fopen just works inside of a loop

Hello,

I currently have a performance problem which should be easy to solve i think. The code here is a minimalistic example of the problem. It´s in 2 seperate .m-files. The "main" function (which is an optimizer) calls the "loop" function with the given values. The "loop" function calls the other m.-file "dothat" in every while-loop.

function main
a = 2;
b = 3;
loop(a, b)
end

function value = loop(a, b)
    while a <= 20 
        dothat
        a = a + b;
    end
end

The "dothat" file needs additional data from a .txt - file in every loop to interpolate a value (in the example just shows a text).

b = b+1;
fid = fopen('example.txt' , 'r');
tline = fgetl(fid)

That works fine, but in the real code the while loop "loops" a few thousand times so i also have to add an fclose command in the "dothat" file to prevent the "too many files open" error. So the code opens and closes the files thousands of times, and the simulation needs minutes instead of seconds. Is there a way to get the fopen-command out of the loop? It just works when fopen is in the "dothat" file or in the "loop" function, but just inside of the while-loop. Otherwise MATLAB cant open the file or doesn´t know "fid".

Thanks for any help, files are in the attachments!

[Stephen23]

"Is there a way to get the fopen-command out of the loop?"

Yes, open it before the loop and pass the file ID.

[Chris Topher]

So why does this work:

function value = loop(a, b)

while a <= 20
    fid = fopen('example.txt' , 'r');
    dothat
    a = a + b;
end
end

but not this:

function value = loop(a, b)
fid = fopen('example.txt' , 'r');

while a <= 20  
    dothat
    a = a + b;
end
end

Doesnt make sense to me

[Chris Topher]

It seems like it only remembers the file for one loop.

[Chris Topher]

I uploaded everything in the original question.

Answer 1

[Image Analyst]

Your dothat.m has a call to fopen() in it. But nowhere do you have a call to fclose(). Why not? You should. If you're opening the same file, you should open it just once, before the loop, and close it after the loop:

function value = loop(a, b)
fid = fopen('example.txt' , 'rt'); % Open text file as 'rt' just once.
while a <= 20  
    dothat % Must not have a call to fopen() in this function.
    a = a + b;
end
fclose(fid)
end

and do NOT have a call to fopen in dothat.m.

If you need to do sequential access on fid using fgetl(), then it's possible you might want to rewind the pointer, depending on if you need to check the file from the beginning or not.

frewind(fid);

[Chris Topher]

Thank you! It now works perfectly with just putting a frewind(fid) at the end of every loop. That´s what I´ve needed and it makes perfectly sense.

[Image Analyst]

OK, but you still need to use fclose() at some point. Don't forget that!!!

Answer 2

[Fangjun Jiang]

Whatever data you need from the text file, you could read it once, store it in a variable and use it in the loop. You wouldn't drive your car to a store 5 times in a roll back and forth to buy 5 items