# Can this for-loop code get faster in some way?

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Martin il 5 Ott 2019
Commentato: darova il 6 Ott 2019
I got a big result table named resTbl.
There I need for each row to grab a timestamp (posix time) and construct a period interval vector (I use a constant, ts_length, to construct this).
Then I need to find all those in period_interval that are represented in the variable data (column 2), and take the sum of those in data (column 7).
The below code works as I want it to:
for i = 1 : size(resTbl ,1)
period_interval = resTbl(i,2) : 60000 : resTbl(i,2) + ts_length;
[hd, he] = ismember(period_interval,data(:,2));
resTbl(i,10) = sum(data(he(hd),7));
end
The problem is that it is slow since resTbl has many rows. Does anyone have a suggestion how to make it faster?
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per isakson il 5 Ott 2019

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### Risposte (1)

darova il 5 Ott 2019
Modificato: darova il 5 Ott 2019
Maybe ismember function can be replaced:
dt = 60000;
period_interval = 0 : dt : ts_length;
n = length(period_interval);
for i = 1 : size(resTbl ,1)
cond = ~mod( data(:,2)-resTbl(i,2),dt ); % multiple by 60 000
mult = (data(:,2)-resTbl(i,2))/dt;
ind = (0 <= mult & mult <= n) & cond; % (0 <= multiplier <= n) and multiple by 60 000
% [hd, he] = ismember(period_interval,data(:,2));
resTbl(i,10) = sum(data(ind,7));
end
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Martin il 6 Ott 2019
Pretty brilliant solution I really have to admit!
It is faster but unfortunately it is however also pretty time consuming and VERY heavy on the memory due to the matrices!
With my current memory, 32 GB, I can not run the full resTbl set (3400 rows) on the dataset which is like 2 million rows and 7 columns.
I tried with smaller resTbl and data set, and for some reason I had to alter n to this: n = length(period_interval)-1; to match my own results.
I am not even sure if there exist a better solution to this problem than yours. I gladly hear from you again, but otherwise I say thank you very much!
darova il 6 Ott 2019
2 million rows and 7 columns
Maybe time is a price in this case

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