error using fprintf invalid file identifier use fopen

I am using fopen and fprintf to read data from array and export to a text file. It works fine when I started running the GUI for several times. However, fprintf started to give error message later on, while the exported file is incomplete too.

error message: Error using fprintf Invalid file identifier. Use fopen to generate a valid file identifier.

May I know what is wrong in the coding:

fid=fopen(fileexportpath,'w'); %Export
fprintf(fid,'%s %s\r\n','Headline: ',Title); %headline
fprintf(fid,'\r\n'); %spacing

for i = 1:TotalFile %total file imported
  for j = 1:Length_Individual_File % length of each file
     fid=fopen(fileexportpath,'a'); %append and write
     fprintf(fid,'%3.3f ',colA(j,i)); 
     fprintf(fid,'%2.5f\r\n',colB(j,i));
  end
  fprintf(fid,'\r\n'); % leave a space
  fclose('all');
end

Answer 1

[Stephen23]

Your code opens the same file multiple times in a loop, without closing it. Once that file is open there is no need to keep opening it again (unless you want to change its read/write mode, in which case you should fclose it first).

Because your code only refers to one file fileexportpath you only need to fopen it once, and fclose it once:

fid = fopen(fileexportpath,'wt');
fprintf(fid,...)
for m = 1:M
    for n = 1:N
        fprintf(fid,...)
    end
end
fclose(fid);

Note that you should fclose using the fid value (rather than using 'all'), as otherwise your code will close any other open files. This is not robust programming if you intend to write useful functions. For this reason you need to pay careful attention to which files you have opened and keep close track of them.

Note that you should avoid using i and j as the loop variable names, as these are both names of the inbuilt imaginary unit.

[Image Analyst]

You'd better check if you want a "friendly" error message rather than have it barf up a bunch of cryptic red error text:

fid = fopen(fileexportpath,'wt');
if fid ~= -1
    fprintf(fid,...)
    for m = 1:M
        for n = 1:N
            fprintf(fid,...)
        end
    end
    fclose(fid);
else
    message = sprintf('Error opening %s for writing', fileexportpath);
    uiwait(warndlg(message));
end

Answer 2

[Star Strider]

If you are creating a new file, see if changing your initial fopen call to:

fid=fopen(fileexportpath,'a+'); %Export

and deleting this line in your loop:

fid=fopen(fileexportpath,'a'); %append and write

does what you want.