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Computer Science (Data Types (Casting (One of the most common reasons for…

- - - - ASCII
        
        ASCII is a 7-bit character set containing 128 characters. It contains the numbers from 0-9, the upper and lower case English letters from A to Z, and some special characters. The character sets used in modern computers, in HTML, and on the Internet, are all based on ASCII.
      - UNICODE
        
        Unicode is a computing industry standard for the consistent encoding, representation, and handling of text expressed in most of the world's writing systems.
  - - - The AND gate performs the boolean AND function:
      - It will only output true if both of its inputs are true.
      - The OR gate performs the boolean OR function:
      - It will output true if either or both of its inputs are true.
      - The NOT gate performs the boolean NOT function:
      - It takes one input and inverts it.
      - Inside a Computer
        
        Underneath the surface computers are electrical devices that process information by controlling the flow of electricity through billions of wires and transistors (electrical switches) inside the central processing unit and other components.
  - - - Binary shift
        
        A binary shift is a technique for performing multiplication or division on a binary number.
    - - If we add 0 + 0 we get 0.
      - If we add 1 + 0 (or 0 + 1) we get 1.
      - If we add 1 + 1, then we cannot use the sym
    - - Binary numbers are stored as a fixed length.
      - If a number is carried past the last place column, then this is called an overflow error.
      - Overflow errors can lead to inaccurate results and software crashes.
- - - - A personal area network is a computer network for interconnecting devices centered on an individual person's workspace. A PAN provides data transmission among devices such as computers, smartphones, tablets and personal digital assistants.
    - - A wide-area network (WAN) is any telecommunications network or computer network that extends over a large geographical distance/place. Wide-area networks are often established with leased telecommunication circuits. E.G the Internet
    - - A local area network is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building.
  - - - Advantages
        
        Faster
        
        Secure
        
        Reliable
      - Disadvantages
        
        Takes up space
        
        Expensive set up costs
    - - Advantages
        
        cheap set-up costs
        
        not tied down to a specific location
        
        can connect multiple devices without the need for extra hardware
        
        less disruption to the building due to no wires being installed
      - Disadvantages
        
        interference can occur
        
        the connection is not as stable as wired networks and can 'drop off'
        
        it will lose quality through walls or obstructions
        
        more open to hacking
        
        slower than wired networks
  - - - Ethernet is a family of computer networking technologies commonly used in local area networks
    - - a facility allowing computers, smartphones, or other devices to connect to the Internet or communicate with one another wirelessly within a particular area.
    - - TCP enables two hosts to establish a connection and exchange streams of data.
    - - UDP (User Datagram Protocol) is an alternative communications protocol to Transmission Control Protocol (TCP) used primarily for establishing low-latency and loss-tolerating connections between applications on the internet.
    - - a digital media transport system that runs over standard IP networks
    - - HyperText Transfer Protocol. HTTP is the underlying protocol used by the World Wide Web and this protocol defines how messages are formatted and transmitted
    - - Hyper Text Transfer Protocol Secure (HTTPS) is the secure version of HTTP
    - - The File Transfer Protocol is a standard network protocol used for the transfer of computer files between a client and server on a computer network
    - - messages distributed by electronic means from one computer user to one or more recipients via a network.
    - - Simple Mail Transfer Protocol is an Internet standard for email transmission.
    - - he Internet Message Access Protocol is an Internet standard protocol used by email clients to retrieve email messages from a mail server over a TCP/IP connection.
  - - - Security protects data
    - - Firewall
        
        Blocks unauthorised files from entering or leaving the computer
      - Mac Address Filtering
        
        Filters away computers not authorised on a network
      - Encryption
        
        Scrambles data so data can only be read with key
      - Authentication
        
        Requires a password and username to access the data
- - - - Cons
        
        Slow on a long list
      - Pros
        
        Easy to implement
      - Concept
        
        If you were looking for a specific piece of paper in a stack of papers, then one way to find it would be to work from the top of the stack to the bottom, checking each paper on the way.
      - How it works
        
        Check the first item in the dataset:
        
        If it is what we are looking for, return it.
        
        Check the second item in the dataset:
        
        If it is what we are looking for, return it.
        
        Continue for rest of items.
        
        If we reach the end of the data set, then the item was not in our data set.
    - - Concept
        
        If you try to find a certain page in a book, it's unlikely that you check every page on the way.You are more likely to split the book in two, then see if the page you need is before or after the split, and repeat.
      - Steps
        
        Find the middle of the dataset.
        
        If the middle value is greater than the target then:
        
        Repeat on first half of dataset.
        
        If the middle value is lesser than the target then:
        
        Repeat on second half of dataset.
        
        If the middle value is the target:
        
        We found it!
        
