Symbolic substitution for variable subscripts

How should I perform the following substitution: ? Can the subs function be used? Or do I have to take a manual approach and use sprintf into a char representation of the expression before converting it to symbolic form?

[Walter Roberson]

Supposing your a is to be 17 x 3, then

a = sym('a', [17 3]);

and then for any given numeric subscript j, sum(a(j,:))

However it is not possible to create the sum when j is symbolic