Getting ready for your A Level Computer Science exam? Test our knowledge by answering all the following questions:
A Level Computer Science RevisionOpen in New Window
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A Level Computer Science Revision
Debugging and refining an algorithm – Q&A
Question 1[4 marks]
Zara has created the following program in her computer science lesson. The aim of this program is to find out if the user is old enough to vote or not, the voting age being 18 years old. If the user is too young to vote, the program will calculate and output in how many years the user will be allowed to vote!Here is her Python code:
#Voting Age Checker
a = int(input("How old are you?"))
if a<=18:
print(You can vote!")
else:
y = 18 - a
print("You will be allowed to vote in " + str(y) + " years!")
Zara made two errors in her code. A syntax error and a logic error. Identify these two errors and explain how these could be fixed.
Syntax Error:
Logic Error:
Logic Error:
Question 2[2 marks]
Explain the difference between a logic error and a syntax error.
Question 3[3 marks]
In order to test her program, Zara is designing a test plan. She wants to use specific input values to complete three types of tests:
- Normal/Valid Test
- Boundary/Borderline/Extreme Test
- Erroneous Test
- Valid/Normal Test Data:
- Boundary Test Data:
- Erroneous Test Data:
Question 4[2 marks]
Identify two purposes of testing.
Question 5[2 marks]
Identify two ways, Zara could use to make her code easier to understand and maintain.
Question 6[5 marks]
Zara would like to create a function using her code. The function should be called checkVotingAge(). It should
- take one parameter (the age of the user as an integer value),
- output the messages as it currently does
- return a Boolean value as to whether the user is allowed to vote or not.
Solution...
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Refining Algorithms – Q&A
Question 1[3 marks]
Somjay has created the following program in his computer science lesson. The aim of this program is to generate and display five random “lucky” numbers every time this program is executed.Here is his Python code:
print("Your five lucky numbers are...")
number = random.randint(1,100)
print(number)
number = random.randint(1,100)
print(number)
number = random.randint(1,100)
print(number)
number = random.randint(1,100)
print(number)
number = random.randint(1,100)
print(number)
Somjay has received feedback from his teacher who asked him to rewrite his code using iteration. Rewrite this code to take into consideration the feedback Somjay received from his teacher.
Question 2[3 marks]
Somjay is investigating a method to save all the numbers from a list into a text file, one number per line. He has typed the following code to do so:
list = [4, 6, 12, 98, 57, 32, 31, 6]
for i in range (0,len(list)):
file = open("stats.txt","a") # "a" -> append mode
file.write(list[i])
file.close()
His teacher believes this code is not as efficient as it could be, especially if the list of numbers was to increase. He believes this code could easily be refined to minimise the number of times the text file is opened and closed when the code is run. Rewrite this code to improve its efficiency based on this feedback.
Question 3[4 marks]
Somjay is now focusing on a piece of code that should ask the user to enter a star rating (a value between 0 and 5) and display an error message if the star rating is invalid.
Here is Somjay’s code:
rating = int(input("Enter a rating between 1 and 5"))
if rating < 0:
print("This rating is to low...")
elif rating > 5:
print("This rating is too high!")
else:
print("This rating is valid!")
Somjay would like to refine this code so that the computer keeps asking the user to enter a rating until a valid rating is being entered. He believes this could be done using a while loop.
Refine this code to meet Somjay’s requirement.
Solution...
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Discount Price Calculator – Q&A
Question 1[10 marks]
Oksana has created the following program in her computer science lesson. The aim of this program is to calculate the price after discount of a list of products bought by a customer.Here is her Python code:
#Discount Price Calculator
def calculateDiscount(price,percentageDiscount):
discount = price * percentageDiscount / 100
discount = round(discount,2)
return discount
#Main Program
item = input("Enter the name of the item to buy? or X to quit!")
while item!="X":
itemPrice=float(input("Enter a price for your item: £"))
percentageDiscount = int(input("Enter a percentage discount for your item: %"))
discount = calculateDiscount(itemPrice, percentageDiscount)
newPrice = itemPrice - discount
print("Discounter price: £" + str(newPrice))
item = input("Enter the name of another item to buy? or X to quit!")
To help Oksana use the right terminology when annotating and explaining what her code does, you will need to answer the following 10 questions:
- What is the identifier of the subroutine defined in this code?