        Stop repeating once the size of our dataset is zero.
      - Pros
        
        Faster than linear search on a large dataset.
      - Cons
        
        Data set must be sorted before starting.
- - - - When someone in an organisation gives away access details or sensitive information.
    - - When someone uses malware or other technical methods to compromise a network's security.
    - - When a person is exploited into giving away critical information that gives access to the network or accounts.
    - - When a hacker eavesdrops on a network by 'sniffing' the data packets.
  - - - Authentication is the name for any methods which allow users to prove that an account is theirs.
      - The most common authentication method is using a password.
      - Many hackers want to find people's passwords, so that they can gain access to accounts:
      - Finding people's passwords is called cracking the password.
    - - A brute force attack tries to crack a password by trying every single combination of letters and numbers until the correct one is found.
        This can take a very long time, although special software is used to do this which makes it possible to make millions of attempts per second.
    - - A quicker form of the brute force attack is the dictionary attack.
      - Rather than attempting every single combination, a dictionary attack tries words from a predetermined list:
      - A common dictionary to use would be a list of common passwords
    - - Writing a network policy which enforces strong passwords can protect against dictionary attacks.
      - Using two-factor authentication can prevent the hacker from logging in, even if they have the password.
      - Restricting the number of failed password attempts before an account is 'locked' for a fixed period of time can deter hackers.
  - - - A denial of service (DoS) attack tries to bring down a server by flooding it with many useless requests.
        The attack aims to overload the server. This stops the server responding to legitimate requests.
    - - To protect against DoS attacks, a server's firewall can blacklist (ban) any traffic from an IP address which is known to perform DoS attacks.
        Firewalls can also monitor traffic in real time. So if a new IP address starts to send too much traffic then traffic limits can be set.
    - - Distributed DoS (DDos) attacks are commonly used to overcome the blacklisting of an IP address because of a high number of requests.
      - In a DDoS attack, the requests are sent from an army of compromised machines, known as a botnet.
      - Botnet machines are infected with malware which allows a hacker to send requests from their computer.
      - The botnet can launch a huge number of simultaneous requests. The owners of the devices in the botnet might not even realise they are taking part.
    - - A botnet is a large number of computer that are controlled by a single hacker. Machines join the botnet when they are infected with
  - - - Malware is malicious software that the victim did not have the intention of installing.
- - - - Imagine that you have a set of cards face up on the desk.
      - If the first two cards are in the wrong order, you swap them.
      - Then do the same for the second and third cards, and continue in this pattern until the end of the pack. This is known as a pass.
      - The highest value will 'bubble' up to the top of the pack each pass.
      - By repeating this enough times, the pack will get sorted.
    - - Compare the first two items of the dataset:
      - If they are in the wrong order, swap them.
      - Continue for the rest of the cards in the deck.
      - Repeat the whole process, until a pass with no swaps happens.
    - - Easy to implement.
      - Does not use much memory.
    - - Poor for efficiency.
  - - - If we have two packs of cards, which are both already in order and want to combine them, what can we do?
      - One solution is to only consider the top card of each pile, and create a new deck starting from the smaller of the two cards.
      - This will give us a combined sorted pack.
      - This is called merge sort.
    - - Split the lists into lists of size one.
      - Merge each pair of sublists by comparing the first value of each list and putting the smaller value into the new list first.
      - Continue merging until there is only one list.
    - - A very efficient algorithm
    - - Can be slower for small lists.
        .
      - Needs additional memory
- - - - The process is that we line the two numbers up (one under the other), then, starting at the far right, add each column, recording the result and possible carry as we go.
      - Here are the possibilities:
      - 0 + 0 = 0
      - 1 + 0 = 1
      - 1 + 1 = 2 which is 10 in binary which is 0 with a carry of 1
      - 1 + 1 + 1 (carry) = 3 which is 11 in binary which is 1 with a carry of 1
      - The carry is involved whenever we have a result larger than 1 (which is the largest amount we may represent with a single binary digit).
    - - With binary subtraction we start to get a little more difficult (But not that difficult). Similar to binary addition, we will work through the numbers, column by column, starting on the far right. Instead of carrying forward however, we will borrow backwards (when necessary).
      - Here are the possibilities:
      - 0 - 0 = 0
      - 1 - 0 = 1
      - 1 - 1 = 0
      - 0 - 1 we can't do so we borrow 1 from the next column. This makes it 10 - 1 which is 1.
    - - Binary multiplication is just about as easy as binary addition. Again it is the same process as we would do with decimal multiplication by hand. Again it is easier as binary only has 0 and 1.
      - We line the two numbers up (similar to addition). Then we multiply the entire top number by each individual digit of the bottom number. As we move across each digit we pad out the result with 0's to line it up. Finally we add all the results together.
      - Here are the possibilities:
      - 0 * 0 = 0
      - 1 * 0 = 0
      - 1 * 1 = 1
- - - - Power the computer
    - - Able to store files
    - - Central Processing unit
    - - Cools components in the computer
    - - Network Interface card
        
        Allows computers to be connected to a network
  - - - Arithmetic Logic Unit
        
        Does number calculations
    - - Control Unit
        
        Controls what each component does
    - - Allows the CPU to write instructions
    - - Read only memory
        
        Can only be read for boot up
    - - Random Access memory
        
        Allows instructions to be written
    - - Very fast memory that can be used easily for the CPU
- - - - Flow diagrams are used to visually represent the steps that make up an algorithm.
        A standard set of shapes are used to represent different types of step
        
        A rectangle is used to represent a process.
        
        A parallelogram is used to represent an input or an output.
        
        An oval is used for the start and end of a program.
        
        A diamond is used to represent a decision:
        
        A decision has two labelled arrows coming out of it.
        
        The 'Yes' arrow is followed if the condition in the diamond was true, otherwise the 'No' arrow is followed.
        
        Arrows represent the flow of control, or what to execute next.
    - - Identifiers
        
        Identifiers are the names of variables, constants, and subroutines.
        
        These often give strong clues about the purpose of an algorithm.
        
        Check what is being input and output in the algorithm for clues about its purpose.
        
        Output messages often format the result of an algorithm in a human readable way.
        
        Comments are descriptions of the code.
        
        Comments will often state the purpose of a given algorithm.
        
        Comments are often the clearest way to identify an algorithm.
        
        The first step for this type of question is to try to identify what the algorithm does.
        
        If you can interpret the algorithm, then it will be much easier to follow it.
        
        Given an algorithm you should be able to read it and explain what it does.
    - - You can draw a table with a column for each variable in the algorithm.
      - Use this table to keep track of the value of each variable, and carefully follow through the code until the end.
      - Finally, the trace table should include your answer.