Random Access FilesRandom Access Files

CSC 171 FALL 2004

LECTURE 23

Sequential File AccessSequential 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

Random/Direct AccessRandom/Direct Access

The middle of the file can be – Retrieved– Modified– Rewritten

without reading/writing other dataGood for data base applications

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

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

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

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

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

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

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

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

File StructureFile Structure

The key to random access is file structureMost 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)

RandomAccessFile ClassRandomAccessFile Class

RandomAccessFile raf = new RandomAccessFile(“products.dat”,”rw”);

– File name– Mode

“r” for read only “rw” for read & write

Pointer PositionPointer 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

Example: Reverse a fileExample: Reverse a file

Consider the problem of reversing a file with sequential access

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();

}}

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

Example: InventoryExample: Inventory

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

// set up the keyboard for string input

InputStreamReader isr =

new InputStreamReader(System.in);

BufferedReader br =

new BufferedReader(isr); 

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);

}

Read & Print the FileRead & Print the FileSystem.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);

}

OUTPUTOUTPUTcd d:/courses/CSC171/CSC171FALL2001/code/bronson/d:/devenv/jdk1.3/bin/javaw ReadRandom   QuantityID. 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

Modify The DatabaseModify The DatabaseSet up KeyboardOpen fileLoop as long as user wants to modify

– Querry for ID number– Look up & display quantity– Querry for modification– Write modified value

Close file

Loop & querry IDLoop & querry ID

while (!acctstring.equals("999")) {

recnum = Integer.parseInt(acctstring) - BASEREC;

position = (recnum - 1) * RECLEN;

Move to the recordMove to the recordraf.seek(position); acct = raf.readInt();

//save loc ready to read/write amntsetbytepos = raf.getFilePointer(); amt = raf.readInt();System.out.println("The current quantity in stock is: " + amt);

UPDATEUPDATE

System.out.print("Enter the new quantity: ");

amtstring = br.readLine();

amt = Integer.parseInt(amtstring);

raf.seek(setbytepos); //reset loc

raf.writeInt(amt);