how to write an opened file in a loop to a new file in a different folder using fwrite?

Hi everyone,

I open a file in a loop using fopen. I make some modifications and then I want to write the file to a new folder but I get the error that the identifier is not valid.

Loop starts >

OpenFile = fopen('BetaWeatherFile.epw','r');

DuplicateATemporary = fopen('temporary.epw','w');

.....

modifications

.....

Then I want to use the fwrite here but it does not work.

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

[Wolfgang McCormack]

Unfortunately, I cannot share the code but I do appreciate any help!

[Mohammad Sami]

check if the file identifier returned from fopen is -1. This would indicate the file could not be opened.

also the you opened "OpenFile" as readonly. are you trying to write to this file ?

[Jorg Woehl]

As Mohammad said, you cannot write to OpenFile because have opened it for reading only. However, you should be able to write to DuplicateATemporary, because it was opened with write permissions.

Inside your loop, the following should work:

OpenFile = fopen('BetaWeatherFile.epw','r');
assert(OpenFile~=-1, 'Cannot open BetaWeatherFile.epw.')
DuplicateATemporary = fopen('temporary.epw','w');
assert(DuplicateATemporary~=-1, 'Cannot open or create temporary.epw.');
.....
modifications
.....
fwrite(DuplicateATemporary, modifications);
fclose(DuplicateATemporary);
fclose(OpenFile);

You can remove the assert statements once your code is running, but they may be helpful for debugging purposes.

[Wolfgang McCormack]

@Mohammad Sami @Jorg Woehl thank you both. So, I am able to modify the file (changing some columns and values | modifications) and even get my desirable output out of it for analyzing purposes and get the analysis going. So, from your fwrite, I am writing it to the same file right? I want to write it (save it) to a new different folder as a repsiotory. For instance, it starts opening 50 files makes the modifications, then save a copy of the final file before numerical alaysis to a diffferent folder. So, I didn't understand why you put modifications in that fwrite? I do really appreciate your answer and writing the code.

[Wolfgang McCormack]

@Mohammad Sami @Jorg Woehl funny story guys, I tried to make it as simple as possible (even getting rid of the dublicate) using the code you suggested. FileOpen goes right to 3! no 1 no 2 neither -1 :D

clear all; clc 
OpenFile = fopen('C:\example\BetaWeatherFile.epw','r')
assert(OpenFile~=-1, 'Cannot open BetaWeatherFile.epw.')
fwrite(OpenFile, 'E:\backup.epw')
fclose('all')

This is what I get:

OpenFile =

3

ans =

0

ans =

0

OpenFile is an array that includes number 3.

[Jan]

Yes, of course the first valid file identifier is 3. 1 is the standard output, 2 the channel for error messages.

With opening the file for reading by 'r', you cannot write to the file. Therefor your fwrite command replies 0 for: no character was written.

Are you sure you want to write the char vector 'E:\backup.epw' into the file? Maybe you have confused FWRITE and COPYFILE?

Answer 1

[Jan]

Yes, of course the first valid file identifier is 3. 1 is the standard output, 2 the channel for error messages.

With opening the file for reading by 'r', you cannot write to the file. Therefor your fwrite command replies 0 for: no character was written.

Are you sure you want to write the char vector 'E:\backup.epw' into the file? Maybe you have confused FWRITE and COPYFILE?

FileName = fullfile(tempdir, 'test.txt')
OpenFile = fopen(FileName, 'w');  % [EDITED, typo fixed]
assert(OpenFile~=-1, 'Cannot open file for writing.')
fwrite(OpenFile, 'Hi', 'char')
fclose(OpenFile);
copyfile(FileName, fullfile(tempdir, 'test2.txt'))

[Wolfgang McCormack]

@Jan sorry I didn't catch up. What I want to do is that I save my modified file to a different folder. So, I am able to change the file as I want and change its lines but I cannot save the output/modified file to a new folder but the modified file works just fine. It's actually outputted as a temporary file in the loop. What I want is that each outputted file in the loop be saved as A(i) in a different folder as backup. Something similar to what we do with excels, xlsreadthen modifications then xlswrite to save it somewhere else.

[Wolfgang McCormack]

so, you are saying that copyfile is like xlswrite not the fwrite?

[Wolfgang McCormack]

@Jan nice, your code gave me the hint to get the job done (my matlab treats that 'w' as part of the file directory). So now another question. This is happening within a loop and every time a 'test.txt' is being written (overwritten). How should i use this copyfile to get a copy of each temp file; will this work?

CopyPhase(i) = copyfile(Thefileis, fullfile(tempdir, 'test.txt'))?

or should I use the i within the name of test.txt?

[Jan]

What do you expect as output of the copyfile command? It is a flag, which tells you if the copy was successful. I do not see a reason to store this in the variable CopyPhase .

Instead of copying the file, you could write directly to the wanted file. Then using an index in the file name is an obvious solution:

for k = 1:10
    file = fullfile(Folder, sprintf('file%03d.txt', k));
    ...
end