- Is this subroutine a procedure or a function? Why?
- How many parameters does this subroutine take?
- What programming construct is this subroutine based on: sequencing, selection or iteration?
- Which line of code will be executed first by the computer when you run this code?
- What programming construct is the main program based on: sequencing, selection or iteration?
- Can you identify three variables used in the main program?
- What is the data type of item?
- What is the data type of discount?
- On which line of code is string concatenation used?
1)
2)
3)
4)
5)
6)
7)
8)
9)
10)
Question 2[4 marks]
Without rewriting the above code, explain how Oksana could improve this code further to calculate and output the total discounted cost of all the items being bought by a customer?
Solution...
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Mode Algorithm using a Hash Table
In this challenge we will compare two methods used to calculate the mode value of a list of numbers. In Maths, The mode is the value that appears most often in a set of data. For instance, considering the following list of numbers:
The mode is 8 as the value 8 appears 4 times. You will notice that it is easier to spot the mode of a list of numbers when the numbers are sorted. e.g.:
Method 1: Iterative Approach, using a sorted list
Python Code:
Method 2: Using a hash table to store the frequency of each value
This method relies on the use of a hash table (a.k.a dictionary data strucutre) to store the frequency of each distinct value of the list.
Python Code:
Your Task
Both of the above algorithm will return the mode of a list. However, on occasion, there could be more than one mode value: if there are multiple numbers that occur with equal frequency, and more times than the others in the set. For instance:
The above list is bimodal as it has two modes: 8 and 30.
Your task is to adapt both of the above algorithms to make sure they identify all the mode values from a given list of numbers.
Tagged with: Hash Table
Job Scheduling Algorithms – Q&A
Question 1[20 marks]
One of the main purpose of the Operating System is to control the hardware and more specifically the CPU. The CPU performs all the jobs/processes requested by the different applications. A scheduler is a program of the Operating System that manages the amount of time that is allocated to each job that the CPU needs to process. To do so the scheduler keeps a list of all the jobs that need to be processed. This is called the job queue. The scheduler also uses a scheduling algorithm to determine in which order these jobs will be processed and the amount of processing time to allocate to each job.
The main job scheduling algorithms are:
- FCFS: First Come First Served
- Round Robin
- Shortest Job First
- Shortest Remaining Time First
- Multilevel Feedback Queues
For each of the following scheduling algorithms:
- Determine the order in which each job will be processed by the CPU (e.g. ABCD).
- Estimate the completion time for each job in the queue.
FCFS: ……………
Shortest Job First: ……………
Shortest Remaining Time First: ……………
Round Robin: ……………
-
Job A: Completion time:
Job B: Completion time:
Job C: Completion time:
Job D: Completion time:
Shortest Job First: ……………
-
Job A: Completion time:
Job B: Completion time:
Job C: Completion time:
Job D: Completion time:
Shortest Remaining Time First: ……………
-
Job A: Completion time:
Job B: Completion time:
Job C: Completion time:
Job D: Completion time:
Round Robin: ……………
-
Job A: Completion time:
Job B: Completion time:
Job C: Completion time:
Job D: Completion time:
Question 2[4 marks]
Some job scheduling algorithms can potentially lead to starvation? Out of the four algorithms listed in question 1, what algorithms can potentially lead to starvation? Explain what would cause this?
Solution...
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Disk Scheduling Algorithms
A hard drive is a magnetic storage device consisting of several disks (a.k.a. platters) where data is stored.
Each disk is divided into many concentric circular tracks. Each track contains several sectors where data is stored. When the Operating System stores a file on a hard drive, this file is split into several clusters. Each cluster will be stored across a few sectors. The typical size of a cluster on a Windows computer is 4KB.
When loading a file into memory, it is necessary to access all the sectors used by this file. This involves physical movement of the Hard Drive arm used to position the Read/Write head on the right track. Once the head is positioned on the right track, the sectors of this track can easily be reached as the disk is constantly rotating. Moving the arm from one track to another takes time which increases the overall seek time: the time taken to locate a specific piece of data stored on a hard disk.
Disk scheduling
Disk scheduling is done by the operating system to schedule Read/Write requests (a.k.a I/O requests) arriving for the disk.
