Final Alevel CS Review Guidance

Data Representation

User-defined data types

Non-composite types

enumerated
- TYPE Courses = (English,Chinese,Maths)
pointer
- TYPE SelectParts = ^Parts // It reference the memory location in which the part is stored

Define & Use composite data types

set, record and class/object
Pseudocode example of class:

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TYPE School
    DECLARE StudentNumber : INTEGEAR
    DECLARE Courses : (English,Chinese,Maths)
    DECLARE StudentList : ARRAY[0:200] OF STRING
ENDTYPE

File organisation and access

Organisation

Serial
Sequential
Random
- Only that the access can be to any record in the file without sequential reading of the file.
- A separate index file is created which has two fields per record.
- The first field has the key field value and the second field has a value for the position of this key field value in the main file.

Access

Sequential
Direct

Floating-point numbers, representation and manipulation

Overflow
Underflow
Rounding errors

Communication and internet technologies

Protocols

Importance

TCP/IP

Bittorrent

file sharing
peer-to-peer

Other protocols

FTP
IMAP, SMTP, POP3
HTTP
……

Circuit switching, packet switching

Packet

Circuit

A method of data transfer in which the message is sent over a dedicated communication channel.

Hardware and Virtual Machines

Processors, Parallel Processing and Virtual Machines

Reduced Instruction Set Computers (RISC)

Complex Instruction Set Computers (CISC)

Parallel processing

SISD
SIMD
MISD
MIMD

Logic Circuits

Half-adder

2 input
2 output
- carry bit
- sum bit

Full-adder

3 input
2 output
- carry bit
- sum bit

Flip-flop

SR

It can be constructed with two NAND gates or two NOR gates.
Used as a storage device for 1 bit in the RAM, since it’s values can be altered
Issue: When the both the input signals are 1 (invalid state) the flip-flop sets the value of Q and Q’ to 0.

JK

The J acts as a set input and the K as a clear input.
There is a clock input to synchronise the inputs.
When both the input signals are one, Q toggles.

Usage

Flip-flops are used to build
- Data storage elements
- Digital circuits

Boolean Algebra

Karnaugh maps

System Software

Purposes of an Operating System (OS)

OS provides utility software, user interface (GUI,CLI), resources management, multi-tasking, a platform to run code……

Translation Software

BNF notation

::=
|

RPN expression

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Origin expression:
0!+123+4*(5*6!+7!/8)/9

RPN expression:
0 ! 123 + 4 5 6 ! * 7 ! 8 / + * 9 / +

Proccess:
大致计算过程：
0入栈
！为运算符，计算栈顶0！，结果入栈
123入栈
+为运算符，将栈顶两元素相加，结果入栈
4入栈
5入栈
6入栈
！为运算符，将栈顶6进行运算，结果入栈
*为运算符，将栈顶6！和5相乘，结果入栈
7入栈
！为运算符，计算7！，结果入栈
8入栈
/为运算符，将栈顶两元素相除，结果入栈
+为运算符，将栈顶两元素相加，结果入栈
*为运算符，将栈顶两元素相乘，结果入栈
......

Security

Encryption, Encryption Protocols and Digital certificates

public key, private key, plain text, cipher text, encryption
symmetric key cryptography and asymmetric key cryptography

Symmetric key encryption

Asymmetric key encryption

Private keys & public key

Digital certificate

Digital signature

Quantum cryptography

SSL & TLS

How to establish a secure connection

Artificial Intelligence (AI)

Please check the note released before🔗

Computational thinking and problem-solving

Algorithms

Please check the note released before🔗

Recursion

Description

have a base case
have a general case
reach the base case aft er a finite (limited) number of calls to itself.

Benefits

More elegant
Use less program code than iterative solutions

Drawbacks

Large amounts of memory usage and processor time

How to implement recursion (Stack)

Each time a subroutine is called, a stack frame is pushed onto the stack.
A stack frame consists of the return address and the values of the local variables.
When a subroutine completes, the corresponding stack frame is popped off the stack.

