GCE Computing. COMP3 Problem Solving, Programming, Operating Systems, Databases and Networking Report on the Examination.

Transcription

1 GCE Computing COMP3 Problem Solving, Programming, Operating Systems, Databases and Networking Report on the Examination 2510 Summer 2014 Version: 1.0

2 Further copies of this Report are available from aqa.org.uk Copyright 2014 AQA and its licensors. All rights reserved. AQA retains the copyright on all its publications. However, registered schools/colleges for AQA are permitted to copy material from this booklet for their own internal use, with the following important exception: AQA cannot give permission to schools/colleges to photocopy any material that is acknowledged to a third party even for internal use within the centre.

3 Question 1 In Part 1 (a) candidates were expected to apply their knowledge of parallel and serial transmission to identify which was most appropriate in three different scenarios. Candidates appeared to find this a more difficult task than to state properties or advantages of the different methods, which is what questions have focused on in previous papers. Almost all students were able to achieve one mark and over a third achieved full marks. Part 1 (b) was well tackled, with over two thirds of students managing to achieve the mark. The most common correct response was that a handshake would be used to check if a device was ready to receive data. Some students gave responses that related to the use of handshakes on networks that were also creditworthy. Answers that suggested that the data payload would be transmitted as part of the handshake were not awarded any marks. Part 1 (c) was also well tackled, with over two thirds of students managing to achieve the mark. Some students gave a generic description of latency and some defined it in terms of data transmission, which was the context used in the question. A small number failed to achieve a mark by giving a response that mixed up the two types of response, such as It is the delay between data being transmitted and its effects being felt. Question 2 Overall, students demonstrated a good understanding of the use of regular expressions. For part 2 (a), students needed to explain that the expression would accept any string that consisted of zero or more b characters followed by a c. Common mistakes were to confuse the + and * operators and therefore to state that one or more b characters would be required, and to apply the * operator to the c to its right rather than the b to its left. Some students wrote any number of b characters which was not enough for a mark. It had to be made clear that zero or more characters were required to achieve the mark. For part 2 (b), students needed to explain that the expression would accept any string that consisted of zero or one b characters followed by a c. Many candidates simply listed the two strings that would be accepted, which were bc and b, which was an acceptable alternative response. Some candidates confused the meaning of the? operator with either the + or the operators. The simplest correct response to part 2 (c) was b(cd)*(e fg), although any valid regular expression was creditworthy. Some students missed marks by using the wrong operator instead of the * or by missing out brackets which were important for defining the scope of the operators. Three quarters of students got some marks for this question part and half got full marks. Question 3 The overwhelming majority of students were able to correctly identify that it was the binary search algorithm that required the list to be sorted for part 3 (a). The trace for part 3 (b) was also well completed with about three quarters of students getting some marks and well over half getting full marks. For part 3 (c), around half of the candidates were correctly able to explain that the value of InnerPointer did not decrease to zero because either the second while loop condition was not 3 of 8

4 satisfied or the number being moved did not need to be inserted at the start of the list. The most common mistake was to state that the list had already been sorted, which was not enough for a mark. For part 3 (d), about two thirds of students correctly identified that the algorithm that they had traced was of time complexity O(n 2 ). Part 3 (e) was poorly answered, with only about one third of students correctly identifying that the algorithm they had traced was an insertion sort. Bubble sort was a far more common but incorrect response. Parts 3 (f) (i), 3 (f) (ii) and 3 (g) were all well answered. The most common error in both parts of 3 (f) was to perform a traversal of the tree instead of using it as a binary search tree. Question 4 As in previous years, candidates demonstrated a very good understanding of floating point representation. Parts 4 (a) and 4 (b) were extremely well tackled, with over three quarters of students achieving full marks. Part 4 (c) was not tackled as well, with less than half of the candidates achieving both marks. Some candidates mistakenly moved the binary point four places to the right instead of the left or miscalculated the exponent to be 12 instead of -4. Others did not know how to deal with the binary point being moved to the left past the sign bit in a negative number and so ended up with a positive number instead of a negative number at the end. Part 4 (d) was well tackled, with the majority of students achieving full marks and two thirds achieving at least two of the three marks. Those students who did not achieve full marks most commonly went wrong when converting 108 in unsigned binary into -108 in two s complement. Another mistake was to write out the binary place values backwards when working out the bit pattern for 108, ie 1,2,4,8 etc. The vast majority of candidates achieved at least one mark for part 4 (e), correctly recognising that overflow occurred when a value was too large to be stored in the number of bits available. The clearest answers explicitly stated that the number of bits available for storing the number was insufficient, rather than making more vague references to, for example, memory space. Some students failed to achieve a mark because they related the number of bits to the mantissa alone. Around two thirds of candidates were able to identify correctly that the situation which might cause overflow was division for part 4 (f). Question 5 Part 5 (a) was well tackled with over two thirds of candidates recognising that a queue was an appropriate data structure to represent the deck of cards because it was a first-in-first out structure. Just over half of the candidates were able to correctly update the pointers to the queue in part 5 (b) (i) and just under half were able to do likewise in part 5 (b) (ii). In the former, the most common mistakes were to change FrontPointer to 10 instead of 11 and to leave QueueSize at 52. In the latter, the most common mistake was to change RearPointer to 54, failing to recognise that this was a circular queue and RearPointer should change to 2. 4 of 8