When a file is loaded into memory, several sectors (and tracks) will need to be accessed. The operating system can use different disk scheduling algorithms to do so. In this post we will compare four main algorithms:
- The FCFS Disk Scheduling Algorithm (First Come First Serve)
- The SCAN Disk Scheduling Algorithm
- The LOOK Disk Scheduling Algorithm
- The SSTF Disk Scheduling Algorithm (Shortest Seek Time First)
FCFS Disk Scheduling Algorithm
Let’s consider a disk consisting of 100 concentric tracks (0-99) and let’s assume the Operating System needs to access sectors on the following tracks: [43,52,24,65,70,48,16,61]. And let’s assume the head is, to start with at track 20.
With a First Come First Served scheduling algorithm, the tracks will be accessed in the order they were requested. So the read/write head will be moving from its current position to track 43, then track 52 then track 24 and so on…
Looking at the above pattern we can estimate the seek time of this approach:
So in this case, we can calculate the number of tracks crossed as follows:
SCAN Disk Scheduling Algorithm
With a SCAN Disk Scheduling algorithm, the disk arm will not access the tracks in the order they were requested. Instead it will move in one direction and service the requests coming in its path. After reaching the end of disk (e.g. track 99), it will reverse its direction and again services the request arriving in its path. Due to the movement of the arm going all the way up and down the number of tracks, the algorithm is also known as elevator algorithm. The aim of this approach is to reduce the average seek time when accessing a list of sectors.
In this case, using the same example as above: Starting Position: Track 20, list of tracks to access [43,52,24,65,70,48,16,61] we can calculate the number of tracks crossed as follows:
On average this algorithm will reduce the overall seek time (compared to a FCFS Disk Scheduling algorithm.
LOOK Disk Scheduling Algorithm
The LOOK disk scheduling algorithm is an improved version of the SCAN disk scheduling algorithm. In this algorithm, instead of reaching the last track (e.g. track 99) before changing direction, this algorithms changes direction when it reaches highest (or lowest) track from the list of track to access.
In this case, using the same example as above: Starting Position: Track 20, list of tracks to access [43,52,24,65,70,48,16,61] we can calculate the number of tracks crossed as follows:
On average this algorithm will reduce the overall seek time (compared to both a FCFS Disk Scheduling algorithm and a SCAN Disk Scheduling algorithm).
SSTF Disk Scheduling Algorithm
With the Shortest Seek Time First algorithm, we once again do not necessary process the requests in order they arrived. Instead we all always process the request which is the closest to the current position (track number) of the head.
In this case, using the same example as above: Starting Position: Track 20, list of tracks to access [43,52,24,65,70,48,16,61] we can calculate the number of tracks crossed as follows:
On average this algorithm will reduce the overall seek time (compared to all the other 3 algorithms we investigated in this blog post).
You Task
Estimate the Seek Time for all 4 scheduling algorithms for the given track lists:
| Starting Position | Track List | FCFS Disk Scheduling Seek Time |
SCAN Disk Scheduling Seek Time |
LOOK Disk Scheduling Seek Time |
SSTF Disk Scheduling Seek Time |
|
| #1 | 50 | [24,60,30,8,52] | ||||
| #2 | 80 | [70,60,10,20,40,30] | ||||
| #3 | 40 | [32,48,64,12,16,8,90] |
Information Board: Javascript Challenge
In this challenge we are creating an information board that displays key messages on a LED screen that consists of 4 rows of 20 characters.
We have created the HTML, and CSS code to render the board on the screen with a welcome message that, for now is hardcoded in the HTML code.
We have also created a Javascript function called clearBoard() to clear the content of the board and have started a new function called inputMessage(). This function will allow the user to input a message to display on the board. However this function is incomplete and your task will be to add the relevant code to convert the message typed by the end-user into a Javascript 2D array called board. This board will then be passed as a parameter to the displayBoard() function that displays the content of this 2D array on the information board.
Note that when displaying a message on the board, if a word cannot fit on a line, then it is added at the start of the next line of the board. (Words are not truncated or hyphenated!)
To complete this challenge, you will need to click on the “Edit on CodePen” button below, and edit the Javascript code from line 36.
See the Pen
Information Board 101 by 101 Computing (@101Computing)
on CodePen.
This challenge is based on a question from the H046 A Level Computer Science June 2017 – Exam paper from OCR
Solution...
