Organisation and structure of data

• How sound can be sampled and stored digitally


  Sound is converted into a digital signal by a process called sampling. This is where hardware, such as a microphone, measures the level of sound many times per second and records this as binary digits.
• Sample rate


  The number of audio samples captured every second.
• Bit depth


  The number of bits available for each clip.
• Bit rate


  The number of bits used per second of audio.
• What are vector images?

  These images do not store the data by pixels, but are a set of instructions for drawing a geometric shape.

  Advantage:

  They can be scaled without loss of quality

  Use less storage space
• Why metadata needs to be included in an image file (including height, width, colour depth)


  The term metadata refers to ‘data about data’. Key properties that are needed to display an image correctly are stored as metadata. Data such as an image’s height, width and colour depth are typical examples of data stored in an image's metadata. Without metadata, a computer system may render an image incorrectly on screen, such as displaying all pixels in one row.
• Binary numbers representing characters

  A character can be a letter, a digit, a space, a punctuation mark or various other symbols. When characters are stored on a computer system, they are stored as a binary number
• ASCII

  In order to allow for data exchange between computer systems, character sets were devised. A character set is a table that maps a character with a unique binary number.

  The problem with using this ASCII character set is that it's only able to represent 128 different characters.

  
• Unicode

  Standard character set that has combined and replaced many others. It was originally an extension to the ASCII character set and it contains many of the characters used around the world – 137,000 characters in total. Each character requires 2 bytes of storage.
• Data structures

  A data structure is a specific way of organising data within memory so it can be processed efficiently.
• Static data structure


  Designed to store a known number of data items. The values of the data can be changed but the memory size is fixed. Example is an array
• Dynamic data structure


  Designed to allow the data structure to grow or shrink at runtime. It is possible to add new elements or remove existing elements without having to consider memory space.
• List


  Data structure that has the data items stored in the order they were originally added to memory. If the list is made up of a set number of data items, it can be a static data structure. If the list can vary in the number of data items, then it will be a dynamic data structure.
• Array


  Data structure that can hold a fixed number of data items, which must be of the same data type, i.e. real, integer, string etc. The data items in an array are known as elements.
• Using one-dimensional arrays

  Can be used to store a list of data in memory that can be used by a program at runtime. There are basic operations that can be carried out on data in a one-dimensional array.
• Two-dimensional arrays


  The data we want to process often comes in the form of a table. The data in a two-dimensional array must all be of the same data type.
• Traversing


  Traversing an array simply means using a loop to use each element of the array in a section of a program.
• Insertion


  You can add an element to an array at a given index.
• Deletion


  You can delete an element from an array.
• Searching


  Arrays can be searched using the index or the value stored at the index.
• Records


  Arrays can only hold data if it is all of the same data type. If you need to hold related data of different data types, you will need to use a data structure called a record. A record will be made up of information about one person or thing.
• Key field


  Each record in a file should have a key field, namely an item of data that is unique and can be used to identify the individual record.
• Files


  Made of records of the same structure and records are made up of fields containing information about one person or item.
• Validation


  A process to check that input data is reasonable
• Presence checks


  Used to prevent further progress if a required field is left blank.
• Format checks


  Used to ensure data matches a specific pattern, such as dd/mm/yyyy for a date. Input masks are often used to create format checks on database forms.
• Length checks


  Used to ensure an input data string is a sensible length, e.g. number of characters in ‘firstName’ to be between 3 and 16
• Type checks


  Used to ensure input data is a particular data type, e.g. quantity ordered to be integer or cost to be real.
• Range checks


  Used to ensure input data lies within a specified range, e.g. over time hours to be > 0 and < 15.
• Verification


  Process for checking data is correct. It can be carried out as a user enters data, via a keyboard for example, and also when data is copied from one part of a system to another. Copying should not change the data.