IO and File Manipulations: Random Access File Processing

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I/O and File ManipulationsRandom Access File
Processing
Wow.. I dont know programming is much easier
than I thought it before. Yes! I can do it.. ?
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You will learn

• Sequential file vs. random access file
• File pointer indicators
• seekp(), seekg(), tellp(), tellg()
• Example of random access file
• write(), read()

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Recap about sequential access file

• Data records are written to, or read from, the
  file in a sequential manner

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Updating Sequential-Access Files

• When we would like to update sequential files
• Risk overwriting other data
• Example change name "White" to "Worthington"
• Old data
• 300 White 0.00 400 Jones 32.87
• Insert new data

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Can you see what ??
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The solution is to use random access file!!!
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Random Access means..

• Moving directly to any location in the file,
  instead of moving through it sequentially..
• Any program will
• Compartmentalize the file (storage space) into
  equi-partition spaces
• maintain an indexing method, which will determine
  the location of data compartment in the file
• Hence, it makes sense to jump directly to that
  location, read and/or modify it

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Of many techniques to create random access files

• Making all the records in a file of same fixed
  length is the easiest technique!
• Goodness
• Make it easy for a program to calculate the exact
  location of any record relative to the beginning
  of the file

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File I/O utilities

• I/O processes are invisible to users
• How can we know that a process runs smoothly?
• Need to monitor the stream condition continuously
  and autonomously

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Stream Error States

• Test state of stream using bits
• eofbit set when EOF encountered
• Function eof returns true if eofbit set
• cin.eof()
• failbit set when error occurs in stream
• Data not lost, error recoverable
• Function fail returns true if set
• badbit set when data lost
• Usually nonrecoverable
• Function bad
• goodbit set when badbit, failbit and eofbit off
• Function good

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Stream Error States (contd)

• Member functions
• rdstate()
• Returns error state of stream
• Can test for goodbit, badbit, etc.
• Better to test using good(), bad()
• clear()
• Default argument goodbit
• Sets stream to "good" state, so I/O can continue
• Can pass other values
• cin.clear( iosfailbit )
• Sets failbit
• Name "clear" seems strange, but correct

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1 // Adapted from Deitels and Deitels text
book 2 // Testing error states. 3
include ltiostreamgt 4 5 using
stdcout 6 using stdendl 7 using
stdcin 8 9 int main() 10 11
int integerValue 12 13 // display
results of cin functions 14 cout ltlt "Before
a bad input operation" 15 ltlt
"\ncin.rdstate() " ltlt cin.rdstate() 16
ltlt "\n cin.eof() " ltlt cin.eof() 17
ltlt "\n cin.fail() " ltlt cin.fail() 18
ltlt "\n cin.bad() " ltlt cin.bad()
19 ltlt "\n cin.good() " ltlt
cin.good() 20 ltlt "\n\nExpects an
integer, but enter a character " 21 22
cin gtgt integerValue // enter character value 23
cout ltlt endl 24
13
25 // display results of cin functions
after bad input 26 cout ltlt "After a bad
input operation" 27 ltlt
"\ncin.rdstate() " ltlt cin.rdstate() 28
ltlt "\n cin.eof() " ltlt cin.eof() 29
ltlt "\n cin.fail() " ltlt cin.fail() 30
ltlt "\n cin.bad() " ltlt cin.bad()
31 ltlt "\n cin.good() " ltlt
cin.good() ltlt endl ltlt endl 32 33
cin.clear() // clear stream 34 35 //
display results of cin functions after clearing
cin 36 cout ltlt "After cin.clear()" 37
ltlt "\ncin.fail() " ltlt cin.fail() 38
ltlt "\ncin.good() " ltlt cin.good() ltlt endl 39
40 return 0 41 42 // end main
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Before a bad input operation cin.rdstate() 0
cin.eof() 0 cin.fail() 0 cin.bad() 0
cin.good() 1   Expects an integer, but enter a
character A   After a bad input
operation cin.rdstate() 2 cin.eof() 0
cin.fail() 1 cin.bad() 0 cin.good()
0   After cin.clear() cin.fail() 0 cin.good() 1
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Sequential File VS Random Access File
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To create, write and read random access files .

• Random access files requires agreement between
  programs used to write and read the files
• Tools are necessary to enable random access
  within the file

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About file position indicator (FPI)/file
position pointer (FPP)

• This indicator/pointer is used to determine the
  position in a file at which the next read or
  write operation will occur
• This helps in both sequential and random access
  files
• 2 types of FPI
• For reading (get pointer)
• For writing (put pointer)
• The get pointer indicates the byte number in the
  file from which the next input (for reading) is
  to occur
• The put pointer indicates the byte number in the
  file at which the next output should be placed  

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Some related functions..

