How to Read Data From Folder in Matlab

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. Information technology covers the following:

• Exporting Data to MAT-Files
• Importing Information from MAT-Files

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

Exporting Data to MAT-Files

This section covers

• Using the save Function
• Saving Structures
• Appending to an Existing File
• Data Compression
• Unicode Character Encoding
• Optional Output Formats
• Storage Requirements
• Saving from External Programs

Using the salvage Function

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

• relieve                          filename                                    

or relieve only those variables that yous specify:

• salvage                          filename                                      var1                                      var2                        ...                          varN                                    

Apply the wildcard character (*) in the variable proper name to save those variables that friction match a specific pattern. For case, the following command saves all variables that showtime with str.

• save strinfo str*          

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

• whos -file strinfo   Name           Size                   Bytes  Class    str2           1x15                      xxx  char array   strarray       2x5                      678  prison cell array   strlen         1x1                        viii  double assortment          

Saving Structures

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

For structure Southward,

• S.a = 12.7;  S.b = {'abc', [iv 5; vi 7]};  S.c = 'Hi!';          

Salve the entire construction to newstruct.mat with the usual syntax:

• save newstruct.mat Due south;  whos -file newstruct   Proper name      Size                   Bytes  Class    Southward         1x1                      550  struct assortment          

Salvage the fields individually with the -struct option:

• save newstruct.mat -struct South;  whos -file newstruct   Proper name      Size                   Bytes  Class    a         1x1                        8  double array   b         1x2                      158  prison cell array   c         1x6                       12  char array          

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

• save newstruct.mat -struct S a c;  whos -file newstruct   Name      Size                   Bytes  Form    a         1x1                        eight  double array   c         1x6                       12  char array          

Appending to an Existing File

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

• For each variable that already exists in the MAT-file, MATLAB overwrites its saved value with the new value taken from the workspace.
• For each variable non plant in the MAT-file, MATLAB adds that variable to the file and stores its value from the workspace.

Note    Saving with the -append choice does not suspend additional elements to any arrays that are already saved in the MAT-file.

Data Compression

MATLAB compresses the data that yous save to a MAT-file. Information pinch can save y'all 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 private save operation, or for all of your MATLAB sessions. Use the -v6 choice with the salvage function to turn off compression on a per-command basis:

• relieve filename -v6          

To disable information 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 Environs documentation for more than information.

Note    You cannot read a compressed MAT-file with MATLAB Version 6 or half-dozen.5. To write a MAT-file that you volition be able to read with i of these versions, save to the file with data compression disabled.

Information returned by the command whos -file is independent of whether the variables in that file are compressed or non. The byte counts returned by this command represent the number of bytes data occupies in the MATLAB workspace, and non in the file the data was saved to.

Evaluating When to Compress. You lot should consider both data set size and the blazon of information being saved when deciding whether or not to shrink the data you save to a file. The benefits of information compression are greater when saving big information 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 non recommended equally information technology slows down the operation of relieve and load significantly, and offers little do good in return.

In full general, information pinch 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 boosted time spent compressing or decompressing. Because loading is typically done more than frequently than saving, load is considered to be the most critical of the ii 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 aforementioned data. Beyond that threshold, however, loading the compressed file is faster.

For example, say that you take a block of data that takes up 100 MB in retention, and this data has been saved to both a ten MB compressed file and a 100 MB uncompressed file. When you load each of these files back into the MATLAB workspace, the starting time x MB of data takes the same corporeality of time to load for each file. Loading the remaining ninety MB from the uncompressed file volition take ix times equally long every bit the first 10 MB, while all that remains to exist 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 as well varies depending on the type of computer existence 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.

Note    Compression and decompression during save and load is done transparently without the use of temporary files on disk. This is of significance to large dataset users in particular.

Unicode Character Encoding

MATLAB saves grapheme data to a MAT-file using Unicode character information encoding. Equally with data pinch, Unicode encoding is optional. If you disable it, MATLAB writes the MAT-file using the default character set for your organization. To disable Unicode character encoding on a per-control footing, use the -v6 selection with the save function:

• save filename -v6          

To disable Unicode encoding for all of your MATLAB sessions, open the Preferences dialog, select General and then MAT-Files, and click Local Grapheme Ready. Meet Full general Preferences for MATLAB in the Desktop Tools and Evolution Environment documentation for more information. When writing grapheme data using Unicode encoding (the default), MATLAB checks if the data is 7-scrap ASCII. If it is, MATLAB writes the 7-bit ASCII character information to the MAT-file using 8 bits per character (UTF-eight format), thus minimizing the size of the resulting file. Whatsoever character data that is non 7-bit ASCII is written in 16-fleck Unicode form (UTF-16). This algorithm operates on a per-string ground.

Note    You lot cannot read a Unicode encoded MAT-file with MATLAB Version half-dozen or half dozen.5. To write a MAT-file that you will be able to read with one of these versions, save to the file with Unicode character encoding disabled.

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

Optional Output Formats

Yous can choose from whatsoever of the post-obit formats for your output file. If yous do not specify a format, MATLAB uses the binary MAT-file format.

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

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

• Each variable to be saved must be either a ii-dimensional double array or a two-dimensional graphic symbol array. Saving a complex double array causes the imaginary role of the data to be lost, as MATLAB cannot load nonnumeric data ('i').
• To read the file with the MATLAB load role, make certain all the variables have the same number of columns. If you lot are using a program other than MATLAB to read the saved data, this brake can be relaxed.
• Each MATLAB character in a character array is converted to a floating-point number equal to its internal ASCII code and written out equally a floating-betoken number string. In that location is no information in the saved file that indicates whether the value was originally a number or a character.
• The values of all variables saved merge into a single variable that takes the name of the ASCII file (minus whatsoever extension). Therefore, it is advisable to relieve simply ane variable at a time.

Saving in Version 4 Format. With the -v4 pick, you can save only those data constructs that are compatible with MATLAB Version four. Therefore, you cannot save structures, cell arrays, multidimensional arrays, or objects. Variable names cannot exceed 19 characters in length. In addition, you must utilize filenames that are supported past 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 eight bytes per real element. Arrays with all integer entries and more than than x,000 elements are saved in the formats shown, requiring fewer bytes per element.

Element Range	Bytes per Chemical element
0 to 255	1
0 to 65535	two
-32767 to 32767	2
-ii31 to ii31-ane	iv
Other	viii

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 admission methods, rather than rely upon the specific MAT-file format, which is likely to change in the future.

Supported File Formats

Importing Information from MAT-Files

© 1994-2005 The MathWorks, Inc.

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