffee since they are types of Coffees Items. These classes at the bottom of the hierarchy represent actual things you can buy. That is, the number of leaves at the bottom of this inheritance tree would be equal to the number things you could actually buy in the coffee shop. Furthermore, The path from a class at the bottom of the inheritance tree, up to the root, really defines what it is. Baker Franke 2005 APCS - 12/10/08 :: 8 of 8