GridBox/packed array behaviour

Question

I have never used low-level formating functions, so this problem might have a simple solution. I'm currently pulling my hair out trying to solve it, however. Consider the following example:

foo = DiagonalMatrix[{1, 1}]
(*{{1, 0}, {0, 1}}*)

bar = {{1, 0}, {0, 1}}
(*{{1, 0}, {0, 1}}*)

GridBox[foo] // DisplayForm
(*GridBox[{{1, 0}, {0, 1}}]*)

GridBox[bar] // DisplayForm
(*gives correctly formated output*)

Why does the DisplayForm depend on the way the list was created?

Edit: I contacted WRI Technical support and got the following answer:

GridBox is doing what it should do. Box forms interpret strings and boxes and not Mathematica expressions. Thus there is no reason to expect GridBox to interpret a packed array which is a compiled expression. Grid uses GridBox for the formatting but unpacks first.

I guess the bottomline is, use Grid instead of GridBox if possible (just as I should have done), or unpack arrays in advance.

Answer 1

Seems like GridBox doesn't work well with packed arrays. Or at least not with DisplayForm

r = RandomInteger[10, {4, 4}];
Developer`PackedArrayQ[r]
rnp = Developer`FromPackedArray[r];
DisplayForm[GridBox[r]]
DisplayForm[GridBox[rnp]]

DiagonalMatrix returns a packed array

Developer`PackedArrayQ[DiagonalMatrix[{1, 2, 3, 4, 5}]]

True

So, I'd say it's a bug, but a workaround to ensure it all works fine is to wrap your variable in FromPackedArray before putting it in the GridBox

foo = DiagonalMatrix[{1, 1}];
GridBox[Developer`FromPackedArray@foo] // DisplayForm

Answer 2

foo = ToExpression[DiagonalMatrix[{1, 1}]]
GridBox[foo] // DisplayForm

works in the docs it states In StandardForm, explicit GridBox objects are output literally.