Computers

Data RepresentationChapter 3, SA

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

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.

Basic data types • Integers – whole numbers• Real numbers – w/ fractional

components• Exponential representation

• Character• ASCII vs EBCDIC

• Boolean –true/false• BLOB (Binary Large Object)

Data structures

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

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.

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.

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.

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.

File Organization

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

•Sequential•Linked List•Indexed•Hashed

Sequential File

addr00 Ayers ACCT01 Buckley MGT

02 Daley ACCT

03 Dejoie MGT

04 Kenderdine MKT

05 Linn FIN

06 Lusch MKT

07 Price MGT

08 Razook MKT

09 Schwarzkopf MGT

Sequential file sorted in alphabetical order. Sequential files are usually sorted in ID sequence order to facilitate batch processing.

Sequential File Processing

Transaction

Old Master New MasterProcess

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.

Linked List

addr pointer

00 Price MGT 01

01 Schwarzkopf MGT 02

02 Kenderdine MKT 0303 Lusch MKT 08

04 Buckley MGT 09

05 Ayers ACCT 06

06 Daley ACCT 07

07 Linn FIN 04

08 Razook MKT ##09 Dejoie MGT 00

Linked list to sort data alphabetically within department. An external reference must point to the start record (05).

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.

Indexed File(sequential index)

addr00 Price MGT Ayers ACCT01 Schwarzkopf MGT Daley ACCT02 Kenderdine MKT Linn FIN03 Lusch MKT Razook MKT04 Buckley MGT Dejoie MGT

ACCT 00ACCT 01FIN 02MGT 00MGT 01MGT 04MKT 03

Index to access data by department abbreviation.

Indexed File Processing

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.

Index

Segmented Index

addr

00 Price MGT Ayers ACCT

01 Schwarzkopf MGT Daley ACCT

02 Kenderdine MKT Linn FIN

03 Lusch MKT Razook MKT

04 Buckley MGT Dejoie MGT

05 Van Horn MGT

addr pointer pointer pointer100 101 Kenderdine 102 Razook 103101 200 Buckley 201 Dejoie 204102 203 Lusch 202103 205 Schwarzkopf 206200 00 Ayers 04 Buckley 201201 01 Daley 04 Dejoie 204202 00 Price 03 Razook 205203 02 Linn 03 Lusch 202204 02 Kenderdine 203205 01 Schwarzkopf 206206 5 Van Horn

Index

Data

Leaf

RootNodes

Indexed File Processing (segmented index)

Data File

Index

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.

Hashing(Prime Number Remainder Algorithm)Pick a prime number to define the file spaceDivide the key by the prime numberPut the result in the location of the remainder

Key = 41 13

3

4139

2

Location = 2

Hashed File Processing

Key Calculation

addr

Contents

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.

Classes and Objects