31 December 2009 0 Comments

Matlab Quick Reference: Input/Ouput

I/O

Extract date from file

1
2
s = dir('junk.m');    
s.date

Open a formatted text file

1
fid = fopen('file.dat','w')

w: write r: read a: append returns -1 if error

If the file is a text file and you want to write to it, consider using ‘t’ as an attribute if you want Windows line break example:

1
fid = fopen('file.txt', 'wt')

Close file

1
fclose(fid)

1
fclose('all') % closes all open files

Filename

1
2
[pathstr, name, ext, versn] = fileparts(filein);      
fileout = fullfile(pathstr, [name, '.out']);

Write to formatted text file (line by line)

1
fprintf(fid,'%2.4f\n',x)

fid: 1 = stdout; 2 = stderr

- omitting fid writes on the screen

d: integer

f: fixed point notation e: exponential notation g: same as f or e but insignificant zeros do not print s: string of characters c: character Remember \n for line break

fprintf tips

Printing strings and numbers together: it looks like you have to separate the printing of strings and numbers into distinct fprintf statements. Example:

1
2
fprintf(fid, '%s', 'test ');
fprintf(fid, '%d %d\n', 10, 20); %printed on the same line as 'test '

Write (numeric) matrices to text file

1
2
A = [1 2 3 4 ; 5 6 7 8];
dlmwrite('my_data.out', A, ';')

‘;’ is the delimiter in my_data.out

“ means no delimiter

Note: not necessary to open the file first

Note: use the following notation if you want Windows line break:

1
dlmwrite('my_data.out', A, 'delimiter', ';', 'newline', 'pc')

To decide the number precision use the ‘precision’ attribute

1
'precision', '%10.5f'

Read ASCII delimited numeric data file

1
2
<p>data = dlmread('myfile.dat'); col1 = data(:,1);</p>
<p> </p>

Read from file containing numbers and strings

1
textscan

example:

Text file scan1.dat contains data in the following form:

Sally Level1 12.34 45 1.23e10 inf NaN Yes
Joe Level2 23.54 60 9e19 -inf 0.001 No
Bill Level3 34.90 12 2e5 10 100 No

Read each column into a variable:

1
2
3
fid = fopen('scan1.dat');
C = textscan(fid, '%s %s %f32 %d8 %u %f %f %s');
fclose(fid);

Specifiers:

%s : string

%n : double

%d : int

Note that textscan returns a cell array. Therefore C{1} is the cell array containing the surnames ‘Sally’, ‘Joe’ and ‘Bill’. C{1}(1) is the cell containing the string ‘Sally’. To convert this cell to a string (i.e. a character array), use the char function:

1
sallyString = char(C{1}(1));

The conversion is necessary e.g. if you want to write out the string using fprintf

Read file containing numbers and NaN

Example:

1
nc = 6; % number of columns of data in the input file    <br />fmt = repmat('%n', 1, nc);     <br />clear m;     <br />[m{1:nc}] = textread(filein, fmt);     <br />m = cat(2,m{:});

Delimiters (textscan and textread)

Default delimiter:

‘ \b\t’ To use a different delimiter:

1
C = textscan(fid, '%s %n', 'delimiter', '\t');

If a nondefault delimiter is specified, repeated delimiter characters are treated as separate delimiters. When using the default delimiter, repeated white-space characters are treated as a single delimiter.

Skipping fields (textscan and textread)

Use asterisk: %*s instead of %s Particularly handy when using textread where you can write the data directly into a list of variables

Error message

1
error('ERROR: wrong format');