I. file input/output (DD 14.3-11, HR A-93-95, A-101-111)
 A. overview
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           FILE PROCESSING IN C++
    (DD 14.3-11, HR A-93-95, A-101-111)

Topics:

    Files and Streams
    Sequential Access files
    Random Access files

Motivating problem:

    power failure at the hotel

Essentials:
     #include <iostream.h>
     #include <fstream.h>

     ofstream outFile("output.txt");
     ifstream inFile("input.txt");

     int i;
     inFile >> i;
     outFile << i;

     outFile.write((char*)&i, sizeof(i));
     inFile.read((char*)&i, sizeof(i));
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 B. files and streams (DD 14.3)
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          FILE STREAMS

def: a *stream* is
    a sequence of characters,
    indefinitely long

Stream n bytes long:

    0 1 2 3 4 5 6 7        n-1
   [ | | | | | | | | ... | | | <EOF>

def: a *file* is a permenent collection
     of data.

def: an *input file stream* is


def: an *output file stream* is


File states:

	nonexistent    not in use
            |       /      ^
            | open /       | close
	    v     v        |
           opened  --> reading/writing

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	How many characters are in cin?
  1. input from files (DD 14.5)
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// countChars.C

#include <iostream.h>
#include <fstream.h>
#include <stdlib.h>

main()
{
  ifstream inFile("test.txt");

  if (!inFile) {
    cerr << "Cannot open " << "test.txt"
         << endl;
    return 1;
  }

  char ch;
  int count = 0;
  while (inFile >> ch) {
    count++;
  }

  cout << "There are " << count
       << " non-whitespace chars in "
       << "test.txt" << endl;

  return 0;
}
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  2. subtyping (DD p. 655)
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       SUBTYPING AMONG I/O TYPES

               ios

       istream       ostream

      ifstream      ofstream


Example:

  extern istream& operator >>(istream& in,
                              char & ch);

  ifstream inFile("my_data.txt");
  inFile >> ch;


def: type S is a *subtype* of T iff
     objects of type S can be used

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  3. output to files (DD 14.4))
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               PROBLEM

Write a program that reads guest info
from the terminal and stores it in a file
guestDB.txt

// Recovery.C

#include <iostream.h>
#include <fstream.h>
#include <string.h>
#include <errno.h>
#include "GuestInfo.h"
#include "IODetails.h"

main()
{
  cout << "Guest Data recovery" << endl;

  const char dbname[] = "guestDB.txt";
















  return 0;
}
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 C. sequential vs. random access files (DD 14.5-10)
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     SEQUENTIAL VS. RANDOM ACCESS FILES
             (DD 14.5-10)

def: in *sequential access*, it takes
    linear time to access each record.

def: in *random access* it takes
    constant time to access each record.


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  1. operations (DD p. 660)
------------------------------------------
   RANDOM ACCESS PRIMITIVES (DD p. 660)

File as a sequence of bytes:

    0 1 2 3 4 5 6 7        n-1
   [ | | | | | | | | ... | | | <EOF>

   file position pointer: [   ]

To start reading from beginning:



To skip to byte n:



To move forward n bytes:



To get the curent position:


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  2. example of random access files (compare DD 14.8)
------------------------------------------
        FIXED SIZE RECORDS

// GuestInfoFixed.h

#ifndef _GuestInfoFixed_h
#define _GuestInfoFixed_h 1

const int STRSIZE = 100;

struct GuestInfoFixed {
  char   name[STRSIZE+1];
  char   address[STRSIZE+1];
  int    roomNumber;
  float	 charges;
};
#endif
------------------------------------------
------------------------------------------
// GuestFileCreation.C

#include <iostream.h>
#include <fstream.h>
#include <string.h>
#include <errno.h>
#include "GuestInfoFixed.h"
#include "IODetails.h"

main()
{
  cout << "Guest File Creation" << endl;

  const char dbname[] = "guestDB.txt";

  ofstream outFile(dbname,
   	      ios::out | ios::noreplace);

  if (!outFile) {
    cerr << "Cannot open " << dbname
         << ": " << strerror(errno)
         << endl;
    return 1;
  }

  GuestInfoFixed blankGuest
    = {"", "", 0, 0.0};

  const int MAX_ROOM_NUM = 99;
  int i;
  for (i = 0; i <= MAX_ROOM_NUM; i++) {
    outFile.write((char*)&blankGuest,
		  sizeof(GuestInfoFixed));
  }

  return 0;
}
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