File AccessStorage of data in variables and arrays is temporary—

such data is lost when a program terminates. Files are used for permanent retention of data.

In this chapter, we explain how data files are created, updated and processed by Matlab programs.

We consider sequential-access files and random-access files.

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Creating a FilePrograms may process no files, one file or several files. Each file used in a program must have a unique name and

will have a different file id returned by fopen. All subsequent file processing functions after the file is

opened must refer to the file with the appropriate file id. Files may be opened in one of several modes.To create a file, or to discard the contents of a file before

writing data, open the file for writing ("w").

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Creating a File To read an existing file, open it for reading (‘r’). To open or create new file for writing (‘w’) To open or create new file, open the file for appending (‘a’). To open a file so that it may be written to and read from, open

the file for updating in one of the three update modes—’r+’, ‘w+’ or ‘a+’.

Mode ‘r+’ opens a file for reading and writing. Mode ‘w+’ creates a file for reading and writing. If the file already exists, the file is opened and the current

contents of the file are discarded. ©1992-2010 by Pearson Education, Inc. All Rights Reserved.

Example – Sequential-access% Create the filefid = fopen(‘myfile.txt', 'w');fwrite(fid, ‘hello’);fclose(fid);

% Read the contents back into an arrayfid = fopen('myfile.txt');str = fread(fid, [1, 5], 'int8=>char');fclose(fid);

©1992-2010 by Pearson Education, Inc. All Rights Reserved.

©1992-2010 by Pearson Education, Inc. All Rights Reserved.

©1992-2010 by Pearson Education, Inc. All Rights Reserved.

Creating a Sequential-Access FilecfPtr = fopen( 'clients.txt','r');if ( cfPtr == -1 ) fprintf( 'File could not be opened\n' );else % display request options request = input( 'Enter request');

Creating a Sequential-Access File %process user's request while ( request ~= 4 ) switch ( request ) case 1 fprintf( '\nAccounts with zero balances:\n' );

%read file contents (until eof) while ( ~feof( cfPtr ) ) %read account, name and balance from file */ account = fscanf( cfPtr, '%d%s%f',[1 3]); if ( account (1,3) == 0 ) fprintf( '%-10d%-13s%7.2f\n',account); end end

Creating a Sequential-Access File case 2 fprintf( '\nAccounts with - balances:\n' ); % read file contents (until eof) */ while ( ~feof( cfPtr ) ) %read account, name and balance from file */ account = fscanf( cfPtr, '%d%s%lf', [1 3]); if ( account(1,3) < 0 ) fprintf( '%-10d%-13s%7.2f\n',account ); end end

Creating a Sequential-Access File case 3 fprintf( '\nAccounts with + balances:\n' ); % read file contents (until eof) */ while ( ~feof( cfPtr ) ) % read account, name and balance from file */ account = fscanf( cfPtr, '%d%s%lf',[1 3] ); if ( account(1,3) > 0 ) fprintf( '%-10d%-13s%7.2f\n',account ); end end end frewind( cfPtr ); % return cfPtr to beginning of file */ request = input('\n?'); end fprintf( 'End of run.\n' ); fclose( cfPtr ); % fclose closes the file */ end

Example – Sequential-accessfid = fopen('bench.dat');

k = 0;while ~feof(fid) curr = fscanf(fid,'%c',1); if ~isempty(curr) k = k+1; benchstr(k) = curr; endend fclose(fid);

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Random-Access FilesRecords in a file created with the formatted output

function fprintf are not necessarily the same length. However, individual records of a random-access file are

normally fixed in length and may be accessed directly (and thus quickly) without searching through other records.

This makes random-access files appropriate for airline reservation systems, banking systems, point-of-sale systems, and other kinds of transaction processing systems that require rapid access to specific data.

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Example – Random-access fid1 = fopen('test1.dat', 'w+'); fwrite(fid1, 'ABCDEFGHIJKLMNOPQRSTUVWXYZ');

fid2 = fopen('test2.dat', 'w+'); fwrite(fid2, 'Second File');

% Seek to the 10th byte ('J'), read 5 fseek(fid1, 9, 'bof'); A = fread(fid1, 5, 'uint8=>char'); fclose(fid1);

% Append to test2.dat fseek(fid2, 0, 'eof'); fwrite(fid2, A); fclose(fid2);

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'bof' or -1 Beginning of file'cof' or 0 Current position

in file'eof' or 1 End of file

Example – Random-access cfPtr = fopen( 'clients.txt','r'); if ( cfPtr == -1 ) fprintf( 'File could not be opened\n' ); else % read record from file */ kayit = fscanf( cfPtr, '%d%s%lf', [3 3]); % update record */ fprintf( '%-10d%-13c%7.2f\n',kayit ); fclose(cfPtr); account = input( 'Enter account to update’); transaction=input('Enter charge(+)or payment(-):'); kayit(3,account) = kayit(3,account)+transaction; cfPtr = fopen( 'clients.txt','w+'); fprintf( cfPtr,'%-10d%-13c%7.2f\n' ,kayit ); fclose(cfPtr); end

Case Study: Transaction-Processing ProgramWe now write a substantial transaction-processing

program using random-access files. The program maintains a bank’s account information. The program updates existing accounts, adds new

accounts, deletes accounts and prints a listing of all the current accounts.