Further Programming

Programming Paradigms

A programming style/classification // characteristics/features that programming language has/uses

Low-level Programming

Programs use the instruction set of a processor

Addressing mode

immediate
direct
indirect
indexed
relative

Imperative Programming (Sequence of commands)

Object Oriented Programming

Declarative Programming (Focus on the result)

The programmer doesn’t tell the computer what to do. To get information, the programmer poses a query (sets a goal). It’s up to the logic programming system to work out how to get the answer.
Declarative programming languages include SQL and Prolog.

File Processing and Exception Handling

Record

Pseudocode

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TYPE CarRecord
    DECLARE VehicleID : STRING // unique identifier and record key
    DECLARE Registration : STRING
    DECLARE DateOfRegistration : DATE
    DECLARE EngineSize : INTEGER
    DECLARE PurchasePrice : CURRENCY
ENDTYPE

DECLARE Car : ARRAY[1:100] OF CarRecord // store the details of 100 cars

Python3

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class CarRecord:              # declaring a class without other methods
    def __init__(self):       # constructor
        self.VehicleID = ""
        self.Registration = ""
        self.DateOfRegistration = None
        self.EngineSize = 0
        self.PurchasePrice = 0.00

ThisCar = CarRecord()         # instantiates a car record
ThisCar.EngineSize = 2500     # assign a value to a field
Car = [CarRecord() for i in range(100)] # make a list of 100 car records
Car[1].EngineSize = 2500 # assign value to a field of the 2nd car in list

File processing

Psedocode

Structured English	Pseudocode
Create a file and open it for writing	OPENFILE <filename> FOR WRITE
Open a file in append mode	OPENFILE <filename> FOR APPEND
Open a file for reading	OPENFILE <filename> FOR READ
Open a file for random access	OPENFILE <filename> FOR RANDOM
Close a file	CLOSEFILE <filename>
Write a record to a file	PUTRECORD <filename>, <identifier>
Read a record from a file	GETRECORD <filename>, <identifier>
Move to a specific disk address within the file	SEEK <filename>, <address>
Test for end of file	EOF(<filename>)

Python3

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file = open("filename",mode=?)   //create a file object

Recommanded mode parameter

'w' for write
'r' for read
'a' for append
'+' let the file can be read and write at the same time
- If the mode is 'w', you cannot use file.read([size]),file.readline([size]) and file.readlines([sizeint])
- If the mode is 'w+', there is no worry of reading the file
'b' for open/write/append a binary file
- for example: 'wb', 'wb+'

Detail mode parameters

模式	描述
t	文本模式 (默认)。
x	写模式，新建一个文件，如果该文件已存在则会报错。
b	二进制模式。
+	打开一个文件进行更新(可读可写)。
r	以只读方式打开文件。文件的指针将会放在文件的开头。这是默认模式。
rb	以二进制格式打开一个文件用于只读。文件指针将会放在文件的开头。这是默认模式。一般用于非文本文件如图片等。
r+	打开一个文件用于读写。文件指针将会放在文件的开头。
rb+	以二进制格式打开一个文件用于读写。文件指针将会放在文件的开头。一般用于非文本文件如图片等。
w	打开一个文件只用于写入。如果该文件已存在则打开文件，并从开头开始编辑，即原有内容会被删除。如果该文件不存在，创建新文件。
wb	以二进制格式打开一个文件只用于写入。如果该文件已存在则打开文件，并从开头开始编辑，即原有内容会被删除。如果该文件不存在，创建新文件。一般用于非文本文件如图片等。
w+	打开一个文件用于读写。如果该文件已存在则打开文件，并从开头开始编辑，即原有内容会被删除。如果该文件不存在，创建新文件。
wb+	以二进制格式打开一个文件用于读写。如果该文件已存在则打开文件，并从开头开始编辑，即原有内容会被删除。如果该文件不存在，创建新文件。一般用于非文本文件如图片等。
a	打开一个文件用于追加。如果该文件已存在，文件指针将会放在文件的结尾。也就是说，新的内容将会被写入到已有内容之后。如果该文件不存在，创建新文件进行写入。
ab	以二进制格式打开一个文件用于追加。如果该文件已存在，文件指针将会放在文件的结尾。也就是说，新的内容将会被写入到已有内容之后。如果该文件不存在，创建新文件进行写入。
a+	打开一个文件用于读写。如果该文件已存在，文件指针将会放在文件的结尾。文件打开时会是追加模式。如果该文件不存在，创建新文件用于读写。
ab+	以二进制格式打开一个文件用于追加。如果该文件已存在，文件指针将会放在文件的结尾。如果该文件不存在，创建新文件用于读写。

