Maybe something like this
z = 1:3;
t = 1:4;
NN = 1:2000;
% Evaluate f(z,t,n) for each element of NN,
% storing all results in cell array C
C = arrayfun(@(n)f(z,t,n),NN,'uni',0);
% Inspect the first and last elements of C
disp(C{1}); disp(C{end});
% Concatenate all elements of C along the 3rd dimension
% and then sum that 3d matrix along the 3rd dimension.
% This will work as long as all elements of C are the
% same size; whether that's true depends on the
% definition of f.
result = sum(cat(3,C{:}),3)
function out = f(z,t,n)
% Some function where z and t are vectors, n is a scalar.
% In this case, out is a matrix of size numel(z)-by-numel(t),
% but out can be any size as long as its size only depends on
% the sizes of z and t.
out = z(:).*t(:).'.*n;
end
