Chapter 16: Data representation

Data types

Built-in data types

• the programming language defines the range of possible values that can be assigned to a variable when its type has been chosen.
• the programming language defines the operations that are available for manipulating values assigned to the variable.

User-defined data types

Non-composite data types

• does not involve a reference to another data type

Enumerated data type

• a non-composite user-defined data type for which the definition identifies all possible values
• a pseudocode example

Composite user-defined data types

• Record
  • collection of related items which may have different data types
• Array
  • (Indexed) collection of items with the same data type
• List
  • (Indexed) collection of items that can have different data types
• Set
  • stores a finite number of different values that have no order // supports mathematical operations
• Class/Structure
  • Gives the properties and methods for an object

Pointer data type

Set data type

• It contains a collection of data values.
• There is no organisation of the data values within the set.
• Duplicate values are not allowed.

File organisation

• the content is stored using a specific binary code
• A file is either a text file or a binary file
• The organisation of a binary file is based on the concept of a record
• A file contains records and each record contains fields

Serial files

• In a serial file each new record is simply appended to the file so that the only ordering in the file is the time order of data entry.

Sequential(顺序的) files

• records that are ordered
• there has to be a key field for which the values are unique and sequential but not necessarily consecutive

Direct-access files(Random-access files)

• 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.

Hashing algorithm

• do not have any order
• different key field values can produce the same remainder and therefore the same address in the file

File access

Real number

Floating-point

• Floating-point representation: a representation of real numbers that stores a value for the mantissa and a value for the exponent
• example:

Fixed-point representations

Precision

• the total number of bits to be used and decide on the split between those representing the mantissa and those representing the exponent
• increasing the number of bits for the mantissa would give better precision for a value
• leave fewer bits for the exponent, which reduces the range of possible values

Normalisation

• For a positive number, the bits in the mantissa are shifted left until the most significant bits are 0 followed by 1
• For a negative number until the most significant bits are 1 followed by 0

Conversion of representations

• Binary floating number to denary

• Denary to binary floating number

  • 
  • 
  • 

Problems with using floating-point numbers

• Floating-point numbers are used in extended mathematical procedures involving repeated calculations.
• Rounding errors
  • if calculations are repeated enough times
  • To avoid, you should increase the precision
• Range of numbers that can be stored
  • overflow error condition
  • underflow error condition