Files and Filenames :: Programming Tips (Programming)

Programming

Files and Filenames

This section covers the following topics:

Naming M-files

M-file names must start with an alphabetic character, may contain any alphanumeric characters or underscores, and must be no longer than the maximum allowed M-file name length (returned by the function namelengthmax).

```
N = namelengthmax
N =
    63
```

Since variables must obey similar rules, you can use the isvarname function to check whether a filename (minus its .m file extension) is valid for an M-file.

```
isvarname mfilename
```

Naming Other Files

The names of other files that MATLAB interacts with (e.g., MAT, MEX, and MDL-files) follow the same rules as M-files, but may be of any length.

Depending on your operating system, you may be able to include certain non-alphanumeric characters in your filenames. Check your operating system manual for information on valid filename restrictions.

Passing Filenames as Arguments

In MATLAB commands, you can specify a filename argument using the MATLAB command or function syntax. For example, either of the following are acceptable. (The .mat file extension is optional for save and load).

load mydata.mat           % Command syntax
load('mydata.mat')        % Function syntax

If you assign the output to a variable, you must use the function syntax.

```
savedData = load('mydata.mat')
```

Passing Filenames to ASCII Files

ASCII files are specified as follows. Here, the file extension is required.

load mydata.dat -ascii            % Command syntax
load('mydata.dat','-ascii')       % Function syntax

Determining Filenames at Run-Time

There are several ways that your function code can work on specific files without you having to hard-code their filenames into the program. You can

Pass the filename in as an argument

```
   function myfun(datafile)
```

Prompt for the filename using the input function

   filename = input('Enter name of file:  ', 's');

Browse for the file using the uigetfile function

   [filename, pathname] = uigetfile('*.mat', 'Select MAT-file');

For more information: See the input and uigetfile function reference pages.

Returning the Size of a File

Two ways to have your program determine the size of a file are shown here.

  -- METHOD #1 --                    -- METHOD #2 --
s = dir('myfile.dat');         fid = fopen('myfile.dat');
filesize = s.bytes             fseek(fid, 0, 'eof');
                               filesize = ftell(fid)
                               fclose(fid);

The dir function also returns the filename (s.name), last modification date (s.date), and whether or not it's a directory (s.isdir).

(The second method requires read access to the file.)

For more information: See the fopen, fseek, ftell, and fclose function reference pages.

Save and Load Input/Output