5 Students achieved a broad range of marks on question part 5 (b) (iii) which required them to write an algorithm for dealing a card. Over two thirds got some marks, but just under 10% scored full marks. The most commonly made mistakes were to try to deal more than one card, to fail to wrap FrontPointer back to the start of the array when it went past the end of it, and to deal a card when the deck was empty. Part 5 (c) was well answered, with most students recognising that in an event-driven program, subroutines would be associated with events, such as a button being clicked or a timer reaching 0, and that the appropriate subroutine would be called when the event occurred. Some responses were not considered creditworthy as they could have applied to any style of programming, for example the program waits for user input before executing instructions. Approximately two thirds of students achieved some marks for comparing a mobile phone operating system and a desktop operating system in part 5 (d). A common mistake was to compare the devices rather than the operating systems, for example stating that the desktop would have more memory that the mobile phone. Many students explained that the mobile phone OS would have lower hardware requirements but it would have been nice to see some more concrete examples of these reduced requirements, and phrases such as smaller and more lightweight were used too often. Some students gave responses to a question from an earlier paper that were not appropriate for this scenario. Question 6 Overall, students demonstrated a satisfactory ability to use SQL in question parts 6 (a) to (c). Part 6 (a) was the worst tackled of the three. Commonly made mistakes when defining the table in this part were to use incorrect SQL data types, to include the field EngineSize, to declare RegistrationNumber to be an integer or to use non-sql syntax. Some students attempted to put a constraint on the PolicyType to ensure that it could only be one of the two valid possibilities. Students were not expected to know how to do this, so incorrect attempts at doing so were not penalised. In part 6 (b), just under half of students got full marks. Many students did not appear to really know the correct syntax of the SQL Update command, but achieved an easy mark by writing the first line of the command as Update Vehicle. A common mistake was to fail to put quotation marks around the registration number or colour values. Responses to part 6 (c) were disappointing, given the frequency that students have been asked to write SQL queries in the exam over the years, and the fact that this was a fairly simple example, involving only two tables. Only one third of candidates achieved full marks, though half managed to achieve three of the four marks. As in part 6 (b), one common mistake was to miss quotation marks around the registration number. Other mistakes were to use the keyword GET instead of SELECT and to miss out the AND operator in the WHERE clause. Pleasingly, most students realised that a condition was needed to link the two tables together. Parts 6 (d) (i) to (iii) were about server side scripts. Just under half of candidates were able to achieve full marks for part (i) by explaining that a server side script was program code that was executed on a server. Some also recognised that the trigger for this could be the requesting of a web page and that the output would be a web page. Part (ii) was the least well tackled part, with only around one third of students achieving any marks. The Request object is used to fetch userinputted data from the web server that it will have been sent when the web page that the data was entered on was submitted. The majority of students believed that the Request object was used to 5 of 8

6 either query a database or that the execution of the command would trigger a request to the user to input data at that time rather than retrieving already input data. Almost all of the candidates got one of the two marks for part (iii) but few clearly explained that the output would be written to a web page which the web server would return to the web browser on the Police Officer s handheld terminal. Part 6 (e) was about extending the design of the database to store safety certificate information. The majority of candidates scored at least two of the three marks. Most candidates correctly identified that a new table would need to be created, what the fields in this table would need to be, and that CertificateNumber would be the primary key. Those candidates who scored two but not three marks usually made a mistake with regard to how the new table would be linked to the existing database, stating that the CertificateNumber would be added to the Vehicle table as a foreign key. This solution would not work as this would only allow one certificate to be associated with each vehicle, and it was required that previous certificates could also be stored. The correct solution was to put the RegistrationNumber from the Vehicle table into the new table as a foreign key. Question 7 More than half of the candidates were able to correctly define abstraction in part 7 (a). The question was asked in the context of data representation, so an appropriate definition would have been to store only those details of a problem/model that were required in the context of the problem being solved. Common mistakes were to state that unnecessary complexity would be hidden from the user, rather than being removed from the model altogether, or to define a simulation instead of abstraction. Part 7 (b) was the question that assessed quality of written communication. As such, it was particularly important the candidates used technical terms accurately when writing their responses. Almost all candidates covered both aspects of the question: the representation as a graph and the representation using arrays as an adjacency matrix or list. In almost all responses, the graph part of the answer was better than the array representation part, with most students being able to correctly explain how the underground railway network could be represented as a graph. Some students lost marks by not using the correct terminology or by drawing diagrams but not explaining points such as that a station would be represented as a node. As the question paper stated, diagrams needed to be fully explained if they were used. A common mistake was to state that stations would be represented by nodes and that railway tracks would be represented by vertices; vertex in a synonym for node but was being mistakenly used instead of the term edge or arc. A small but not insignificant number of students misinterpreted the term graph and explained how the network might be represented as a bar chart or scattergraph. The part of the question relating to the representation using arrays as an adjacency matrix or list was poorly tackled. Many students discussed how the representation could be drawn out as a grid or list on paper instead of how it could be represented using arrays in a programming language, or described solutions that were not really either an adjacency matrix or list. Those who decided to represent the data as an adjacency matrix often suggested that the station names would be written into the array rather than that stations would be associated with a number that would be used as an index into the array. That said, pleasingly, most students who suggested a matrix representation recognised that the distances would be filled into the array at the 6 of 8

