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Saving and Loading MAT-Files

This section explains how to save the variables in your MATLAB session to a binary file called a MAT-file, and how to load them back into your MATLAB workspace. It covers the following:

MAT-files are double-precision, binary, MATLAB format files. They can be created on one machine and later read by MATLAB on another machine with a different floating-point format, retaining as much accuracy and range as the different formats allow. They can also be manipulated by other programs external to MATLAB.

Exporting Data to MAT-Files

This section covers

Using the save Function

To export workspace variables to a binary or ASCII file, use the save function. You can save all variables from the workspace in a single operation (if you omit the filename, MATLAB uses the name matlab.mat):

or save just those variables that you specify:

Use the wildcard character (*) in the variable name to save those variables that match a specific pattern. For example, the following command saves all variables that start with str.

Use whos -file to examine what has been written to the MAT-file:

Saving Structures

When saving a MATLAB structure, you have the option of saving the entire structure, saving each structure field as an individual variable in the MAT-file, or saving specific fields as individual variables.

For structure S,

Save the entire structure to newstruct.mat with the usual syntax:

Save the fields individually with the -struct option:

Or save only selected fields using -struct and specifying each field name:

Appending to an Existing File

You can add new variables to those already stored in an existing MAT-file by using save -append. When you append to a MAT-file, MATLAB first looks in the designated file for each variable name specified in the argument list, or for all variables if no specific variable names are specified. Based on that information, MATLAB does both of the following:

Data Compression

MATLAB compresses the data that you save to a MAT-file. Data compression can save you a significant amount of storage space when you are working with large files or working over a network.

Data compression is optional, however, and you can disable it either for an individual save operation, or for all of your MATLAB sessions. Use the -v6 option with the save function to turn off compression on a per-command basis:

To disable data compression for all of your MATLAB sessions, open the Preferences dialog, select General and then MAT-Files, and click Save uncompressed. See General Preferences for MATLAB in the Desktop Tools and Development Environment documentation for more information.

Information returned by the command whos -file is independent of whether the variables in that file are compressed or not. The byte counts returned by this command represent the number of bytes data occupies in the MATLAB workspace, and not in the file the data was saved to.

Evaluating When to Compress.   You should consider both data set size and the type of data being saved when deciding whether or not to compress the data you save to a file. The benefits of data compression are greater when saving large data sets (over 3MB), and are usually negligible with smaller data sets. Data that has repeating patterns or more consistent values compresses better than random data. Compressing data that has a random pattern is not recommended as it slows down the performance of save and load significantly, and offers little benefit in return.

In general, data compression and decompression slows down all save and some load operations to some extent. In most cases, however, the resulting reduction in file size is worth the additional time spent compressing or decompressing. Because loading is typically done more frequently than saving, load is considered to be the most critical of the two operations. Up to a certain threshold (relative to the size of the uncompressed MAT-file), loading a compressed MAT-File is slightly slower than loading an uncompressed file containing the same data. Beyond that threshold, however, loading the compressed file is faster.

For example, say that you have a block of data that takes up 100 MB in memory, and this data has been saved to both a 10 MB compressed file and a 100 MB uncompressed file. When you load each of these files back into the MATLAB workspace, the first 10 MB of data takes the same amount of time to load for each file. Loading the remaining 90 MB from the uncompressed file will take 9 times as long as the first 10 MB, while all that remains to be done with the compressed file is to decompress the data, and this takes a relatively short amount of time.

The loading size threshold is lower for network files, and also varies depending on the type of computer being used. Network users loading compressed MAT-files generally see faster load times than when loading uncompressed files, and at smaller data sizes than users loading the same files locally.

Unicode Character Encoding

MATLAB saves character data to a MAT-file using Unicode character data encoding. As with data compression, Unicode encoding is optional. If you disable it, MATLAB writes the MAT-file using the default character set for your system. To disable Unicode character encoding on a per-command basis, use the -v6 option with the save function:

To disable Unicode encoding for all of your MATLAB sessions, open the Preferences dialog, select General and then MAT-Files, and click Local Character Set. See General Preferences for MATLAB in the Desktop Tools and Development Environment documentation for more information. When writing character data using Unicode encoding (the default), MATLAB checks if the data is 7-bit ASCII. If it is, MATLAB writes the 7-bit ASCII character data to the MAT-file using 8 bits per character (UTF-8 format), thus minimizing the size of the resulting file. Any character data that is not 7-bit ASCII is written in 16-bit Unicode form (UTF-16). This algorithm operates on a per-string basis.

For more information on how MATLAB saves specific ASCII data formats, and on preventing loss or corruption of character data, see Writing Character Data in the MATLAB External Interfaces documentation.

Optional Output Formats

You can choose from any of the following formats for your output file. If you do not specify a format, MATLAB uses the binary MAT-file format.

Output File Format
Binary MAT-file (default)
save filename
8-digit ASCII
save filename -ascii
8-digit ASCII, tab delimited
save filename -ascii -tabs
16-digit ASCII
save filename -ascii -double
16-digit ASCII, tab delimited
save filename -ascii -double -tabs
MATLAB Version 4 compatible
save filename -v4

Saving in ASCII Format.   When saving in any of the ASCII formats, consider the following:

Saving in Version 4 Format.   With the -v4 option, you can save only those data constructs that are compatible with MATLAB Version 4. Therefore, you cannot save structures, cell arrays, multidimensional arrays, or objects. Variable names cannot exceed 19 characters in length. In addition, you must use filenames that are supported by MATLAB Version 4.

Storage Requirements

The binary formats used by save depend on the size and type of each array. Arrays with any noninteger entries and arrays with 10,000 or fewer elements are saved in floating-point formats requiring 8 bytes per real element. Arrays with all integer entries and more than 10,000 elements are saved in the formats shown, requiring fewer bytes per element.

Element Range
Bytes per Element
0 to 255
0 to 65535
-32767 to 32767
-231 to 231-1

Saving from External Programs

The MATLAB External Interfaces documentation provides details on reading and writing MAT-files from external C or Fortran programs. It is important to use recommended access methods, rather than rely upon the specific MAT-file format, which is likely to change in the future.

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