How to save an array output to file every 1000 iterations?

My array is 201 by 201. And I want to watch it evolve by looking at every 1000 steps in time. Basically I have an initial array and then every time step it evolves based on an operator that I defined for it. I don't have enough memory in matlab to save every iteration of the array, so I've only got the last 5 saved at a time. What I want to do is when the time step reaches 1000,2000,3000, etc to save in a seperate file (possibly a .cvs file or a .txt file) the values of that array. That way I can look at how my function is evolving over long times. Does anyone know how to do this?

If you need to see my code I can add it, but it's rather long so I've ommitted it for now.

EDIT:

I've tried using

for n = 1:n_max
   (lots of stuff defining f at each n value)

     if ~(mod(n,1000))
        export(mat2dataset(f(:,:,n))) 
     end
end

which seems to convert f to the appropriate type of dataset and then exports it to a text file which is great. The only problem with this is that I seem to be writing over the same document for each n that I get. Is there a way that I can give each new file a new name without having to write a new line in my script for each n?

Answer 1

[Wayne King]

You can certainly insert a write operation to catch every 1000 iterations by inserting an if statement on your counter.

Here is just a very simple example.

fid = fopen('test.txt','w');
for nn = 1:3000
    x = nn;
if ~(mod(nn,1000))
  fprintf(fid,'%4.2f\n',x);
end
end

Obviously this very simple example simply writes 1000.00, 2000.00, and 3000.00 to a text file because the value of the variable equals the value of the loop counter.

So basically you insert an if statement that catches when your counter hits 1000, 2000, etc.

You may actually want to just think about catching those matrices as elements of a cell array or something similar and handling the write operation (of just those matrices) after your simulation executes.

[sasha]

Would this give a different name to every saved file? Also, I'm a little confused on how to use fprintf. All of the examples I've found print things out as columns. But my array is 201 by 201 and I want it printed out in that way. Not as a column. would that be possible?

Basically if I had a matrix:

A = [A1 A2 A3; A4 A5 A6; A7 A8 A9]

I would want it to print out in the txt file as something with 3 rows and 3 colums

Answer 2

[Chad Greene]

If you can keep your analysis in Matlab, saving to a .mat file tends to be a smoother process. The syntax is

save myfile

to save all the variable in your workspace or

save variable1 variable2 someothervariable

to save specific variables. If you prefer text files, dlmwrite can do it as well.

If you're looping some solution, say you're going through the loop 10,000 times, you can save every time n is divisible by 1000 with

for n = 1:10000 
    myVariable = 5 + n; % or whatever calculations you're doing 
    if mod(n,1000)==0
       save(['solution_number',num2str(n),'.mat'],'myVariable'); 
    end
end

[sasha]

Ideally I would like to save this as a .txt file or an exel file so that I can send the data to someone who uses a different program and we can both work on it separately.

Also could you just elaborate how the ['solution_number',num2str(n),'.mat'] part works and exactly what it does?

[Chad Greene]

When n=1000, the ['solution_number',num2str(n),'.mat'] saves a .mat file called solution_number1000.mat. When n=2000, the ['solution_number',num2str(n),'.mat'] saves a .mat file called solution_number2000.mat and so on.

[Chad Greene]

You could apply the same concept to Wayne's solution if you want to write multiple text files with different names. Instead of his

 dlmwrite('testfile.txt',x,'-append');

You could use

 dlmwrite(['solution_number',num2str(n),'.txt'],x)

Answer 3

[Wayne King]

If its just a matrix, then you can use dlmwrite() with the '-append' option that will simply append the data. By default, dlmwrite() will comma separate the elements of the matrix, but you can control that by specifying a different delimiter.

for nn = 1:3000
  x = randn(4,4);
if ~(mod(nn,1000))
 dlmwrite('testfile.txt',x,'-append');

end
end

Note that fopen() also has a 'a+' option that appends.

[sasha]

I don't want to append the matrizes onto eachother. It ends up being a really really big matrix with the separate n value matrices not really separated from eachother. What I really would want in the above senario is 3 separate files with names like Variable_{n} with n replaced by the appropriate n value

Answer 4

[Wayne King]

OK, so simply change the filename each time. In the following example I just create a file called Iteration1000.txt, Iteration2000.txt, etc and just write that 4x4 matrix.

for nn = 1:3000
  x = randn(4,4);
  if ~(mod(nn,1000))
    filename = ['Iteration' num2str(nn) '.txt']; 
    dlmwrite(filename,x);

    end
  end

Thanks for accepting my answer if I have helped you.