| Date | May 2019 | Marks available | 4 | Reference code | 19M.2.HL.TZ0.17 |
| Level | HL | Paper | 2 | Time zone | no time zone |
| Command term | Construct | Question number | 17 | Adapted from | N/A |
Question
The organizing school arranges for visiting students to stay with a host family. The information about each of the visiting students is stored in a file. The student records are sorted by name. Some of the other variables included are school, gender, age and host family name, as shown in the UML diagram below.
A program needs to be written to match visiting students with host families. The matching process requires data to be manipulated extensively (adding, editing, deleting). The file will be read into RAM.
This program will be used for different events with different numbers of visitors. Therefore, it will be implemented using a dynamic data structure.
It has been decided to use a single linked list named guests to store and manipulate the Visitor objects.
Consider the following diagram which represents the list guests.
The following recursive method has been written to act on the list guests.
public void recursive(int k, char a)
{
if (k == guests.size())
{
output ("no such record");
}
else
{
Visitor current = guests.get(k);
if ((current.getGender() == a) && (current.getAge() > 15))
{
output current.getName();
}
else
{
recursive(k + 1,a);
}
}
}
Define the term object reference.
Outline one reason why a linked list may be more suitable than a binary tree in this particular situation.
Construct the code needed to instantiate an object guests of the LinkedList class.
Construct the code for the method penultimate() that returns the second to last element in the linked list guests. You may assume that guests is locally accessible.
Using the data provided in the diagram, trace the call recursive(0,'F'), clearly showing the levels of recursion.
Outline one reason why the use of a recursive method may be inappropriate for linked lists.
Due to unforeseen circumstances, schools may cancel their participation in an event at the last minute. Therefore, a method is needed to remove all the visiting students of one school from the linked list guests.
Construct the method removeSchool that takes the name of a school as parameter and removes all students of that school from the list guests.
Markscheme
Award [1 max].
A reference is a variable whose value points to the location of an object (in memory);
Award [2 max].
The ease of which pointers or references can be manipulated in a linked list;
Allows easier addition/deletion of objects (compared to a binary tree);
It can be difficult to manipulate pointers or references in a binary tree;
Which makes it more difficult to add or delete objects in a binary tree;
Award [1 max]. Allow missing final parentheses.LinkedList<Visitor> guests = new LinkedList<Visitor>();
Award [4 max].
Award [1] for correct signature.
Award [1] for using size() or finding the correct size of the linked list *
Award [1] for correctly returning the penultimate element.
Award [1] for correctly returning null if there is no penultimate element in the list.
public Visitor penultimate(LinkedList guests) // parameter
is optional
{
Visitor result = null;
if(guests.size() > 1) {
result = guests(guests.size()-2);
}
return result;
}
*Note to examiners – allow a method that will iterate through the class as long as a valid LinkedList class methods are used.
Award [4 max].
Award [1] for showing the second call to the method
Award [1] for showing the third call to the method (and no subsequent calls)
Award [1] for including the correct test outcome
Award [1] for correct output
recursive(0,'F')
k = 0
current is Ana
test incorrect
recursive(1,'F')
k = 1
current is Ben
test incorrect
recursive(2,'F')
k = 2
current is Defne
test correct
output Defne Tasci
Award [2 max].
For a large linked list, this would require a large number of recursive calls / become very memory intensive;
Which may cause stack overflow;
A linked list only allows for sequential access;
Therefore, using recursion would not lead to any gains in efficiency;
Award [6 max].
Award [1] for the correct initialisation of index.
Award [1] for correct loop/iterator declaration and initialisation
Award [1] for the correct assignment to current.
Award [1] for the correct comparison of school (allow use of =)
Award [1] for the correct remove (allow correct use of next pointer)
Award [1] for the decrementing index after remove.
Award [1] for correct increment of index / use of iterator.next(). Within context.
Example answers:
public void removeSchool(String school) {
int index = 0;
Visitor current;
while(index < guests.size()) {
current = guests.get(index);
if(current.getSchool().equals(school)) {
guests.remove(index);
index--;
}
index++;
}
}
public void removeSchool(String school) {
Visitor current;
for(int index = 0; index < guests.size();index++) {
current = guests.get(index);
if(current.getSchool().equals(school)) {
guests.remove(index);
index--;
}
}
}
public void removeSchool(String school) {
Iterator<Visitor> itr = guests.iterator();
Visitor current;
while(itr.hasNext()){
Visitor current = itr.next();
if(current.getSchool().equals(school)) {
itr.remove();
}
}
}
Examiners report
Not many candidates related this term to a location in the memory.
A binary tree may be ideal when searching for items, but when extensive editing is required then the ability of a linked list to easily manipulate its pointers makes this structure the preferred option.
Reasonably well answered.
When answering questions regarding a library class then the use of the correct methods are expected. Not many candidates were familiar with the use of the size method.
This was a quite straightforward trace of a recursive algorithm which most candidates completed well.
Some candidates alluded to the excessive use of memory if recursion was used.
Many candidates made a reasonable attempt but too many tried to use generic methods instead of ones specifically pertaining to this class.
Syllabus sections
- 17N.2.HL.TZ0.19a: Explain why recursion is a suitable tool to use when performing this check.
- 18N.2.HL.TZ0.16d.i: Define the term recursion.
