Computer science

• User-defined types


  New data types created from existing data types to extend the flexibility of the programming language
• Non-composite types


  • Can be defined without referencing another data type
  • Can be a primitive type or a user-defined type
• Enumerated


  A user-defined non-composite data type with a list of all possible values that is ordered
• Enumerated type


  • TYPE TMonth = (Jan,Feb,Mar,Apr,May,Jun,Jul,Aug,Sep,Oct,Nov,Dec)
  • DECLARE ThisMonth : TMonth
  • ThisMonth ← May
  • ThisMonth > Feb
• Pointer


  A user-defined non-composite data type that stores addresses/memory locations and indicates the type of data stored
• Pointer type


  • TYPE TMyPointer = ^Integer
  • DECLARE IntegerPointer : TMyPointer
  • ValuePointedTo ← IntegerPointer^
• Composite data types


  A user-defined data type that is a collection of data that can consist of multiple elements of different or the same data types
• Set


  A data type that allows storing a finite number of different values that have no order
• Record


  A collection of related items that may have different data types
• Record type


  • TYPE TStudentRecord
  • DECLARE FirstName : STRING
  • DECLARE LastName : STRING
  • DECLARE Absences : INTEGER
  • DECLARE Class : STRING
  • ENDTYPE
  • DECLARE Student1 : TStudentRecord
  • Student1.FirstName ← "John"
  • Student1.LastName ← "Doe"
  • Student1.Absences ← 0
  • Student1.Class ← "A2 Level"
• Class/object


  A data type that has properties and methods for an object
• Choose and design an appropriate user-defined data type for a given problem
• Serial file organisation


  • Records stored in chronological order/order they were added in
  • New records added to end of file
  • Sequential Access only
  • Reorganisation not needed when new record added
• Sequential file organisation


  • Records (in the file) are ordered based on the key field
  • A new version (of the file) has to be created to update the file
• Random file organisation


  • Records are stored in no particular order within the file
  • There is a relationship between the key of the record and its location within the file
  • Updates to the file can be carried out directly
• Sequential access


  • Start at the beginning of the file
  • Check records linearly until the desired record is found
  • Process/update records as required
  • EOF found
• Direct access


  Used for sequential and random files
• Describe and use different hashing algorithms to read from and write data to a random/sequential file
• Binary floating-point real numbers

  • Use two's complement form
  • Increase in precision as the number of bits in the mantissa has increased
  • Reduction in precision as the number of bits in the mantissa has decreased
  • Increase in range as the number of bits in the exponent has increased
  • Reduction in range as the number of bits in the exponent has decreased
• Normalisation of floating-point numbers


  • To store the maximum range of numbers in the minimum number of bytes/bits
  • Minimises the number of leading zeros/ones represented
  • Maximises the number of significant bits/potential precision/accuracy of the number for the given number of bits
  • Enables very large/small numbers to be stored with accuracy
  • Avoids the possibility of many numbers having multiple representations
• Real numbers can have a fractional part, but the fixed length of storage means binary numbers can't represent all fractions precisely
• Underflow


  Following an arithmetic/logical operation, the result is too small to be precisely represented in the available system
• Overflow


  Following an arithmetic/logical operation, the result is too large to be precisely represented in the available system
• Protocols


  • Set a standard for communication
  • Enable communication/compatibility between devices from different manufacturers/platforms
• If two devices were sending messages to each other but using different protocols, they would not be able to communicate properly
• Protocol implementation as a stack

  Each layer has its own functionality
• TCP/IP protocol suite


  • Four Layers: Application, Transport, Internet, Link
  • Application provides user services
  • Transport handles packets, sequences them, adds headers, sends to Internet layer
  • Internet identifies intended network and host, routes packets independently
  • Network Access handles how data is physically sent, ensures correct protocols are followed, enables connection to the physical medium
• Protocols and their purposes

  • HTTP(S) for sending and receiving web pages/hypertext documents
  • FTP for sending and receiving files over a network
  • SMTP for sending/uploading emails
  • POP(3) for receiving/downloading emails
  • IMAP for receiving/downloading emails
  • BitTorrent for peer-to-peer file sharing
• Circuit switching


  • A dedicated circuit is established before transmission starts and released after
  • Data is transferred using the whole bandwidth over the same route
  • Used where a dedicated path needs to be sustained, for real-time communication, private data networks
• Circuit switching benefits


  • Whole bandwidth available
  • Dedicated communication channel increases quality
  • Data transmitted at fixed rate
  • No waiting time at switches
  • Suitable for long continuous communication
  • Fast data transfer
  • Data arrives in same order as sent
  • Data can't get lost
  • Better for real-time
  • Simple data transfer
• Circuit switching drawbacks


  • Dedicated connection makes it impossible to transmit other data even if channel is free
  • Not very flexible
  • No alternative route in case of failure
  • Time required to establish physical link can be too long
  • Dedicated channels require whole bandwidth, can't be shared
• Packet switching


  Data is split into packets, each given its own route, routing depends on congestion, packets may not arrive in order
• Packet switching benefits


  • Ensures accurate delivery of message
  • Missing packets can be detected and re-sent
  • Resilient to network changes
  • Path available to other users, doesn't use whole bandwidth
  • Better security as packets hashed and sent by different routes
• Packet switching drawbacks


  • Time delays to correct errors
  • Network problems may introduce errors in packets
  • Requires complex protocols for delivery
  • Unsuitable for real time transmission
• Router function in packet switching

  • Examines packet header
  • Reads destination IP address
  • Accesses routing table to determine next hop
• Types of electronic communication

  • email
  • text messages
  • documents
  • VOIP
• Benefits of electronic communication

  • Accuracy - Ensures accurate delivery of the message
  • Completeness - Missing packets can be easily detected and a re-send request sent so the message arrives complete
  • Resilience - if a network changes the router can detect this and send the data another way to ensure it arrives
  • Path also available to other users // Doesn't use whole bandwidth //allows simultaneous use of channel by multiple users
  • Better security as packets hashed and sent by different routes
• Drawbacks of electronic communication

  • Time delays to correct errors // Network problems may introduce errors in packets
  • Requires complex protocols for delivery
  • Unsuitable for real time transmission applications
• Function of a router in packet switching


  1. Examines the packet's header

  2. Reads the IP address of the destination (from the packet header)

  3. Has access to a routing table

  4. Containing information about, e.g., available hops / netmask / gateway used

  5. And the status of the routes along the route

  6. Decides on the next hop / best route

  7. Sends the packet on its next hop
• How packet switching is used to pass messages across a network, including the internet


  1. A large message is divided up into a group of smaller chunks of the same size called packets

  2. The packet has a header and a payload

  3. The header contains a source IP address, destination IP address (and sequence number)

  4. Each packet is dispatched independently

  5. And may travel along different routes / paths

  6. The packets may arrive out of order

  7. And are reassembled into the original message at the destination

  8. If packets are missing / corrupted a re-transmission request is sent