finding numbers below a threshold and doing calculations on them

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 

Answer 1

[Voss]

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.

[C.G.]

I hadn't realised that 0.115 would be used in both. Thank you for identifying that.

I've used the wrong columns in the ResV calculation so should be T1, T2, T3, but I corrected that since I posted this.

I now want to calculate the average granular temperature (velocity of each particle - mean velocity of whole segment) for each segiment. I've tried to adapt your code to do this but I cant get it to work

for a = 1:num_files
    T = table2array(readtable(filelist(a).name)); %read in the values
    S1 = T(:,4) >0.13; %identify all the rows in column 4 over 0.13
    p1(a) = sum(S1); %number of particles in seg1
    gd1 = diff(p1); 
    e1 = abs(gd1-1); %efflux out of seg1
    V1(a) = mean(sqrt(sum(T(S1,1:3).^2,2))); %mean resultant velocity in seg1
    GT1(a)= mean(sqrt(sum(T(S1,1:3).^2,2))-V1); %mean GT in seg1
end

Unable to perform assignment because the indices on the left side are not compatible with the size of the right side.
 

[Voss]

Try changing V1 to V1(a) in the line where V1 is used to calculate GT1(a).

[C.G.]

Thank you for your help!