-
18N.2.HL.TZ0.16d.ii:
Trace the call
process(runway1,0)given the diagram ofrunway1as drawn above. Copy and complete the following table.
-
18M.2.HL.TZ0.19a:
Identify two essential features of a recursive algorithm.
-
19M.2.HL.TZ0.17a:
Define the term object reference.
-
19M.2.HL.TZ0.17d:
Using the data provided in the diagram, trace the call
recursive(0,'F'), clearly showing the levels of recursion. -
19M.2.HL.TZ0.17e:
Outline one reason why the use of a recursive method may be inappropriate for linked lists.
- 18M.2.HL.TZ0.18a: Outline why a linked list structure has been chosen for allCustomers.
-
18M.2.HL.TZ0.19b:
Copy and complete the following table to show the output when the method is called by:
recursionEx(3);
-
19M.2.HL.TZ0.17b:
Outline one reason why a linked list may be more suitable than a binary tree in this particular situation.
-
19M.2.HL.TZ0.17c.i:
Construct the code needed to instantiate an object
guestsof theLinkedListclass. -
17N.2.HL.TZ0.18a:
Outline one benefit that the use of either of these choices will have over the original array structures.
-
17N.2.HL.TZ0.18c:
Explain how a binary tree structure would allow a more efficient search for the
Itemobject. -
17N.2.HL.TZ0.19b:
The method
palindrome()is called byboolean t = palindrome(word);
wherewordis a String variable.Without writing code, describe the recursive method
palindrome()that returns whether or not a word is a palindrome. -
18N.2.HL.TZ0.16a:
Outline an advantage of using a library.
-
18N.2.HL.TZ0.16b:
Evaluate the use of dynamic linked lists to static arrays when managing arriving airplanes.
-
19N.2.HL.TZ0.17a:
Define the term object reference.
-
19N.2.HL.TZ0.17b:
Using object references, construct the method
public Cart removeFirst()that removes the firstCartNodeobject fromlist. The method must return theCartobject in that node ornulliflistis empty. -
20N.2.HL.TZ0.16c:
Identify the steps that would be involved in taking the data from this two-dimensional array and creating a binary search tree.
-
20N.2.HL.TZ0.17b:
A stack can be implemented using an array or a singly linked list.
Compare the use of these two data structures for creating a stack. -
19N.2.HL.TZ0.17c:
Sketch the linked list
listafter running the following code fragment wherecart1, cart2, cart3, cart4andcart5have been instantiated as objects of the classCart.POSlist queueList = new POSlist()
queueList.addLast(cart2);
queueList.addLast(cart1);
queueList.addLast(cart4);
queueList.removeFirst();
queueList.addLast(cart5);
queueList.addLast(cart3);
queueList.removeFirst(); -
18N.2.HL.TZ0.16c:
Sketch the resulting linked list when an airplane with ID RO225, STA 12:05 and delay 0 is added to this list.
-
19N.2.HL.TZ0.17f:
Explain the importance of using coding style and naming conventions when programming.
-
18M.2.HL.TZ0.19d:
Identify three features which should be included in either program code or UML diagrams to help programmers understand and modify programs in the future.
-
19N.2.HL.TZ0.17d:
Outline one feature of the abstract data structure queue that makes it unsuitable to implement customers waiting in line.
-
17N.2.HL.TZ0.18b:
Construct the method
changePrice(). -
18M.2.HL.TZ0.19c:
Explain what would happen if the method was called by
recursionEx(-3). -
20N.2.HL.TZ0.16b:
Outline why searching for the registration plate in a binary tree would be faster than looking for it in the two-dimensional array.
-
18M.2.HL.TZ0.18b:
Construct the method
goldMails()that will return anArrayListcontaining the email addresses of all current “Gold” members. You should make use of any previously defined methods. -
19M.2.HL.TZ0.17f:
Due to unforeseen circumstances, schools may cancel their participation in an event at the last minute. Therefore, a method is needed to remove all the visiting students of one school from the linked list
guests.Construct the method
removeSchoolthat takes the name of a school as parameter and removes all students of that school from the listguests. -
19N.2.HL.TZ0.17e:
Using object references, construct the method
public Cart leaveList(int n)that removes the nthCartNodeobject fromlist. The method must return theCartobject in that node.You may assume that the nth
CartNodeobject exists in the list.
You may use any method declared or developed. -
20N.2.HL.TZ0.17a:
Construct the
staffRemoveCarmethod. -
18N.2.HL.TZ0.16e:
In case of bad weather it is possible that one of the runways has to be closed and the two linked lists (
runway1andrunway2) need to be merged using a methodmergeLists()into one new linked listrunway.Construct the method
public LinkedList<Arrival> mergeLists()that merges the two sorted linked lists in to one sorted linked listresultwhich needs to be returned. You may assume thatrunway1andrunway2are accessible to the method and do not need to be passed. The original two lists are allowed to becomenullas a result of the merging.In answering this question, you may use the following methods from the JETS
LinkedListclass, in addition to any method provided or developed in this exam paper.addFirst(<object>) // adds the object at the head of the list
addLast(<object>) // adds the object at the tail of the list
getFirst() // returns the head object in the list
getLast() // returns the tail object in the list
removeFirst() // removes and returns the head object
removeLast() // removes and returns the tail object
size() // returns the number of objects in the list
isEmpty() // returns true if the list is empty
