In Grid and using Frame, how can I only frame the header cells?

Question

Suppose that I use Grid to display a table tab. I did the following, which frames all cells:

tab = {{"a", "b"}, {"1", SpanFromLeft}, {"2", SpanFromLeft}, {"3", SpanFromLeft}};
Grid[tab, Frame -> All]

and gives this output:

But, how do I programatically generate a table in which only the header cells are framed?

(I created the above by erasing cell dividers manually, using Paint, but for larger tables, I would like to do this programatically.) Thanks.

Answer 1

You can explicitly frame only those elements and not frame the rest. For example:

Grid[tab, Frame -> {None, None, {{1, 1} -> True, {1, 2} -> True}}]

To extend this to grids with $n$ columns in the header, you could replace the last element of the RHS of the Frame option with

Table[{1, i} -> True, {i, n}]

where n is an integer.

Answer 2

A slight variation:

Grid[MapAt[Item[#, Frame -> True] &, tab, {{1, 1}, {1, 2}}]]

Answer 3

A slight variation of a slight variation:

frRow = Item[#, Frame -> 1] & /@ # &;

MapAt[frRow, tab, 1] // Grid