# How to Sum the N newest data

1 view (last 30 days)
Hao Zhang on 9 Sep 2019
Edited: KALYAN ACHARJYA on 9 Sep 2019
Hi,
Say I have a code to simulate the time evolution of a very large 3D array E (the electric field), for instantce in finite-difference time-domina simulations. So, in each time step, E will be updated to a new value. Now I want to find the sum of the N (N=5 for example) newest E during each time of my simulation, what is the most efficient, that requires the least ammount of memory, to do this?
So, more intuitively,
at t=t1, E is updated to E1.
at t=t2, E is updated to E2.
at t=t3, E is updated to E3.
at t=t4, E is updated to E4.
at t=t5, E is updated to E5.
.
.
.
at t=tn, E is updated to En.
What I require is the following:
at t=t1, I require S=E1.
at t=t2, I require S=E1+E2.
at t=t3, I require S=E1+E2+E3.
at t=t4, I require S=E1+E2+E3+E4.
at t=t5, I require S=E1+E2+E3+E4+E5.
at t=t6, I require S=E2+E3+E4+E5+E6.
at t=t7, I require S=E3+E4+E5+E6+E7.
at t=t8, I require S=E4+E5+E6+E7+E8.
.
.
.
at t=tn, I require S=E_{n-4}+E_{n-3}+E_{n-2}+E_{n-1}+E_{n}
I do not want to use a new vaiable to keep storing the 5 newest data since E is a very large 3D matrix. Is there any other way to do it?
Many thanks!!

KALYAN ACHARJYA on 9 Sep 2019
Edited: KALYAN ACHARJYA on 9 Sep 2019
One way: % Assuming E1,E2,E3...are scalars
n= %total_data t1,t2,t3........
s=0;
E=zeros(1,);
for t=1:n % t time represents in your case
E(t)= ....% Updated
S=sum(E(1:t))
end
Any issue let me know?

KALYAN ACHARJYA on 9 Sep 2019
If so??
%First calculate E(1) to E(5) then
for t=5:n
E(t)=sum(E(t-4:t)) ....% Updated
.....
end
E1,E2, are scalars or vectors?
Hao Zhang on 9 Sep 2019
E1 E2... are also very large 3D matrix. Even the 5 latest value of E I do not want to store them in memory. Because the 5 is just a number I use for this example. It can be much larger in the real calculation.
Any way to only store the result of summation without explicitly storing the 5 latest E?
KALYAN ACHARJYA on 9 Sep 2019
E1 E2... are also very large 3D matrix,, If you want to latest 5 also..
Then you have to use (save) E{1}, E{2} as cell array elements and call the respective to compute the required
Save the last 5 iterartion E in cell array and call the cell elemets to sum the data, after 6 iteration replace the first cell array elements ...so ..on....