A shorter way to express this?

[Geoff Canyon Rev]

As David suggested, you should definitely use for each line... As your data
grows it will operate in linear time, while using line x of... will be
something like quadratic time I think.

If fully numerical indexes for your array are acceptable, this would work

   put 1 into i
   repeat for each line L in fld 2
      split L using tab
      put L into x[i]
      add 1 to i
   end repeat