Iteration anomalu

Surely this is a FAQ but I can't figure out how I would search for an
explanation for this behavior. In short, this code

  a = [[1,2,3], [4,5,6], [7,8,9]]

  for x in a do
    print x, "\n"
  end

  for i in [0..a.length] do
    print a[i], "\n"
  end

yields this result:


123
456
789
123456789

Why does the second iteration evaluate all the prints before printing
the newline?

I have some puzzling variations on this too, but I am sure that the
answer to this will clear them up. Thanks!

FWIW, the reason I even consider the second iteration style, old-school
that it is, is because I want to do a double-nested iteration through a,
in this way:

  for i in [0..a.length] do
    for j in [i+1..a.length] do
      # etc... processing lower-left-hand triangle of possibilities...

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