Import Text Data Files with Low-Level I/O
Low-level file I/O functions allow the
most 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,
importdata. For more information on the
high-level functions that read text files, see Import Text Files.
If the high-level functions cannot import your data, use one of the following:
fscanf, which reads formatted data in a text or ASCII file; that is, a file you can view in a text editor. For more information, see Reading Data in a Formatted Pattern.
fgets, which read one line of a file at a time, where a newline character separates each line. For more information, see Reading Data Line-by-Line.
fread, which reads a stream of data at the byte or bit level. For more information, see Import Binary Data with Low-Level I/O.
For additional information, see:
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 Data in a Formatted Pattern
To import text files that
read, consider using
requires that you describe the format of your file, but includes many
options for this format description.
For example, create a text file
shown. The data in
mymeas.dat includes repeated
sets of times, dates, and measurements. The header text includes the
number of sets of measurements,
Measurement Data N=3 12:00:00 01-Jan-1977 4.21 6.55 6.78 6.55 9.15 0.35 7.57 NaN 7.92 8.49 7.43 7.06 9.59 9.33 3.92 0.31 09:10:02 23-Aug-1990 2.76 6.94 4.38 1.86 0.46 3.17 NaN 4.89 0.97 9.50 7.65 4.45 8.23 0.34 7.95 6.46 15:03:40 15-Apr-2003 7.09 6.55 9.59 7.51 7.54 1.62 3.40 2.55 NaN 1.19 5.85 5.05 6.79 4.98 2.23 6.99
Opening the File
As with any of the low-level I/O functions, before reading,
open the file with
obtain a file identifier. By default,
files for read access, with a permission of
When you finish processing the file, close it with
Describing the Data
Describe the data in the file with format specifiers, such as
'%d' for an integer, or
a floating-point number. (For a complete list of specifiers, see the
fscanf reference page.)
To skip literal characters in the file, include them in the
format description. To skip a data field, use an asterisk (
in the specifier.
For example, consider the header lines of
Measurement Data % skip the first 2 words, go to next line: %*s %*s\n N=3 % ignore 'N=', read integer: N=%d\n % go to next line: \n 12:00:00 01-Jan-1977 4.21 6.55 6.78 6.55 ...
To read the headers and return the single value for
N = fscanf(fid, '%*s %*s\nN=%d\n\n', 1);
Specifying the Number of Values to Read
fscanf reapplies your format
description until it cannot match the description to the data, or
it reaches the end of the file.
Optionally, specify the number of values to read, so that
not attempt to read the entire file. For example, in
each set of measurements includes a fixed number of rows and columns:
measrows = 4; meascols = 4; meas = fscanf(fid, '%f', [measrows, meascols])';
Creating Variables in the Workspace
There are several ways to store
the MATLAB workspace. In this case, read the values into a structure.
Each element of the structure has three fields:
fscanf fills arrays with numeric values in
column order. To make the output array match the orientation of numeric
data in a file, transpose the array.
filename = 'mymeas.dat'; measrows = 4; meascols = 4; % open the file fid = fopen(filename); % read the file headers, find N (one value) N = fscanf(fid, '%*s %*s\nN=%d\n\n', 1); % read each set of measurements for n = 1:N mystruct(n).mtime = fscanf(fid, '%s', 1); mystruct(n).mdate = fscanf(fid, '%s', 1); % fscanf fills the array in column order, % so transpose the results mystruct(n).meas = ... fscanf(fid, '%f', [measrows, meascols])'; end % close the file fclose(fid);
Reading Data Line-by-Line
MATLAB provides two functions that read lines from files
and store them as character vectors:
fgets function copies the line along with the
newline character to the output, but
The following example uses
fgetl to read
an entire file one line at a time. The function
whether a given character sequence (
in each line. If it does, the function prints the entire line preceded
by the number of times the literal appears on the line.
function y = litcount(filename, literal) % Count the number of times a given literal appears in each line. fid = fopen(filename); y = 0; tline = fgetl(fid); while ischar(tline) matches = strfind(tline, literal); num = length(matches); if num > 0 y = y + num; fprintf(1,'%d:%s\n',num,tline); end tline = fgetl(fid); end fclose(fid);
Create an input data file called
Oranges and lemons, Pineapples and tea. Orangutans and monkeys, Dragonflys or fleas.
To find out how many times
'an' appears in
this file, call
2: Oranges and lemons, 1: Pineapples and tea. 3: Orangutans and monkeys, ans = 6
Testing for End of File (EOF)
When you read a portion of your data at a time, you can use
check whether you have reached the end of the file.
a value of
1 when the file pointer is at the end
of the file. Otherwise, it returns
Opening an empty file does not move the
file position indicator to the end of the file. Read operations, and
move the file position indicator.
Testing for EOF with feof
For example, suppose that the hypothetical file
the following form, with no information about the number of measurement
sets. Read the data into a structure with fields for
12:00:00 01-Jan-1977 4.21 6.55 6.78 6.55 9.15 0.35 7.57 NaN 7.92 8.49 7.43 7.06 9.59 9.33 3.92 0.31 09:10:02 23-Aug-1990 2.76 6.94 4.38 1.86 0.46 3.17 NaN 4.89 0.97 9.50 7.65 4.45 8.23 0.34 7.95 6.46
To read the file:
filename = 'mymeas.dat'; measrows = 4; meascols = 4; % open the file fid = fopen(filename); % make sure the file is not empty finfo = dir(filename); fsize = finfo.bytes; if fsize > 0 % read the file block = 1; while ~feof(fid) mystruct(block).mtime = fscanf(fid, '%s', 1); mystruct(block).mdate = fscanf(fid, '%s', 1); % fscanf fills the array in column order, % so transpose the results mystruct(block).meas = ... fscanf(fid, '%f', [measrows, meascols])'; block = block + 1; end end % close the file fclose(fid);
Testing for EOF with fgetl and fgets
If you use
fgets in a control loop,
not always the best way to test for end of file. As an alternative,
consider checking whether the value that
is a character vector.
For example, the function
in Reading Data Line-by-Line includes
while loop and
y = 0; tline = fgetl(fid); while ischar(tline) matches = strfind(tline, literal); num = length(matches); if num > 0 y = y + num; fprintf(1,'%d:%s\n',num,tline); end tline = fgetl(fid); end
This approach is more robust than testing
fgetsfind data, they return a character vector. Otherwise, they return a number (
After each read operation,
fgetscheck the next character in the file for the end-of-file marker. Therefore, these functions sometimes set the end-of-file indicator before they return a value of
-1. For example, consider the following three-line text file. Each of the first two lines ends with a newline character, and the third line contains only the end-of-file marker:
Three sequential calls to
fgetlyield the following results:
t1 = fgetl(fid); % t1 = '123', feof(fid) = false t2 = fgetl(fid); % t2 = '456', feof(fid) = true t3 = fgetl(fid); % t3 = -1, feof(fid) = true
This behavior does not conform to the ANSI specifications for the related C language functions.
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.
If you do not specify an encoding scheme when opening a file for reading,
fopen uses auto character-set detection to determine the encoding. If
you do not specify an encoding scheme when opening a file for writing,
fopen defaults to using UTF-8 in order to provide interoperability
between all platforms and locales without data loss or corruption.
To determine the default, open a file, and call
fopen again with the
[filename, permission, machineformat, encoding] = fopen(fid);
If you specify an encoding scheme when you open a file, the
following functions apply that scheme:
For a complete list of supported encoding schemes, and the syntax
for specifying the encoding, see the