Structured binary files

Text files have no structure. They are just a stream of characters. Programmers must give special meaning to certain characters to recognise the pages and lines in text files.

Structured binary files have a definite structure:

• elements are all the same type (usually records)
• elements are stored in the same binary representation as they have in computer memory
• no LINE oriented operations

Since structured files are not stored as a sequence of characters:

• cannot read/write them with text editors
• I/O performed using special I/O packages (not TEXT_IO)
• read/write by program only

Choosing to use structured files

when?

• program designed for specific data
  • eg database
• program executed often with same data
  • eg payroll
• scratch  files
  • temporary storage during execution

why?

• read/write entire element at once
  • fast
  • simple code
• efficient data representation
  • save space

Types of structured files

There are two forms of structured files. The difference is in how their elements may be accessed:

• elements in sequential files can only be processed in sequence, starting at the beginning
• elements in direct access files can be processed in any order (including sequential)

Comparisons

Direct files vs sequential files:

• Direct files have the overhead of working out file sizes and locations within the file. Sequential files do not.
• Access to an individual element can be immediate with direct files. With a sequential file, all previous elements must be read before a given element is reached.

Which to use?

• Sequential files are appropriate when you want to read most or all of the records in the file every time you run your program.
• Direct access files are appropriate when only a few records will be processed.

Structured files vs arrays:

• In holding multiple elements of data, each of the same type, structured files have something in common with arrays.
• The comparisons are summarised in the following tables:

  	Structured file	Array
  Access speed	Slow (Secondary storage)	Fast (Primary memory)
  Size limit	Unlimited	Bounded
  Life time	Permanent	Transient

  	Access method
  Array	Direct or sequential
  Sequential file	Sequential
  Direct access file	Direct or sequential