n=359;

a=[];

b=[];

c=[];

% for loop

for i=1:n  

    filename=sprintf('output_%d.dat',i);

    fileinfo = importdata(filename);

    ux=fileinfo.data(:,7);   % extract column data

    uy=fileinfo.data(:,8);

    uz=fileinfo.data(:,9);

    a=[a,ux];   % put all Ux in an array

    b=[b,uy];

    c=[c,uz];

end

aver_Ux=sum(a,2)/n;  % sum of array elements

aver_Uy=sum(b,2)/n;

aver_Uz=sum(c,2)/n;

*********

- [[https://www.mathworks.com/help/matlab/ref/sum.html?searchHighlight=sum&s_tid=doc_srchtitle][sum]]