• For istream
• tellg()
• Get position of the get pointer
• seekg()
• Set position of the get pointer
• For ostream
• tellp()
• Get position of the put pointer
• seekp()
• Set position of the put pointer

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seekg()

• istream seekg( streampos pos )
• istream seekg( streamoff off, iosseek_dir dir
  )
• Parameters
• pos
• The new position value streampos is a typedef
  equivalent to long.
• off
• The new offset value streamoff is a typedef
  equivalent to long.
• dir
• The seek direction. Must be one of the following
  enumerators
• iosbeg Seek from the beginning of the stream.
• ioscur Seek from the current position in the
  stream.
• iosend Seek from the end of the stream.
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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tellg()

• streampos tellg()
• Function
• Gets the value for the streams get pointer.
• Return Value
• A streampos type, corresponding to a long
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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seekp()

• ostream seekp( streampos pos )
• ostream seekp( streamoff off, iosseek_dir dir
  )
• Parameters
• pos
• The new position value streampos is a typedef
  equivalent to long.
• off
• The new offset value streamoff is a typedef
  equivalent to long.
• dir
• The seek direction. Must be one of the following
  enumerators
• iosbeg Seek from the beginning of the stream.
• ioscur Seek from the current position in the
  stream.
• iosend Seek from the end of the stream.
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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tellp()

• streampos tellp()
• Remarks
• Gets the position value for the stream.
• Return Value
• A streampos type that corresponds to a long.
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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Examples

• ofstream fout(orders.dat)
• ifstream fin(comparts.dat)
• fout.seekp(100,iosbeg)
• fin.seekg(-countsizeof(compart),iosend)
• Take note
• Line 3 will write at the location 100 bytes
  away from the beginning of the file
• Line 4 moves FPI for reading countsizeof(compar
  t) bytes towards the beginning of the file,
  starting from the end of the file
• Offset value can be positive or negative,
  indicating the direction away from or toward the
  beginning of the file
• After the FPI is positioned at the desired place,
  reading and writing will take place

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Example

• include ltiostreamgt
• include ltfstreamgt
• using namespace std
• int main ()
• long begin,end
• ifstream myfile ("example.txt")
• begin myfile.tellg()
• myfile.seekg (0, iosend)
• end myfile.tellg()
• myfile.close()
• cout ltlt "size is " ltlt (end-begin) ltlt "
  bytes.\n"
• return 0

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Creating a Random-Access File

• Usually write entire struct or object to file
• This will determine the fixed-length chunk of
  bytes youre writing to/reading from the file

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Functions involved in random access file
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Example

• Problem statement
• Struct compart containing part information and
  its quantity
• Variable count will update how many compart
  structures have been created
• This information will be used for writing to and
  reading from the file

compart1
compart2
compart3
.
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Example

• include ltiostreamgt
• include ltiomanipgt
• include ltfstreamgt
• include ltcstdlibgt
• using namespace std
• struct compart
• char part10
• int qty
• int main()
• int count0
• compart newpart
• ofstream fcout("comparts.dat",iosoutiosbin
  ary)
• if(!fcout)
• cerrltlt"Error in file opening for writing!"
• exit(1)
• coutltlt"Enter part and quantity or Ctrl-Z to
  stopgt"ltltendl
• while(cingtgtnewpart.partgtgtnewpart.qty)

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• ifstream fcin("comparts.dat",iosiniosbinary)
  
• if(!fcin)
• cerrltlt"Error in file opening for reading!"
• exit(1)
• if(countgt3)
• coutltlt"\nLast 3 records written to the file
  gt"ltltendl
• coutltlt"\nPart Quantity"ltltsetiosflags(ios
  left)ltltendl
• fcin.seekg((count-3)sizeof(compart),iosbe
  g)
• int c0
• while(clt3 !fcin.eof())
• fcin.read((char)newpart,
  sizeof(compart))
• c
• if(clt3)
• coutltltsetw(10)ltltnewpart.partltltsetw(5)ltlt
  newpart.qtyltltendl

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Review on Example

• Take note of the binary mode chosen when creating
  the file object for both read and write operation
• Line 20
• FPI for writing is positioned by seekp() at the
  location countsizeof(compart)bytes away from the
  beginning of the file
• Line 21
• Writes a data record stored in newpart to the
  file
• By referring to the format of write(), (char)
  type cast need to be applied to the address of
  newpart before passing it to write()
• Line 36
• FPI for reading is positioned by seekg() at the
  location count-3sizeof(compart)bytes away from
  the beginning of the file
• Line 40
• Reads a data record stored in file to newpart

