# How to write a for loop that saves every 100th value?

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Lauren Miller on 30 Apr 2021
Edited: Scott MacKenzie on 30 Apr 2021
I have a code that has a for loop that calculates the eccentricity of an orbit as time passes, and I need to plot the results. Saving every single value of accentricity would be very large task, so I need to save every 100th value of eccentricity, along with the corresponding time, to some type of file.
This is what I have so far but it very much doesn't work, any advce would be greatly appreciated.
m = 0;
for i = 1: m
m = m + 1;
if m == 100
C{i} = emag;
C2{i} = t;
E(i,1) = emag;
E(i,2) = t;
matrixToWrite = [t emag];
writematrix(matrixToWrite, 'myData.txt')
m = 0;
t = t + delta;
end
end
Lauren Miller on 30 Apr 2021

Scott MacKenzie on 30 Apr 2021
Edited: Scott MacKenzie on 30 Apr 2021
If I understand your quesiton correctly, a for-loop is not necessary. It seems you have an array emag with many values and a corresponding array t with the timestamp for each value. Here's some code (using mock arrays) that saves every 100th value in t and emag in a file mydata.txt:
emag = rand(1,23456);
t = rand(1,23456);
n = length(emag);
emag100 = emag(1:100:n);
t100 = t(1:100:n);
writematrix([emag100' t100'], 'mydata.txt');
Lauren Miller on 30 Apr 2021
Thank you!!

EmirBeg on 30 Apr 2021
First of all you set m=0 and then start a loop from 1 to m. So i guess your loop does nothing.
You need to set a new variable for the amount of iterations your loop will go through. After that you should also set m to 0 again in your If-statement so you get a value every 100th iteration and not just the 100th.
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Lauren Miller on 30 Apr 2021
Thank you so much that's really helpful!