1. File Concept, File Management, File
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2. Recap
In the last class, you have learnt:
• File and File operations
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4. Objective(s)
On completion of this period, you would be able to:
• Understand the different file access methods
• Introduction to directory
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5. Introduction
• Files store information
• This information must be accessed and read into
computer memory for use
• There are different access methods
• Some systems support only one access method
• Other systems support different access methods
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6. Types of Access Methods
• Sequential Access
• Direct Access
• Other Access Methods
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7. Sequential Access
• Simplest access method
• Information processed in order
• Most common
• Used by editors and compilers
• Based on a tape model of a file
• Works well on sequential access devices and
random access devices
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8. Sequential Access
current position
beginning end
rewind
read or write
Fig : Sequential Access File
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9. Most Common Operations on Files
• Read
• Write
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10. File Pointer
• Whenever a file is opened for read or write
operation a file pointer is maintained to keep
track of the current position in the file
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11. Read operation
• Reads the next portion of the file
• Automatically advances file pointer
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12. Write operation
• Appends data to the end of the file
• Advances pointer to the new end of file
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13. Reset
• Reset to beginning
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14. Skip
• Skip n records forwards or backwards
( supported by few systems )
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15. Applications for sequential access
• Payroll of employees
• Student data processing
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16. Direct Access
• Also known as Relative Access
• Fixed length logical records
• No particular order for read and write
• Based on disk model of a file
• File viewed as a numbered sequence of blocks or
records
• Arbitrary block read or written
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17. Direct Access
• Useful for immediate access to large volumes of
data
• Most suitable for database applications
• Easy to read, write and delete a record
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18. Direct Access
•Address of arbitrary record N
Let length of logical record = L
Address of record N = L*(N-1)
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19. Simulate Sequential Access
• Easy to simulate sequential access on direct
access file
• Keep a variable cp i.e. Current position
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20. Simulate Sequential Access
Sequential Access Implementation for
direct access
reset cp := 0
read next read cp
cp := cp + 1
write next write cp
cp := cp + 1
Fig : Simulation of sequential access on a direct access file
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21. Applications for Direct access
• Airline reservation system
• Customer account processing in a bank
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22. Other Access Methods
• Built on top of direct access method
• Require the construction of an index for the file
• Index is a small table stored in memory
• The index contains pointers to various blocks
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23. To find an entry in the file…
• Search the index
• Use the pointer to access the file directly
for the desired entry
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24. To find an entry in the file
logical record
last name
number
Adams
Arthur
Asher Smith, John SSN
Age
.
.
.
Smith relative file
index file
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25. Primary index and Secondary index
• With large files index file itself too large to
be in memory
• Solution is create index for the index file
• Primary index points to secondary index
• Secondary index points to actual data
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26. Other subject materials
• Web designing
• Micro processors
• C++ tutorials
• java
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