Reading files from different directories

I am new with Matlab, want to read a file from a different directory, see below for the hirarchy:

Project_Folder/
    ->dat/
        -> file1.dat
    ->src/
        -> myFile.m

I must read

file1.dat

from

myFile.m

so I type (as found online):

../dat/file1.dat

But it says the file does not exist but if I put it in the same folder and type just

file1.dat

it works! (I am on a MACOS)

More exactly this is the error I get:

File does not exist: ../dat/controlled_config_1_train.dat
Error opening file: Invalid file identifier. Use fopen to generate a valid file identifier.
Output argument "Hist" (and maybe others) not assigned during call to "dct_coef_hist_from_list".

Here is the directory view:

Thanks for your help!

Here is the peace of code:

% file is defined as a parameter and contains a string like: "dat/file1.dat"
function [Hist, file_path] = dct_coef_hist_from_list(file)
%   This function extracts DCT coefficient histograms of images provided in
%   a file
%   Input:
%       file: the file containing all paths
%       channel: 1 (luminance), 2 (chrominance)
%       Nc: number of analyzed DCT coeff in each 8x8 blocks
%       BT: the limit of histogram ==> number of bins = 2*BT + 1
%   Output:
%       Hist: the nxd histogram matrix; n is the number of images; d is the
%       number of histogram bins
%       file_path: a list containing file paths (used to split data later)


    Nc = 9;         % number of AC coefficients (zig zag scan)
    BT = 20;        % maximum bin value => number of bins = 2*BT+1

if exist(file, "file")
    fprintf("File exists: %s\n", file);
else
    % this message is displayed    
    fprintf("File does not exist: %s\n", file);
end
try
    fid = fopen(file, 'r'); % here the code fails
    file_path = textscan(fid, '%s', 'delimiter', '\n', 'whitespace', '');
    file_path = file_path{1};
    N = length(file_path);
    fprintf('\nNumber of images: %d \n', N);
    
    Hist = zeros(N, (2*BT+1)*Nc);
    for i = 1:N
        [~,file_name,ext] = fileparts(file_path{i});
        fprintf('process file: %s\n', [file_name ext]);
        fprintf('process file: %s\n', file_path{i});
        Hist(i,:) = dct_coef_hist(file_path{i});
    end
catch me
    fprintf("Error opening file: %s\n", me.message());
    % this message prompts: "File does not exist: ../dat/file1 
    % Error opening file: Invalid file identifier. Use
    % fopen to generate a valid file identifier."
end

Answer 1

[dpb]

if exist(file, "file")

will only find the file if it is in the current working directory or on the search path and only if the variable file (which isn't defined in the code we can see so we don't know what it is) is defined to contain a filename that is in one of those locations.

The syntax "../dat/file1" only works if you're currently in a folder on the same level in the hierarchy as dat; the two dots take you a level higher than currently are before looking below that level. We can't tell what cd would return at the time you tried the above.

In general, it's far better to use fully-qualified filenames built via the fullfile function to explicitly locate and subsequently open the file than to rely on partial paths; they lead to issues like the above where the result is wholly dependent upon the current working directory.

Establish a base root directory and then create all names explicitly from that location -- if you want/need for user to be able to use different working areas, then use uigetdir to set a default location for that session.

[Bojan Poletan]

Thanks for your answer. I was told that the cose works as it is but it doesn't. I added the code in the question and a pic describing my working directory.

When you talk about search path you mean path added from

Enviroment > Set Path

menu? If so, the entire project folder has been added. In this case:

../dat/file1

should return the correct path? The 2 files are on the same level. And what in case they are not on the same level? I have also tried to pass the full file name in the system like:

/Users/irene/Desktop/sharing-chains-reconstruction-main/dat/file1.dat

(where file1 has been arranged to match my filename)

PS: when you talk about fullfile function, you mean something like this: ?

file = fullfile("../", file);

[dpb]

Get rid of the "../" stuff -- it's not useful here (and almost never is except in make files or the like).

Set a root directory and then use if with fullfile() to build the complete path.

I'm sure the code worked for whoever wrote it with their particular arrangement of working directory and path settings, but as you're discovering, that's not a robust way to address files when something is changed.

[Bojan Poletan]

Thanks, so if I want to build the path for feature_extraction.m, in the matlab terminal I type:

>> matlabroot

ans =

'/Applications/MATLAB_R2020a.app'

>> which feature_extraction.m
/Users/irene/Desktop/matlab_class/feature_extraction.m

I should write something like:

my_file_path = fullfile(matlabroot, "/Users/irene/Desktop/matlab_class/feature_extraction.m");
fid = fopen(my_file_path, "r");

Right ?