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Import Binary Data with Low-Level I/O

Low-Level Functions for Importing Data

Low-level file I/O functions allow the most direct control over reading or writing data to a file. However, these functions require that you specify more detailed information about your file than the easier-to-use high-level functions. For a complete list of high-level functions and the file formats they support, see Supported File Formats for Import and Export.

If the high-level functions cannot import your data, use one of the following:

    Note:   The low-level file I/O functions are based on functions in the ANSI® Standard C Library. However, MATLAB® includes vectorized versions of the functions, to read and write data in an array with minimal control loops.

Reading Binary Data in a File

As with any of the low-level I/O functions, before importing, open the file with fopen, and obtain a file identifier. When you finish processing a file, close it with fclose(fileID).

By default, fread reads a file 1 byte at a time, and interprets each byte as an 8-bit unsigned integer (uint8). fread creates a column vector, with one element for each byte in the file. The values in the column vector are of class double.

For example, consider the file nine.bin, created as follows:

fid = fopen('nine.bin','w');
fwrite(fid, [1:9]);
fclose(fid);

To read all data in the file into a 9-by-1 column vector of class double:

fid = fopen('nine.bin');
col9 = fread(fid);
fclose(fid);

Changing the Dimensions of the Array

By default, fread reads all values in the file into a column vector. However, you can specify the number of values to read, or describe a two-dimensional output matrix.

For example, to read nine.bin, described in the previous example:

fid = fopen('nine.bin');

% Read only the first six values
col6 = fread(fid, 6);

% Return to the beginning of the file
frewind(fid);

% Read first four values into a 2-by-2 matrix
frewind(fid);
two_dim4 = fread(fid, [2, 2]);

% Read into a matrix with 3 rows and
% unspecified number of columns
frewind(fid);
two_dim9 = fread(fid, [3, inf]);

% Close the file
fclose(fid);

Describing the Input Values

If the values in your file are not 8-bit unsigned integers, specify the size of the values.

For example, consider the file fpoint.bin, created with double-precision values as follows:

myvals = [pi, 42, 1/3];

fid = fopen('fpoint.bin','w');
fwrite(fid, myvals, 'double');
fclose(fid);

To read the file:

fid = fopen('fpoint.bin');

% read, and transpose so samevals = myvals
samevals = fread(fid, 'double')';

fclose(fid);

For a complete list of precision descriptions, see the fread function reference page.

Saving Memory

By default, fread creates an array of class double. Storing double-precision values in an array requires more memory than storing characters, integers, or single-precision values.

To reduce the amount of memory required to store your data, specify the class of the array using one of the following methods:

  • Match the class of the input values with an asterisk ('*'). For example, to read single-precision values into an array of class single, use the command:

    mydata = fread(fid,'*single')
  • Map the input values to a new class with the '=>' symbol. For example, to read uint8 values into an uint16 array, use the command:

    mydata = fread(fid,'uint8=>uint16')

For a complete list of precision descriptions, see the fread function reference page.

Reading Portions of a File

MATLAB low-level functions include several options for reading portions of binary data in a file:

Testing for End of File

When you open a file, MATLAB creates a pointer to indicate the current position within the file.

    Note:   Opening an empty file does not move the file position indicator to the end of the file. Read operations, and the fseek and frewind functions, move the file position indicator.

Use the feof function to check whether you have reached the end of a file. feof returns a value of 1 when the file pointer is at the end of the file. Otherwise, it returns 0.

For example, read a large file in parts:

filename = 'largedata.dat';		% hypothetical file
segsize = 10000;

fid = fopen(filename);

while ~feof(fid)
    currData = fread(fid, segsize);
    if ~isempty(currData)
        disp('Current Data:');
        disp(currData);
    end
end
    
fclose(fid);

Moving within a File

To read or write selected portions of data, move the file position indicator to any location in the file. For example, call fseek with the syntax

fseek(fid,offset,origin);

where:

  • fid is the file identifier obtained from fopen.

  • offset is a positive or negative offset value, specified in bytes.

  • origin specifies the location from which to calculate the position:

    'bof'

    Beginning of file

    'cof'

    Current position in file

    'eof'

    End of file

Alternatively, to move easily to the beginning of a file:

frewind(fid);

Use ftell to find the current position within a given file. ftell returns the number of bytes from the beginning of the file.

For example, create a file five.bin:

A = 1:5;
fid = fopen('five.bin','w');
fwrite(fid, A,'short');
fclose(fid);

Because the call to fwrite specifies the short format, each element of A uses two storage bytes in five.bin.

Reopen five.bin for reading:

fid = fopen('five.bin','r');

Move the file position indicator forward 6 bytes from the beginning of the file:

status = fseek(fid,6,'bof');

Read the next element:

four = fread(fid,1,'short');

The act of reading advances the file position indicator. To determine the current file position indicator, call ftell:

position = ftell(fid)

position = 
     8 

To move the file position indicator back 4 bytes, call fseek again:

status = fseek(fid,-4,'cof');

Read the next value:

three = fread(fid,1,'short');

Reading Files Created on Other Systems

Different operating systems store information differently at the byte or bit level:

  • Big-endian systems store bytes starting with the largest address in memory (that is, they start with the big end).

  • Little-endian systems store bytes starting with the smallest address (the little end).

Windows® systems use little-endian byte ordering, and UNIX® systems use big-endian byte ordering.

To read a file created on an opposite-endian system, specify the byte ordering used to create the file. You can specify the ordering in the call to open the file, or in the call to read the file.

For example, consider a file with double-precision values named little.bin, created on a little-endian system. To read this file on a big-endian system, use one (or both) of the following commands:

  • Open the file with

    fid = fopen('little.bin', 'r', 'l')
  • Read the file with

    mydata = fread(fid, 'double', 'l')

where 'l' indicates little-endian ordering.

If you are not sure which byte ordering your system uses, call the computer function:

[cinfo, maxsize, ordering] = computer

The returned ordering is 'L' for little-endian systems, or 'B' for big-endian systems.

Opening Files with Different Character Encodings

Encoding schemes support the characters required for particular alphabets, such as those for Japanese or European languages. Common encoding schemes include US-ASCII or UTF-8.

The encoding scheme determines the number of bytes required to read or write char values. For example, US-ASCII characters always use 1 byte, but UTF-8 characters use up to 4 bytes. MATLAB automatically processes the required number of bytes for each char value based on the specified encoding scheme. However, if you specify a uchar precision, MATLAB processes each byte as uint8, regardless of the specified encoding.

If you do not specify an encoding scheme, fopen opens files for processing using the default encoding for your system. To determine the default, open a file, and call fopen again with the syntax:

[filename, permission, machineformat, encoding] = fopen(fid);

If you specify an encoding scheme when you open a file, the following functions apply that scheme: fscanf, fprintf, fgetl, fgets, fread, and fwrite.

For a complete list of supported encoding schemes, and the syntax for specifying the encoding, see the fopen reference page.

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