Random Access Files

1
Random Access Files

• CSC 171 FALL 2004
• LECTURE 23

2
Sequential File Access

• Sequential access
• Data in files are accessed one item after another
• The 4th item cannot be read without reading the
  first 3 items
• Imagine updating the 1000000th item and then
  updating the 999999th

3
Random/Direct Access

• The middle of the file can be
• Retrieved
• Modified
• Rewritten
• without reading/writing other data
• Good for data base applications

4
(No Transcript)
5
(No Transcript)
6

• In a ___________________ file access, a file is
  processed a byte at a time, in order.

7

• In a ____sequential____ file access, a file is
  processed a byte at a time, in order.

8

• ______________ access allow access at arbitrary
  locations in the file, without first reading the
  bytes preceding the access location.

9

• __Random______ access allow access at arbitrary
  locations in the file, without first reading the
  bytes preceding the access location.

10

• A file _____________ is a position in a
  random-access file.

11

• A file ____pointer___________ is a position in a
  random-access file.

12

• Because files can be very large a file pointer is
  of type ____________.

13

• Because files can be very large a file pointer is
  of type ___long___.

14
File Structure

• The key to random access is file structure
• Most commonly
• Fixed length records consisting of
• Fixed length items
• Example Inventory control (16 byte record)
• Product ID code (int 4 bytes)
• Quantity in stock (int 4 bytes)
• Price (double 8 bytes)

15
(No Transcript)
16
RandomAccessFile Class

• RandomAccessFile raf
• new RandomAccessFile(products.dat,rw)
• File name
• Mode
• r for read only
• rw for read write

17
Pointer Position

• Each random access stream establishes an internal
  pointer position
• The pointer keeps track of where the next byte is
  to be accessed
• The seek(long i) method permits the programmer to
  move to any byte position
• 1st byte _at_ position 0

18
Example Reverse a file

• Consider the problem of reversing a file with
  sequential access

19

• RandomAccessFile raf
• new RandomAccessFile(fileName,"rw")
• last raf.length()
• position last - SIZEOFCHAR
• while (position gt 0)
• raf.seek(position)
• ch (char)raf.readByte()
• System.out.print(ch"")
• position position - SIZEOFCHAR
• raf.close()

20

• test.dat
•  
• This is a test.
•  
• OUTPUT
•  
• cd d/courses/CSC171/CSC171FALL2001/code/
• d/devenv/jdk1.3/bin/javaw DisplayReversed
•  
• .tset a si sihT
• Process DisplayReversed finished

21
Example Inventory

• Inventory control (16 byte record)
• Product ID code (int 4 bytes)
• Quantity in stock (int 4 bytes)
• Price (double 8 bytes)

22

• // set up the keyboard for string input
• InputStreamReader isr
• new InputStreamReader(System.in)
• BufferedReader br
• new BufferedReader(isr)
•  

23

• for(int i 1 i lt 5 i)
• System.out.print("Enter the identification
  number ")
• acctstring br.readLine()
• acct Integer.parseInt(acctstring)
• raf.writeInt(acct)
• System.out.print("Enter the quantity in
  stock ")
• amtstring br.readLine()
• amt Integer.parseInt(amtstring)
• raf.writeInt(amt)
• System.out.print("Enter the price ")
• pricestring br.readLine()
• price Double.parseDouble(pricestring)
• raf.writeDouble(price)

24
Read Print the File

• System.out.println(" Quantity")
• System.out.println("ID. No. In Stock Price")
• System.out.println("------- --------
  ------")
•  
• // read and print the data
• for(int i 1 i lt 5 i)
• acct raf.readInt()
• amt raf.readInt()
• price raf.readDouble()
• System.out.println(" " acct " "
• amt " " price)

25
OUTPUT

• cd d/courses/CSC171/CSC171FALL2001/code/bronson/
• d/devenv/jdk1.3/bin/javaw ReadRandom
•  
• Quantity
• ID. No. In Stock Price
• ------- -------- ------
• 1001 476 28.35
• 1002 240 32.56
• 1003 517 51.27
• 1004 284 23.75
• 1005 165 32.25
•  
• Process ReadRandom finished

26
Modify The Database

• Set up Keyboard
• Open file
• Loop as long as user wants to modify
• Querry for ID number
• Look up display quantity
• Querry for modification
• Write modified value
• Close file

27
Loop querry ID

• while (!acctstring.equals("999"))
• recnum Integer.parseInt(acctstring) - BASEREC
  
• position (recnum - 1) RECLEN

28
Move to the record

• raf.seek(position)
• acct raf.readInt()
• //save loc ready to read/write amnt
• setbytepos raf.getFilePointer()
•  
• amt raf.readInt()
• System.out.println("The current quantity in stock
  is " amt)

29
UPDATE

• System.out.print("Enter the new quantity ")
• amtstring br.readLine()
• amt Integer.parseInt(amtstring)
• raf.seek(setbytepos) //reset loc
• raf.writeInt(amt)