Computers

1
Computers

• Data Representation
• Chapter 3, SA

2
Data Representation and Processing

• Data and information processors must be able
  to
• Recognize external data and convert it to an
  appropriate internal format
• Store and retrieve data internally
• Transport data among internal storage and
  processing components

3
Binary Representation of Data

• Computers represent data using binary numbers.
• Binary numbers correspond directly with values in
  boolean logic.
• Computers combine multiple digits to form a
  single data value to represent large numbers.

4
Basic data types

• Integers whole numbers
• Real numbers w/ fractional components
• Exponential representation
• Character
• ASCII vs EBCDIC
• Boolean true/false
• BLOB (Binary Large Object)

5
Data structures

• Defined in software
• Arrays
• Lists
• Records
• Tables
• Files
• Indices
• Objects

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Data Structures

• A data structure is a related group of
  primitive data elements that is organized for
  some type of processing.
• Data structures are defined and manipulated
  within software.

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Data Structures

• Virtually all data structures make extensive
  use of pointers and addresses.
• Pointer a data element that contains the
  address of another data element.
• Address the location of some data element
  within a storage device.

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Arrays and Linked Lists

• Linked List
• A linked list is a data structure that uses
  pointers so list elements can be scattered among
  nonsequential storage locations.

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Records and Files

• A record is a data structure composed of other
  data structures or primitive data elements.
• Records are used as a unit of input and output to
  files or databases.

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File Organization
Physical arrangement of the records of a file on
secondary storage devices

• Sequential
• Linked List
• Indexed
• Hashed

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Sequential File
Sequential file sorted in alphabetical order.
Sequential files are usually sorted in ID
sequence order to facilitate batch processing.
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Sequential File Processing
Old Master
New Master
Process
Transaction
Sequential files must be recopied from the point
of any insertion or deletion to the end of the
file. They are commonly used in batch processing
where a new master file will be generated each
time the file is updated.
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Linked List
Linked list to sort data alphabetically within
department. An external reference must point to
the start record (05).

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Linked List File Processing
The next record in a linked list is found at the
address stored in the record. Records are added
at any location in the DASD and pointers adjusted
to include them. Deletions are not erased, but
pointers changed to omit the deleted record.
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Indexed File(sequential index)
Index to access data by department abbreviation.
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Indexed File Processing
Index
Index
Data File
When a record is inserted or deleted in a file
the data can be added at any location in the data
file. Each index must also be updated to reflect
the change. For a simple sequential index this
may mean rewriting the index for each insertion.
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Segmented Index
Index
Root
Nodes
Leaf
Data
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Indexed File Processing (segmented index)
Index
Data File
Data can be inserted or deleted at any location
in the data file. The index(es) must be updated
for each change, but only the affected segments
need to be rewritten.
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Hashing

• (Prime Number Remainder Algorithm)
• Pick a prime number to define the file space
• Divide the key by the prime number
• Put the result in the location of the remainder

Location 2
Key 41
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Hashed File Processing
Calculation
Key
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Records and Files

• A sequence of records on secondary storage is
  called a file.
• A sequence of records stored within main memory
  is called a table.
• Sequential files suffer the same problems as
  contiguous arrays when inserting and deleting
  records.
• To eliminate this problem, linked lists and
  indexed files are used.

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Classes and Objects