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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.

share|improve this question
Wow, that's weird! – Eli Lansey Apr 25 '12 at 20:05
Seems like GridBoxes don't work well with packed arrays. Another packed array bug? – Rojo Apr 25 '12 at 20:07
It's also contagious. If you set foo1=foo the weirdness happens with foo1 as well. – Eli Lansey Apr 25 '12 at 20:08
Please tell me you can reproduce that...? – sebhofer Apr 25 '12 at 20:08
foo===bar yields True... – faleichik Apr 25 '12 at 20:10
up vote 11 down vote accepted

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

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

DiagonalMatrix returns a packed array

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


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
share|improve this answer
Good catch, would never have thought of that... – sebhofer Apr 25 '12 at 20:17
Thanks @sebhofer. I had context of a similar bug that I found in a Mr.Wizard's answer, that ended up in him asking this question. Seems like there are some corner cases when packed arrays are a little buggy, and one can only be safe by unpacking them manually. Hope they get fixed – Rojo Apr 25 '12 at 20:23
I'll report it as a bug when I find the time... – sebhofer Apr 25 '12 at 20:32
Although it's not a bug, you definitely found the culprit :) Thanks again! – sebhofer Apr 26 '12 at 19:23
@sebhofer, why is it not considered a bug? – Rojo Apr 26 '12 at 19:37
foo = ToExpression[DiagonalMatrix[{1, 1}]]
GridBox[foo] // DisplayForm

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

share|improve this answer
Seems like applying ToExpression to packed arrays serves to unpack them – Rojo Apr 25 '12 at 20:15

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