Sequence file processing

To store records in binary file, we can use module pickle.

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# 3 most important codes
import pickle
pickle.dump(item,file)
pickle.load(file_content)

You need to import the module.
Read/write a file object via the binary mode (mode='wb'/'rb'/etc.)

Code example

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import pickle # this library is required to create binary files
Car = [CarRecord() for i in range(100)]

CarFile = open('CarFile.DAT', 'wb') # open file for binary write
for i in range(100): # loop for each array element
    pickle.dump(Car[i], CarFile) # write a whole record to the binary file

CarFile.close() # close file

CarFile = open('CarFile.DAT', 'rb') # open file for binary read

Car = [] # start with empty list
while True: # check for end of file
    Car.append(pickle.load(CarFile)) # append record from file to end of list

CarFile.close()

Some file fuctions

序号	方法及描述
1	file.close() 关闭文件。关闭后文件不能再进行读写操作。
6	file.read([size]) 从文件读取指定的字节数，如果未给定或为负则读取所有。
7	file.readline([size]) 读取整行，包括 "\n" 字符。
8	file.readlines([sizeint]) 读取所有行并返回列表，若给定sizeint>0，返回总和大约为sizeint字节的行, 实际读取值可能比 sizeint 较大, 因为需要填充缓冲区。
9	file.seek(offset[, whence]) 移动文件读取指针到指定位置
10	file.tell() 返回文件当前位置。
11	file.truncate([size]) 从文件的首行首字符开始截断，截断文件为 size 个字符，无 size 表示从当前位置截断；截断之后后面的所有字符被删除，其中 windows 系统下的换行代表2个字符大小。
12	file.write(str) 将字符串写入文件，返回的是写入的字符长度。
13	file.writelines(sequence) 向文件写入一个序列字符串列表，如果需要换行则要自己加入每行的换行符。

Exception

Code language

Pseudocode

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TRY
    <statementsA>
EXCEPT
    <statementsB>
ENDTRY

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# With FINALLY
TRY
    <statementsA>
EXCEPT
    <statementsB>
FINALLY
    <statementsC>
ENDTRY

TRY
EXCEPT
FINALLY - The statements (<statementsC>) in this block will be executed regardless of whether there was an exception or not.

Python3

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## Basic try and exception examples
file = open("test.txt",mode='r')


try:
    file.readlines()
except EOFError:
    print("reach the end of file")
    file.close()

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try:
    file = open("test.txt",mode='w')
    file.write("line 1")
    file.close()
except IOError:
    print("cannot find the file")

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try:  
    runoob()  
except AssertionError as error:  
    print(error)  
else:  
    try:  
        with open('file.log') as file:  
            read_data = file.read()  
    except FileNotFoundError as fnf_error:  
        print(fnf_error)  
finally:  
    print('这句话，无论异常是否发生都会执行')

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import sys  

try:  
    f = open('myfile.txt')  
    s = f.readline()  
    i = int(s.strip())  
except OSError as err:  
    print("OS error: {0}".format(err))  
except ValueError:  
    print("Could not convert data to an integer.")  
except:  
    print("Unexpected error:", sys.exc_info()[0])

More info about exception in Python3

Python3 错误和异常 | 菜鸟教程 (runoob.com)🔗