"Is there a clever and convenient way to accomplish this without using, for instance, fieldnames in combination with a for loop?"
Not really, because fields are not designed to represent a particular sequence - their order can be changed, without changing their names. A sequence is best represented by an index: its order is fixed, simply by definition of the index itself. This means the two are not equivalent, and there is no general solution to convert between fieldnames and indices.
If you want to access something with indices, then why are you using fieldnames? Probably you should instead be using a non-scalar structure with efficient indexing, and avoid using slow numbered fieldnames altogether.
