Week 14 Reading a File

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Week 14Reading a File

• Loops

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Files
File - collection of records
Logical records - represent entities (persons,
places, objects, things, etc.)
1234567890123456 John Smith 12000 500.00
1234654321098765 Richard Roe 12000 500.00
Fields - describe properties of the entities
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Basic Concepts
DASD (direct access storage device)

• Performance
• Seek time
• Rotational delay
• Transfer time

Track
Magnetic disks
Cylinder (vertical alignment of tracks)
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Access Data
Tracks
? Rotational delay
?Seek time
Locating the beginning sector
Locating the right track
? Transfer time Transferring data from the disk
to memory
Read/write Head
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Basic Concepts
DASD (direct access storage device)
Tracks
Physical records (blocks, pages)
Block overhead
Logical records
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File Organization and Access Methods Progression
Magnetic tape

• Heap (sequential) files
• Hashed
• Hashed files
• Sorted
• Indexed sequential
• B trees (e.g., VSAM files)

Sequential access method
Hash methods
DSAD
Secondary storage devices
Indexed access methods
Applies to accessing records, not just storing
them
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File Organization and Access Methods

• File OrganizationThe physical arrangement of
  data in a file into records and pages on
  secondary storage
• Types
• Heap (unordered)-sequential
• Ordered (ordered by value)
• Hash
• Access methodThe steps involved in storing and
  retrieving records from a file

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Heap or Sequential Files
Queue
Records are accessed in the same sequence they
were placed into the file (first in, first out).
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