The solution for this challenge is available to full members!Find out how to become a member:
➤ Members' Area
Tagged with: JavaScript
The 4 Stages of the Compilation Process
The compilation process involves converting high-level source code into machine code that a computer can execute. It’s a multi-step process, and each stage plays an important role in ensuring that the code runs as expected. Here are the four main stages of the compilation process:
1. Lexical Analysis
The first step in the compilation process is lexical analysis. During this stage, the source code is read and converted into a sequence of tokens. A token is a basic unit of meaning, like keywords (if, while), variables, constants, operators (+, -), and punctuation.
During lexical analysis, the compiler also removes comments, annotations (like // or /*...*/), and whitespace (such as spaces, tabs, and newlines). These are ignored because they are not needed for understanding the logic of the program, but they help in writing human-readable code.
This step is performed by the lexer or scanner, which breaks down the source code into these tokens.
2. Syntax Analysis
The next stage is syntax analysis (also known as parsing). The compiler takes the tokens generated in the previous stage and checks whether they are arranged in a valid way according to the grammar (syntax) of the programming language.
A parser is used to create a parse tree or abstract syntax tree (AST). This tree represents the structure of the program, showing how different parts of the code relate to each other according to the language rules. For example, the parser will check if statements are correctly formed, like ensuring that an if statement has the proper condition and body.
The purpose is to ensure the code follows the language’s syntax rules and that the sequence of tokens makes sense logically in the structure of the program.
During the syntax analysis, if a syntax error is detected, the compilation process stops here and an error message is generated.
3. Code Generation
Once the program passes the syntax analysis and is considered structurally correct, the compiler moves to code generation. In this step, the compiler translates the high-level source code (like if (x > 5)) into machine code or intermediate code (such as bytecode for languages like Java).
The compiler uses the abstract syntax tree (AST) from the syntax analysis stage to generate the target code. This code can be directly executed by the computer’s processor or can be further processed into an executable program.
The purpose of code generation is to create an efficient machine-readable output (binary code) that the computer can execute.
4. Code Optimisation
Code optimisation is an optional but very important step where the generated code is improved for efficiency. The optimiser attempts to reduce the program’s runtime, memory usage, or both, without changing its functionality.
The optimisation process might involve eliminating redundant calculations, simplifying loops, or using more efficient data structures or algorithms. In some cases, it can rearrange the order of instructions to make the program run faster or take up less space.
The goal is to make the code run faster or more efficiently, improving its performance while keeping the logic of the program intact.
To Sum It Up…
- Lexical Analysis: Breaks down the source code into tokens and removes comments, annotations, and whitespace.
- Syntax Analysis: Checks the tokens against the syntax rules of the language, generating a parse tree or abstract syntax tree (AST).
- Code Generation: Translates the valid code into machine-readable or intermediate code.
- Code Optimisation: Improves the efficiency of the generated code, making it run faster or consume less memory.
Each of these stages is crucial in turning human-readable high-level code into a functional and efficient machine-executable program.
Task 1: Drag and Drop
Can you identify and describe the 4 stages of the compilation process?Complete the fill in the gaps activity below to label the 4 stages of the compilation process with for each stage their input and output:
Compilation ProcessOpen in New Window
Task 2: Q&A
Answer the following question:Question 1[12 marks]
Can you identify and describe the 4 stages of the compilation process:
Stage 1:
Stage 2:
Stage 3:
Stage 4:
Stage 2:
Stage 3:
Stage 4:
Network Design – Drag & Drop
Can you identify some of the main hardware components that you will find in a small Client-Server Local Area Network?
Before competing this activity, you may want to read more about network design concepts.
Task 1: Network Design Drag and Drop
Complete the fill in the gaps activity below to label this network diagram:Network DesignOpen in New Window
Task 2: Q&A
Answer the following questions, in the context of the above Local Area Network.Question 1[6 marks]
Can you describe the purposes of the following network components used in the above Local Area Network:
Router:
Switch:
Firewall:
WAP:
File Server:
E-Mail Server:
Switch:
Firewall:
WAP:
File Server:
E-Mail Server:
Question 2[2 marks]
The above network is a Client Server network as it includes servers which provide a dedicated services to all the computers/workstations connected to the network. Can you identify two other types of servers, other than File Server and E-Mail Server, that can be used on a Client-Server network?
Tagged with: Networks
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