7 appropriate location and that a null value would be used where there was no route. Some students used an array containing only 0s and 1s instead of distances. Students who chose to present an adjacency list solution rarely explained how this would be achieved in a programming language, focussing instead on how it could be depicted diagrammatically. Another common mistake was to talk about pointers, without explaining how these would work in the context of the array. Question 8 Question part 8 (a) asked students to explain what inheritance was. Just under two thirds of students were able to do this. Those students who failed to achieve the mark usually did not make clear that the relationship was between a parent class and a sub class or, if they did this correctly, failed to explain the nature of the relationship, ie that the sub class could share some of the methods or properties of the parent. Students had to draw an inheritance diagram in part 8 (b). Almost 90% of candidates achieved at least two of the three available marks, but only half of the candidates achieved full marks. The failure to achieve the third mark was usually because a candidate failed to style the diagram correctly, either not drawing arrowheads, drawing them at the wrong end of the lines, or not enclosing the names of the classes in some type of box. A very good range of responses was made to question part 8 (c). The most common mistakes that students made were to fail to identify that the new class was a sub class of the Selector class, to fail to redefine the SelectItemFromList procedure so that it was overridden, to add in extra unnecessary functions and procedures, and to redefine the data items from the parent class. Question 9 Question part 9 (a) was a trace of the execution of a Turing machine. This was very well tackled with over three quarters of students achieving full marks. Part 9 (b) required students to explain the overall effect of three of the rules of the Turing machine s transition function. The overall effect was that the tape head would move right along the string until the end of the string was found, without changing the contents of the tape. When the end of the string was located, the state would change to S CO and the head would move left. The most common mistake that students made was to explain what each individual rule did rather than what the purpose of the rules taken together was. Part 9 (c) was poorly tacked, with only slightly over a quarter of students achieving any marks. A universal Turing machine can be seen to work as an interpreter because it reads instructions in order from a tape and executes them in sequence. This is similar to how an interpreter reads instructions in order from memory and executes them in sequence. Many students either defined what a universal Turing machine was, or answered the question that has been asked on a previous paper about the importance of them. Question 10 Question parts 10 (a) (i) to (iii) all required students to determine IP addresses. In each part, approximately three quarters of students did so correctly. Common mistakes were to write IP 7 of 8

8 addresses that were formed from three octets instead of four, to write IP addresses that were not appropriate for the segment or to give a value of 0 for the last octet of an IP address. Part 10 (b) was well tackled with the vast majority of students achieving at least one mark and just under half achieving both marks. Many students discussed collisions or the effects of cable failure. Students who missed out on the second mark usually did so because they failed to explain their answer in enough technical detail. Parts 10 (c) (i) and (ii) were about Software as a Service, or SaaS. The vast majority of students achieved one mark on each part, although only just over a third achieved both marks for part (i). For part (i), the most commonly seen correct responses related to the fact that the software could be used on Internet-connected computers outside of the office and that the hardware requirements would be lower for SaaS. Some candidates made good points about the company not having to update the software, but others mistakenly suggested that the company would only have to update the software once, confusing SaaS with a thin client system. With SaaS, the company would not have to update the software at all as this would be handled by the provider of the SaaS. For part (ii), the most commonly seen correct response was that unreliability of the Internet connection or service would make the software inaccessible. Part 10 (d) was poorly tackled, with just under half of students achieving any marks. The most commonly seen correct response was that a LAN would use baseband and a WAN broadband. Other valid responses included that a LAN would have faster transmission speeds and lower latency than a WAN and that more security issues might need to be dealt with on a WAN. Some students compared the communication media that would be used. Those who did so in detail often achieved a mark, but many made vague or incorrect points such as that a LAN would be wireless and a WAN wired. Some students assumed that a WAN and the Internet were the same thing and gave responses relating to IP addresses that did not really answer the question. Mark Ranges and Award of Grades Grade boundaries and cumulative percentage grades are available on the Results Statistics page of the AQA Website. Converting Marks into UMS marks Convert raw marks into Uniform Mark Scale (UMS) marks by using the link below. UMS conversion calculator 8 of 8