# Square bracketed matrices in Mathematica - precision markers?

I'm trying to display Matrices with square brackets in Mathematica.

I've found this post from a mailing list in 2009 http://forums.wolfram.com/mathgroup/archive/2009/Aug/msg00458.html and it seems to work fine for whole numbers and symbols.

NotebookWrite[InputNotebook[],
TemplateBox[{GridBox[{{a, b}, {c, d}}]}, "Identity",
DisplayFunction -> (RowBox[{StyleBox["[",
SpanMaxSize -> \[Infinity]], #1,
StyleBox["]", SpanMaxSize -> \[Infinity]]}] &)]]

(* Outputs: *)
Identity[{
{a, b},
{c, d}
}]

(* Which displays correctly as a square matrix. *)


But as soon as I try to input floating point numbers, I get floating point precision markers displayed. For example:

NotebookWrite[InputNotebook[],
TemplateBox[{GridBox[{{2.1, 1}, {1, 1.2}}]}, "Identity",
DisplayFunction -> (RowBox[{StyleBox["[",
SpanMaxSize -> \[Infinity]], #1,
StyleBox["]", SpanMaxSize -> \[Infinity]]}] &)]]

(* Outputs: *)
Identity[{
{2.1000000000000001, 1},
{1, 1.2}
}]

(* Which displays correctly as a square matrix. *)


I don't see any function in there that would force the numbers to be evaluated into machine precision form. I think using HoldForm or others could solve this issue but I'm not too sure where that can be placed since GridBox needs a list and RowBox needs a box - evaluating them individually displays decimal numbers just fine.

For clarification, I'm looking for something like this:

(* In the square bracketed matrix display form of course *)
Identity[{
{2.1, 1},
{1, 1.2}
}]


Any help for a beginner? Thank you!

• Obviously I think this has to do with floating point representation inaccuracies in binary forms but I want Mathematica to display those numbers symbolically without evaluating them, if that makes sense. – Michael Yoo Apr 28 '16 at 10:03
• And if I use Defer[] in one of the matrix elements, I get an error message saying An unknown box name (Defer) was sent as the BoxForm for the expression. Check the format rules for the expression. – Michael Yoo Apr 28 '16 at 10:18
• Okay, so using ToBoxes[Defer[2.1]] instead of Defer[2.1] seems to have solved the problem. Now just have to figure out how to eliminate the backtick precision marker... – Michael Yoo Apr 28 '16 at 10:20
• What about makeBrackMat[mat_?MatrixQ] := DisplayForm[RowBox[{"[", GridBox[mat], "]"}]]? – user31159 Apr 28 '16 at 10:21
• Probably better: makeBrackMat[mat_?MatrixQ] := DisplayForm[ RowBox[{StyleBox["[", SpanMaxSize -> \[Infinity]], GridBox[mat], StyleBox["]", SpanMaxSize -> \[Infinity]]}]]. – user31159 Apr 28 '16 at 10:24

makeBrackMat[mat_?MatrixQ] :=
DisplayForm[
RowBox[{StyleBox["[", SpanMaxSize -> \[Infinity]],
GridBox[mat],
StyleBox["]", SpanMaxSize -> \[Infinity]]}
]
];


Exact numbers:

mat1 = Partition[Range[12], 3];
makeBrackMat[mat1]


Machine-precision numbers:

mat2 = {{1.3, 2.9}, {9.5, 8.4}, {7.6, 0.2}};
makeBrackMat[mat2]