How can I save the output in a loop to an external text file?

In my loop, I'm trying to save the outputted values of eccentricity to a text file so that I can use this to plot the answers for my results but it's just not working and I have no clue where I'm going wrong.

I don't want to save every value that comes out because there'll be a lot, I just want to save 1 value for every 100 so I've set up a for loop but I'm not very experienced with MATLAB so I think it's ended up a bit of a mess. Any help would be incredibly appreciated

The loop:

   D=dot(P(2,:), V(2,:))
e1=norm(V(2,:));
e1=e1^2
mu=G*M(1)
e2=e1/mu
r1=norm(P(2,:))
e3=e2/r1
e4=e3*P(2,:)
e5=D/mu
e6=e5*V(2,:)
e=e4-e6
emag=norm(e)
emag=emag/1.0e+10
    %t = t + steps*delta;
steps = 100;
delta = 0.5;
    update_plot(P,H,t)
for i = 1: n
    m = 0;
    m = m + 1;
    if m == 100
        C{i} = emag;
        C2{i} = t;
        E(i,1) = emag;
        E(i,2) = t;
        dlmwrite('local filename.txt',emag,'-append', '\t')
        dlmwrite('local filename.txt',t,'-append', '\t')
        %m = 0;
t = t + steps*delta;
    end   
end

Answer 1

[Benjamin Großmann]

There are a few problems inside your loop. In each step you set m equal to zero and increment it to one. So m is not growing as it is intended to be. m=0 should be prior to the beginning of the loop. Furthermore, inside the loop you are using dlmwrite to write emag and t to text files, but emag is not changing inside the loop.

Basically, you are not writing anything because m never reaches 100. If you change this and m reaches 100, 200, ... then you are appending the same emag to the file.

Howerer, if you have some data in the variable myData, you can write every 100th value to a text file as follows:

% Generate some random data
myData = rand(1e6,1);

% write every 100th data point
writematrix(myData(1:100:end), 'myData.txt')

Edit: If you want to save the variable for further processing in matlab, than I would recommend to save the variable as mat-File instead of a text file. Mat-Files are compressed, a lot faster to write and read and I think you do not neccesseraly need to downsample your data. The one-liner

save('myMatFile', 'myData')

does the job. Be careful, that your variable names appear in appostrophes, i.e. as char array.

[Lauren Miller]

Thank you so much! How would you suggest I incorporate emag and time into the writematrix function?

[Benjamin Großmann]

You're welcome. I do not know how your data is shaped, maybe you should provide a MWE. Generally you can do this as follows:

% Parameters for data generation
T_start = 0;
T_step = 0.1;
T_stop = 100;

% Generate random data for MWE:
% time vector t and emag as column vector, i.e. of size (steps x 1)
t = (T_start:T_step:T_stop)';
emag = rand(size(t));

% We need a single matrix as input for writematrix(). Thus, we combine
% the column vectors to a matrix
matrixToWrite = [t emag];
writematrix(matrixToWrite, 'myData.txt')

[Lauren Miller]

Thank you so much for your help! In the writematrix part, is there a way to make it so that the results are just added on the end and not overwrited?

[Benjamin Großmann]

Yes, there is (see the writematrix doc page at "Name-Value Pair Arguments"):

writematrix(matrixToWrite, 'myData.txt', 'WriteMode', 'append') % Default write mode is 'overwrite'

[Lauren Miller]

You have been such a big life saver and made it so easy to understand, thank you so much for taking the time to help

[Benjamin Großmann]

I have seen your other questions regarding the same problem... Please do not use the for loop to save every 100th element. You can achive this with an one-liner:

matrixToWrite = [t(1:100:end) emag(1:100:end)];
writematrix(matrixToWrite, 'myData.txt', 'WriteMode', 'append') % Default write mode is 'overwrite'

[Brandon Gaydusek]

Years later and still helping out, thanks this was extremely useful, was looking for a way to append to a file a new matrix on every loop in my CFD code!