finding numbers below a threshold and doing calculations on them

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I have a model of a box filled with particles. I have the velocity and coordinates of each particle in the box. I have divided the box into 4 segments based on coordinates and want to do calculations on the particles in each segment.
I have written a code which tells me how many particles are in each segment in each time step. But I now it to identify which particles are in each segment of the model, and calculate the average velocity per segment based off individual particel velocities.
file = dir('*.csv');
num_files = length(file);
[~, index] = natsort({file.name});
filelist = file(index);
rows = zeros(1, num_files);
for a = 1:num_files
T = table2array(readtable(filelist(a).name)); %read the data into matlab
%sum how many particles are in each of the 4 segments of the model
S1(a) = sum(T(:,4) >0.13); %first segment thresholds
S2(a) = sum(T(:,4)>= 0.115 & T(:,4) <= 0.13); %second segment thresholds
S3(a) = sum(T(:,4)>= 0.1 & T(:,4) <= 0.115); %third segment thresholds
rows(a) = height(T); %record how many rows are in the whole table (e.g. how many total grains in pile)
ResV(a) = sqrt((T(:,2).^2) + (T(:,3).^2) + (T(:,4).^2)); %calculate the resultant velocity of each particle in each time step
end

Accepted Answer

Voss
Voss on 13 Oct 2021
idx = T(:,4) > 0.13; % similarly for other thresholds
ResV(a) = mean(sqrt(sum(T(idx,2:4).^2,2)));
In your original code, note that a value of 0.115 in T(:,4) would be counted in S2 and S3 both and note that T(:,1) is not used. I don't know whether these are intentional.
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