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write()

• ostream write( const char pch, int nCount )
• ostream write( const unsigned char puch, int
  nCount )
• ostream write( const signed char psch, int
  nCount )
• Parameters
• pch, puch, psch
• A pointer to a character array.
• nCount
• The number of characters to be written.
• Remarks
• Inserts a specified number of bytes from a buffer
  into the stream. If the underlying file was
  opened in text mode, additional carriage return
  characters may be inserted. The write function is
  useful for binary stream output.
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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read()

• istream read( char pch, int nCount )
• istream read( unsigned char puch, int nCount )
• istream read( signed char psch, int nCount )
• Parameters
• pch, puch, psch
• A pointer to a character array.
• nCount
• The maximum number of characters to read.
• Remarks
• Extracts bytes from the stream until the limit
  nCount is reached or until the end of file is
  reached. The read function is useful for binary
  stream input.
• Reference http//msdn.microsoft.com/library/defau
  lt.asp

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Text File VS Binary File
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(No Transcript)
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Goodness of Random Access File

• Provides rapid access
• Beneficial to some applications such as
• Banking system
• Airline-reservation systems
• ATM
• Can perform further manipulations of the records
  efficiently
• How to retrieve the records will depend on your
  design
• Previous example writes in consecutive places
• Example in deitel and deitel, they are using
  account number as indexing
• More examples refer to deitel and deitel
  slides (in my website) and the given handout

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MORE istream Member Functions peek, putback and
ignore

• ignore()
• Discards characters from stream (default 1)
• Stops discarding once delimiter found
• Default delimiter \n
• putback()
• Puts character obtained by get() back on stream
• peek()
• Returns next character in stream, but does not
  remove

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1 // Adapted from Deitels and Deitels text
book 2 // Writing to a random access file. 3
include ltiostreamgt 4 5 using
stdcerr 6 using stdendl 7 using
stdcout 8 using stdcin 9 using
stdios 10 11 include ltiomanipgt 12
13 using stdsetw 14 15 include
ltfstreamgt 16 17 using stdofstream 18
19 include ltcstdlibgt 20 include
"clientData.h" // ClientData class definition 21

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22 int main() 23 24 int
accountNumber 25 char lastName 15 26
char firstName 10 27 double
balance 28 29 ofstream outCredit(
"credit.dat", iosbinary ) 30 31 //
exit program if ofstream cannot open file 32
if ( !outCredit ) 33 cerr ltlt "File
could not be opened." ltlt endl 34 exit(
1 ) 35 36 // end if 37 38
cout ltlt "Enter account number " 39 ltlt
"(1 to 100, 0 to end input)\n? " 40 41
// require user to specify account number 42
ClientData client 43 cin gtgt
accountNumber 44 client.setAccountNumber(
accountNumber ) 45
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46 // user enters information, which is
copied into file 47 while (
client.getAccountNumber() gt 0 48
client.getAccountNumber() lt 100 ) 49 50
// user enters last name, first name and
balance 51 cout ltlt "Enter lastname,
firstname, balance\n? " 52 cin gtgt setw(
15 ) gtgt lastName 53 cin gtgt setw( 10 )
gtgt firstName 54 cin gtgt balance 55
56 // set record lastName, firstName
and balance values 57 client.setLastName(
lastName ) 58 client.setFirstName(
firstName ) 59 client.setBalance(
balance ) 60 61 // seek position in
file of user-specified record 62
outCredit.seekp( ( client.getAccountNumber() - 1
) 63 sizeof( ClientData ) )
64 65 // write
user-specified information in file 66
outCredit.write(
67 reinterpret_castlt const char gt(
client ), 68 sizeof( ClientData ) )
69
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70 // enable user to specify another
account number 71 cout ltlt "Enter account
number\n? " 72 cin gtgt accountNumber 73
client.setAccountNumber( accountNumber
) 74 75 // end while 76 77
return 0 78 79 // end main
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Enter account number (1 to 100, 0 to end input) ?
37 Enter lastname, firstname, balance ? Barker
Doug 0.00 Enter account number ? 29 Enter
lastname, firstname, balance ? Brown Nancy
-24.54 Enter account number ? 96 Enter lastname,
firstname, balance ? Stone Sam 34.98 Enter
account number ? 88 Enter lastname, firstname,
balance ? Smith Dave 258.34 Enter account
number ? 33 Enter lastname, firstname, balance ?
Dunn Stacey 314.33 Enter account number ? 0