Development Environment

Opening Files

Before reading or writing a text or binary file, you must open it with the fopen command.

• fid = fopen('filename','permission')
  

Specifying the Permission String

The permission string specifies the kind of access to the file you require. Possible permission strings include

• r for reading only
• w for writing only
• a for appending only
• r+ for both reading and writing

Note    Systems such as Microsoft Windows that distinguish between text and binary files might require additional characters in the permission string, such as 'rb' to open a binary file for reading.

Using the Returned File Identifier (fid)

If successful, fopen returns a a nonnegative integer, called a file identifier (fid). You pass this value as an argument to the other I/O functions to access the open file. For example, this fopen statement opens the data file named penny.dat for reading:

• fid = fopen('penny.dat','r')
  

If fopen fails, for example if you try to open a file that does not exist, fopen

• Assigns -1 to the file identifier.
• Assigns an error message to an optional second output argument. Note that the error messages are system dependent and are not provided for all errors on all systems. The function ferror can also provide information about errors.

Test the file identifier each time you open a file in your code. For example, this code loops until a readable filename is entered.

• fid=0;
  while fid < 1 
     filename=input('Open file: ', 's');
     [fid,message] = fopen(filename, 'r');
     if fid == -1
       disp(message)
     end
  end
  

Now assume that nofile.mat does not exist but that goodfile.mat does exist. On one system, the results are

• Open file: nofile.mat
  Cannot open file. Existence? Permissions? Memory? . . .
  
  Open file: goodfile.mat
  

Opening Temporary Files and Directories

The tempdir and tempname commands assist in locating temporary data on your system.

Function	Purpose
tempdir	Get temporary directory name.
tempname	Get temporary filename.

You can create temporary files. Some systems delete temporary files every time you reboot the system. On other systems, designating a file as temporary can mean only that the file is not backed up.

A function named tempdir returns the name of the directory or folder that has been designated to hold temporary files on your system. For example, issuing tempdir on a UNIX system returns the /tmp directory.

MATLAB also provides a tempname function that returns a filename in the temporary directory. The returned filename is a suitable destination for temporary data. For example, if you need to store some data in a temporary file, then you might issue the following command first.

• fid = fopen(tempname, 'w');
  

Note    The filename that tempname generates is not guaranteed to be unique; however, it is likely to be so.

Using Low-Level File I/O Functions

Reading Binary Data