fopen a file from another directory location

Hi,

I am trying to read a .txt file in another directory than my matlab workspace by using fopen. I found a lot of thing on mathworks but nothing work. Someone can help me? It mustn't be that difficult I suppose...

[Paolo]

Have you tried specifying the path of the file in the filename? Like this:

 fopen('C:\Users\Paolo\Documents\download.txt')

[Yohann Cornilliere]

Yes I tried this but the output variable of the fopen is 3 (I don't know why) by specifying the path and if I open the same file in the matlab workspace that variable is full of informations;but I would like to read that file in another folder of course...

[Ameer Hamza]

@Yohann, fopen does not load the content of the file. To read the content use a function like textscan(), fscanf() etc

[Stephen23]

"Yes I tried this but the output variable of the fopen is 3 (I don't know why)"

Because 3 is the file identifier for the open file, exactly as the fopen documentation describes: "opens the file...and returns an integer file identifier equal to or greater than 3."

To know what functions and operators do is why you should read the documentation.

Answer 1

[Ameer Hamza]

You can use a relative path as follow

f = fopen('../../filename');

symbol .. means one folder up. For example, if you have a structure like this

folder1
  file1.txt
  folder2
    file2.txt
    folder3
      current directory <---- you are here

To access the file1 use

f = fopen('../../file1.txt')

and to open file2.txt use

f = fopen('../file2.txt')

this is especially useful if you want to move your project folder as this will not break the paths on any other system.

The second option is to use absolute paths as given by @Paolo in the comments.

[Guillaume]

I would recommend always using absolute paths, so you don't have to rely on what the current directory may (or may not) be.

It is trivial to construct an absolute path from relative paths:

rootdir = 'C:\somewhere\rootdir';
relativefolder = 'mysubfolder';
fid = fopen(fullfile(rootdir, relativefolder, 'somefile.txt'));

Using relative paths is a sure way to have your program fail at some point because the user changed the current directory. (or the user gets upset because you changed their current directory).

[Ameer Hamza]

That is mostly dependent on the preference of the user. In absolute path case, it will be necessary to change rootdir whenever moving the project. Whereas in relative case, it is always easy to construct a full path by using pwd without asking the user to change anything.

[Stephen23]

@Ameer Hamza: pwd is not required, simply specify the current directory using '.'.

Answer 2

[Guillaume]

the output variable of the fopen is 3

That is expected and means that fopen is succesful.

if I open the same file in the matlab workspace that variable is full of informations

Then you're not using fopen. fopen, as documented opens a file for reading (or writing) and returns a file identifier. It never returns the content of the file. You then have to read the content with some other functions such as textscan or fread.

If you don't want the hassle, there are plenty of other ways to import a file in one go.

Regardless of the function you use, all of them support using absolute or relative paths as explained by others.

[priyanshu ashiya]

im facing the same problem

i m running gazealyse matlab toolbox.

error i get is

Error using textscan

Invalid file identifier. Use fopen to generate a valid file identifier.

tried to run matlab as admin didn't work.

any insight is helpful.

[Image Analyst]

@priyanshu ashiya What is the value of fid? If it's -1, the file was not able to be opened for some reason.