Question: if and for loops (nested)

At the moment I'm trying to find the smallest difference between two things (of form E[i] i goes from 1 to n)  and input them in a list. To do this I need to use nested loops but I'm not sure of the best kind/way to do this. I have been experimenting with for and if loops so far

for instance, this is my most recent effort.
 

for i from 2 to n do;

a := abs(E[i]-E[1]);

for j from 2 to n do;

if i <> j then ;

b := abs(E[i]-E[j]);

end if;

if b < a then a := b end if;

end do;

N := [op(N), a] ;

end do;
 

 (was planning on writing a seperate loop for i=1 case here).
 I am really struggling to find a way to make it work though, particularly as I need to do all cases E[i]-E[j] i,j=1...n i<>j. I think my trouble lies in the original defining of a (and what to set it as/how to reset it for the next i case), and when to put the command N := [op(N), a] ; into the loop

Can anyone help me with this problem?