Error using fgets Invalid file identifier. Use fopen to generate a valid file identifier.

this is my pgm

function [filenames, classIDs] = ReadOutexTxt(txtfile)

 i= 1;
fid = fopen(txtfile,'r');
%tline = fgetl(fid); % get the number of image samples
while 1
    tline = fgetl(fid);
    if ~ischar(tline)
       break,  end
    %disp(tline)
    index = findstr(tline,'.');

    filenames(i) = str2double(tline(1:index-1)); % the picture ID starts from 0, but the index of Matlab array starts from 1
    classIDs(i) = str2double(tline(index+5:end));
    i = i+1;
end
fclose(fid)

Answer 1

[Thorsten]

Add

if fid == -1
 error('Cannot open file.')
end

You probably misspelled txtfile or it is not in the current directory.