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Thursday, May 26, 2016

Working with Files in MATLAB - Part 1

In this post we will discuss some basic functions to work with external files using MATLAB. Matlab can interact with many popular file fromats. The common file formats are Excel® spread sheets, text, images, audio and video, XML documents and scientific data formats. In addition to the functions to directly work with these file format, there are low-level file I/O functions which work with data files in any format.

Open( ) - Open the files with extension

The open command is used to open a file available in the Matlab path. The file name is specified with the extension. This function is used to open almost all common file types. The syntax is:
open NAME
where NAME is the filename with extension. If the extension is not specified, it will search for a variable with name NAME and if not found only look for the files. The path for the file also can be used in the command. If the file is not found in the Matlab path of the specified path, the function will give an error message.
Examples:
Figure shows the outputs of the different open commands.
open test.txt
open test.docx
open test.pptx
open test.xlsx



If the file extension is not specified it will give an error message
open test
Error using open (line 102)
File 'test' not found.
 

fopen( ) - Open file with access permission


fopen( ) is the function used to open the files located in the current folder or Matlab path with assigned permission modes. The syntax is;
FID = fopen(‘Filename’, ’Permission’)
The command opens the file Filename in the mode specified by Permission. Default mode of permission is ‘read access’. The file name is specified as a string with extension. The file name may include the path of the file. FID is a scalar MATLAB integer valued double, called a file identifier. The codes used for the permission are given below.
         'r'     open file for reading
        'w'     open file for writing; discard existing contents
        'a'     open or create file for writing; append data to end of file
        'r+'    open (do not create) file for reading and writing
        'w+'    open or create file for reading and writing; discard
                existing contents
        'a+'    open or create file for reading and writing; append data
                to end of file
        'W'     open file for writing without automatic flushing
        'A'     open file for appending without automatic flushing

no = [1 2 3];
id = [331560  331566  332563];
A= [no; id];
fileID = fopen('test.txt','w');
fprintf(fileID,'%6s %12s\n','No','ID');
fprintf(fileID,'%6.0f %12.0f\n',A);
fclose(fileID);

The script open the file test.txt and write the data in to the file. 
The fclose( ) function is used to close the file.  
If the file is opened in update mode ('+'), we have to use fseek( ) or frewind( ) between other commands for entering data(input) to or reading(output) data from the file.

[FID, MESSAGE] = fopen('Filename') can be used to return a system dependent error message if the open is not successful.
[FID, MESSAGE] = fopen('data.txt')
FID =
    -1
MESSAGE =
No such file or directory
  

fclose( ) - Close the file

  
fclose(FID) closes the file associated with file identifier FID. The FID must me the same variable name used with the fopen( ) command.  If FID does not represent an open file, or if it is equal to 0 (standard input), 1 (standard output), or 2 (standard error), fclose(FID) returns an error.
 >> s=fopen(FID)
s =
C:\Users\skoya\Google Drive\Matlab Work\test.txt
s = fclose('all')

closes all open files, except 0, 1 and 2.


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