Low Level File I/O (Updated 4/29/98)

This URL is http://www.stewart.cs.sdsu.edu/cs205/module12/

Our text discusses high-level file I/O on p. 27 with the

save data_1 x y

to save the vectors x and y computed in a script to the data file data_1.mat.

save data_2.dat z /ascii

to save the vector z to a file data_2.dat as a text file, i.e. ascii file that can be read by humans.

load data_1;

restores the data that was saved previous by a script.
A useful example is presented in the MATLAB code below. Given the file "exp.txt" exists and contains data that can be read with the %g format, these commands will read the data, 2 numbers per row, until the end of the file is reached.

fid = fopen('exp.txt')
a = fscanf(fid,'%g %g',[ 2 inf]) % fill the 2 by inf matrix
% In the example above, [2 inf] = 2 by inf means 2 elements per row,
% read until end of file
a = a'; % transpose
fclose(fid) % close the file

You can make this less formal by directly using the name of the file, without the fopen command

a = fscan('exp.txt', '%g %g', [2 inf])

There is a MATLAB script file available for you to run (and examine) today. At the MATLAB prompt:

explore_eps

explore_eps2

The new script (explore_eps2.m) is a slightly modified version of the example (explore_eps.m) we used earlier in the semester when we investigated Unit Roundoff in explore_eps, unitround examples. When you now run explore_eps2, you will notice there is no output, where did it go? How would you find out?

type explore_eps2

or look at the source code explore_eps2.m and its accompanying output file explore_eps2.out.
To balance this treatment of Low Level I/O, we have been looking at formatted input and output, i.e. numbers that people can read. What about machine readable (binary) data?

MATLAB provides the fread and fwrite commands to manipulate binary data.
When would this be an appropriate way to handle data? An external file is actually a sequence of records, often separated by an EOL, End of Line symbol for a text file. When you begin working with external files, you also have the opportunity (responsibility) to manipulate them. Each time you reference an external file, you do so through the file pointer, an operating system provided mechanism which initially points to the very first record of the file. MATLAB provides a uniform set of commands to rewind a file, test for the end of a file, feof, find out exactly where you are positioned in the file, ftell, and many other low-level operations that might be handy.

MATLAB documentation pages for I/O Details

fprintf
Formatted file I/O - Write data to a file
sprintf
Write formatted data to a string. For example
fscanf
Formatted file I/O - Read data from a file
fget1
Read formatted data from a file, discard newline character
fgets
Read formatted data from a file, keep newline character
sscanf
Read a string under format control. For example
s = '2.7183 3.1416';
A = sscanf(s,'%f') % creates the two-element vector containing poor approximations of e and pi
fclose
feof
ferror
fopen
format
fread
frewind
fseek
ftell
fwrite