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FILES

External Storage structures

General

Files provide NON VOLATILE data storage (ie remain when power off)

There are 2 main classifications of files - Sequential Access and Random Access.

In order to access a Sequential access file, the file must be read from the beginning. To find a particular record in a sequential access file, you must begin your search from the beginning. In the worst case, the entire file might be searched to discover the required record is the last one. Clearly if the file is large, this may be very time consuming. A good analogy is an Audio Cassette - to find the song you want on a cassette, you start searching from the beginning of the tape.

Accessing a Random access file is fundamentally different - each record is indexed, meaning you can access any record you like instantly. A suitable analogy is an audio CD - you can get to any song (be it the first or the last) with equal speed of access.

In Pascal, there are 2 main types used - PASCAL DATA FILES (which are random access), and TEXT FILES (which are sequential access).


PASCAL DATAFILES

As a Random Access File, each part is accessible without reference to any other part (compared with sequential access files where the entire file has to be read from the beginning until the part being searched for is found)

The Operating System views random access files as objects, it is not interested in file structure, knows its size, type, date of save, and cluster info (FAT). Can manipulate whole file (copy, rename, delete...)

Lazarus has no direct knowledge of the total file size or location on disk, no knowledge of file status. Pascal views a Pascal Data File as comprising COMPONENTS. Each component is organised due to its type (any type except file). Only ONE component can be accessed at one time. Has ability to share intimate knowledge of data structure within each component.

Pascal datafiles generally

  • resides in secondary memory (disk, cartridge, tape)
  • ONE component accessible at once
  • Pascal allows an 'unlimited' number of components
  • Pascal must READ a component from disk or WRITE it back
  • Pascal and DOS must cooperate - the file 'belongs' to DOS so Pascal must 'ask' OS to do work for it.

Because the Operating system (OS) owns the file, different procedures for establishing a new file (REWRITE) or using an existing file (RESET). these can't be interchanged --> OS will intervene and may 'crash' your program.

Setting Up a New Data File In Lazarus

      Var   HighestScore    : integer;
            ScoreFile       : file of integer;

      Begin
            HighestScore    := 0;
            assignfile (ScoreFile,'A:\DATA\HISCORE.DAT');
            rewrite(ScoreFile);
            write(ScoreFile,HighestScore);
            closefile(ScoreFile);

assignfile = Pascal Identifier <---> OS filename link establishment
rewrite = OPEN a new file, named in assign
write = dump integer 'highestscore' into file (2 bytes) - OS does this, noting clusters etc in FAT
closefile = OS asked to place an EOF marker and perform file housekeeping

Using an Existing Pascal Data File

            assignfile (ScoreFile,'A:\DATA\HISCORE.DAT');
            reset(ScoreFile);
            read(ScoreFile,HighestScore);
            closefile(ScoreFile);
               :
               :         {the rest of the game}
               :
            reset(ScoreFile);
            write(ScoreFile,HighestScore);
            closefile(ScoreFile);

reset = open (if not already open) an existing file and place file pointer on the first component of the file
write = dump a value, then advance the file pointer to point at the next component

IT IS A LOGICAL ERROR to rewrite a file that already exists, and unpredictable damaging results will be the result of such an attempt

Files With Multiple Components

      write(destination,value) general form of output statement

...where destination could be lst (printer) or pascal file name

      while not eof(filename)    
    allows you to continually read from a disk file until the end of file marker is encountered
    successive read/writes advance file pointer to next component after action.

reset repoints to first component again

Finding a Particular Component

choices : loop sequentially (horribly inefficient and akin to sequential access) or seek

   eg:suppose we want the 16th component from a file, then

            assignfile(filename,'a:\DOSNAME.TXT');
            reset(filename);
            seek(filename,15);
            read(filename,Sixteenth_component);
            closefile(filename)

NOTE that to point the file pointer at the 16th component, we had to SEEK number 15 - that is because file contents begin at component 0.


Storing Tables In Files

Suppose we have the following table stored in memory:

             Name           Rank       Sernum
             Moncrieff G    Lc         362426
             Nigglerbater G Gen        42
             Parrts O       Pt         NCC1701
             Grable B       Lt         386222

with the following definitions:

      Type  range = 1..20;
            soldiers =   record
                            name     : string[30];
                            rank     : string[5];
                            sernum   : 0..maxint
                         end;
      Var   table : array [range] of soldiers;
            soldier, nsold : range;
            soldierfile : file of soldiers;
      Begin
         {input section}

         {write a new file}
         assignfile(soldierfile,'d:\SOLDIERS.DAT');
         rewrite(soldierfile);
         for soldier := 1 to nsold do
            write(soldierfile,table[soldier]);
         closefile(soldierfile);

         {get back whole file - unknown # of components}
         reset(soldierfile);
         soldier :=0;
         while not eof(soldierfile) do
            begin
               read(soldierfile,table[soldier+1]);
               inc(soldier)
            end;
         closefile(soldierfile);

NOTE: If access to individual elements in the saved table is not required, then the entire table can be saved in a single write statement, or read with one read:

      write(soldierfile,table)

      read(soldierfile,table)

Modifying a particular row in a file

To modify existing contents of a data file (so long as each row has been sequentially saved and assign already done), we could use code similar to the folowing:

   write('Which soldier number? :'); readln(soldier);
   reset(soldierfile);
   seek(soldierfile,soldier-1);  {position file pointer}
   read(soldierfile,table[soldier]); {get table row}

   with table[soldier] do
      rank := 'Capt';

   seek(soldierfile,soldier-1);   {reposition file pointer}
   write(soldierfile,table[soldier]); {put back row}
   closefile(soldierfile);

Using the first record for special purposes

There are many 'clever' things that can be built into Pascal data files to make their management easier.

Prior to writing into a new file, seek the second component (thus stepping over the first. Write all the components sequentially from there, then seek the first component and place the number of written components there. This allows you, the programmer to access the number of records there are, rather than rely on an indefinite loop to do the same thing. This method is commonly employed by programmers, but relies on the components all being the same type (ie. a numeric field would need to be part of the structure), and remembering to update this number each transaction.


TEXT FILES

A text file is a sequential access file, that can be read and created by many different applications. Most word processors, spreadsheet programs and other utilities can use TEXT - this makes this file type more generally applicable.

The drawback of using TEXT files is the speed of access - especially during searching and editing

They are declared thus:

           Var   name : textfile;

A text file is a file containing sequences of characters delimeted by called end of line (or EOLN) markers. Also, the file terminates with an end of file marker (EOF).

NOTE: you cannot randomly access a text file, you must access the stream of characters SEQUENTIALLY

eoln(filename) - answers true if the end of line in the file has been encountered, otherwise it is false
eof(filename) -answers true if end of file encountered
read(filename, variable {,variable}) gets values from a file

note: the read statement in a text file retrieval does TYPING (ie. interprets the incoming character stream to be of a particular data type) depending on the types of variables the text is being read into

if the file contained the following ASCII characters:

      abc 123 45.67

then the folowing read would be legal...

      read(filename,char1, gstring, int1, real1)

where the following values would be retrieved:

         char1 := 'a'    ;  gstring := 'bc'
         int1 := 123     ;  real1 := 45.67

readln(filename, variable {,variable}) reads values for variables listed, then moves to the next line (skipping over that text not read on the line)
write(filename, variable {,variable}) behaves as a write to the screen does, including field width and decimal places, but places text in the file without an end of line marker
writeln(filename, variable {,variable}) writes text as above but also places an eol

Compound Data Structures - FILES
 

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