Difference between revisions of "Matlab IO"

Revision as of 12:25, 22 September 2008

Input in MATLAB

Input can be accomplished in several ways in Matlab:

• Interactive input from the user. This is accomplished using the input command.

• Reading files from disk. There are two common methods to do this: using fscanf command and using the load command.

The INPUT command

The basic syntax for the input command is:

x = input('Enter the value of x: ');

This outputs the given text to the command window. The user can then enter a value and it will be stored in x.

Loading variables using LOAD

This section is a stub and needs to be expanded. If you can provide information or finish this section you're welcome to do so and then remove this message afterwards.

The FSCANF command

There are three steps to read from a file:

1. Open the file using fopen
2. Read from the file using fscanf
3. Close the file using fclose

The fscanf command is used primarily for reading from files. The basic syntax is:

value = fscanf( fid, format, size );

Opening and Closing Files

Before reading from a file, we must open it. This is done using the fopen command:

fid = fopen('filename','r');  % opens a file named "filename" for reading.

This assigns an identifier to fid that can be used in subsequent file operations (reading, closing, etc.). The 'r' indicates that this file is to be read from. Options are:

After you are done with a file, you should close it using the fclose command:

fclose(fid);  % closes the file referred to be "fid"

A Simple Example

Fred 64
Bob   32
Julia  49

file = fopen('ages.dat','r');
name = fscanf( file, '%s', 1 );
age  = fscanf( file, '%s' );
fclose(file);

If we wanted to read the whole file, we would need a few additional things:

• A cell array to hold the names
• An array to hold the ages.
• A way to determine when we hit the end of the file. This is done using the feof(fid) command.

Using these concepts, we can now read the file.

fid = fopen('ages.dat','r');      % opens the file "ages.dat" for reading.
i = 1;
names = {};
ages = [];
while ~feof(fid)
  names{i} = fscanf(fid,'%s',1);  % read the name.
  age(i)   = fscanf(fid,'%i');    % read the age.
  i = i+1;                        % get ready to read the next line
end
fclose(fid);                      % close the file.

Output in MATLAB

DISP

The disp command is the simplest output command. It can be used to output variable contents or text strings:

  T = linspace(50,250,5);
  disp('the temperatures (C) are:');
  disp( T );

FPRINTF

The fprintf command provides significantly more control over output. The basic usage of this command is:

fprintf( fid, 'formatting & text', v1, v2, ... vn );

This prints several variables, interspersed with text, to a file with the identifier fid. If no file id is specified, the output goes to the screen. The formatting is controlled by the formatting string.

Formatting Output

fprintf('Hello,\nWorld\n');

fprintf('\tline 1 \n line 2\n');

fprintf('Here is a backslash: \\');
fprintf('A backslash: \\\nand then a new line\n');

a = pi;
fprintf('The value of ''a'' is %1.3f\n',a);

score = 0.75;
fprintf('Your score is %2.0f%%\n',100*score);

fprintf('The value of pi to 6 decimal places is: %1.6f\n',pi);

year = [ 1750 1800 1850 1900 1950 2005 ];
pop = [ 791000 978000 1262000 165000 2518629 6453628 ];
for i=1:length(year)
  fprintf('The world population in %1.0f was %1.2e\n',year(i),pop(i) );
end

names = {'Bob','Jill','Carl','Steve'};
for i=1:length(names)
  fprintf('%s\n',names{i});
end

Left Justifying Output

Be default, output is right-justified. To left-justify the output, insert a "-" sign between the % and the formatting string. For example,

fprintf('There were %10.1e people in %4.0f\n',2518629,1950);

There were    2.5e+06 people in 1950

fprintf('There were %-10.1e people in %4.0f\n',2518629,1950);

There were 2.5e+06    people in 1950

fprintf('%10s %10s\n','pi','sin(pi)');      % right justified
fprintf('--------------------\n');
fprintf('%10.5f %10.5f\n',pi,sin(pi))       % right justified

        pi    sin(pi)
--------------------
   3.14159    0.00000

fprintf('\n%-10s %-10s\n','pi','sin(pi)');  % left justified
fprintf('--------------------\n');
fprintf('%-10.5f %-10.5f\n',pi,sin(pi));    % left justified

pi         sin(pi)   
--------------------
3.14159    0.00000   

File Output

The fprintf command may be used to write formatted output to files. This requires three steps:

1. Open the file for writing. Use the fopen command to open files.
2. Write to the file using the fprintf command.
3. Close the file using the fclose command.

Here is an example of writing data to a file.

x = linspace(-pi,pi);
y = sin(x);

% step 1: open the file
out = fopen('myOutput.dat');

% step 2: write data to the file
fprintf(out,'%-10s %-10s\n','x','sin(x)');
for i=1:length(x)
  fprintf(out,'%-10.5f %10.5f\n',x(i),y(i));
end

% step 3: close the file
fclose(out);

x          sin(x)    
---------------------
-3.14159     -0.00000
-2.44346     -0.64279
-1.74533     -0.98481
-1.04720     -0.86603
-0.34907     -0.34202
0.34907       0.34202
1.04720       0.86603
1.74533       0.98481
2.44346       0.64279
3.14159       0.00000

Note that the file "myOutput.dat" will be saved in the directory where the script was run from.

Saving Variables with SAVE

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Exporting Matlab Figures

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