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Random Access Files

Random Access Files. CSC 171 FALL 2004 LECTURE 23. Sequential File Access. Sequential access Data in files are accessed one item after another The 4 th item cannot be read without reading the first 3 items Imagine updating the 1000000 th item and then updating the 999999 th.

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Random Access Files

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

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

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

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

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

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

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

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

  12. 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)

  13. RandomAccessFile Class RandomAccessFile raf = new RandomAccessFile(“products.dat”,”rw”); • File name • Mode • “r” for read only • “rw” for read & write

  14. 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 @ position 0

  15. Example: Reverse a file • Consider the problem of reversing a file with sequential access

  16. RandomAccessFile raf = new RandomAccessFile(fileName,"rw"); last = raf.length(); position = last - SIZEOFCHAR; while (position >= 0) { raf.seek(position); ch = (char)raf.readByte(); System.out.print(ch+"|"); position = position - SIZEOFCHAR; } raf.close(); } }

  17. test.dat This is a test. OUTPUT cd d:/courses/CSC171/CSC171FALL2001/code/ d:/devenv/jdk1.3/bin/javaw DisplayReversed .|t|s|e|t| |a| |s|i| |s|i|h|T| Process DisplayReversed finished

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

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

  20. for(int i = 1; i <= 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); }

  21. 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 <= 5; i++){ acct = raf.readInt(); amt = raf.readInt(); price = raf.readDouble(); System.out.println(" " + acct + " " + amt + " $" + price); }

  22. 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

  23. 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

  24. Loop & querry ID while (!acctstring.equals("999")) { recnum = Integer.parseInt(acctstring) - BASEREC; position = (recnum - 1) * RECLEN;

  25. 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);

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

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