TikZ-matrix: add brackets around some blocks inside

Question

How can I get brackets (say "normal parenthese math brackets") inside a TikZ-matrix around some blocks?
For example here at the marked places:

Is there a decoration or something?

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}[align=center]
\matrix[matrix of nodes,
row sep=5mm, column sep=3mm,
]{
a11 & a12 & a13 & a14 & a15 \
a21 & a22 & a23 & a24 & a25 \
a31 & a32 & a33 & a34 & a35 \
a41 & a42 & a43 & a44 & a45 \
a51 & a52 & a53 & a54 & a55 \
a61 & a62 & a63 & a64 & a65 \
a71 & a72 & a73 & a74 & a75 \
};
\end{tikzpicture}
\end{document}

Answer 1

For information, here is what you can do with {NiceMatrix} of nicematrix. That package uses PGF (which is a sublayer of Tikz) but does not load Tikz and does not use the library matrix of Tikz.

However, each cell is a PGF node that the user can use with standard Tiks instructions.

There is a command \SubMatrix to add brackets. The brackets are the standard brackets of the mathematical expressions of LaTeX.

\documentclass{article}
\usepackage{nicematrix}
\begin{document}
\renewcommand{\arraystretch}{1.5}
$\begin{NiceMatrix}
a11 & a12 & a13 & a14 & a15 \
a21 & a22 & a23 & a24 & a25 \
a31 & a32 & a33 & a34 & a35 \
a41 & a42 & a43 & a44 & a45 \
a51 & a52 & a53 & a54 & a55 \
a61 & a62 & a63 & a64 & a65 \
a71 & a72 & a73 & a74 & a75 \
\CodeAfter [delimiters/color=blue]
  \SubMatrix({1-4}{2-5})
  \SubMatrix({5-1}{7-3})
\end{NiceMatrix}$
\end{document}

You need several compilations because of the PGF nodes.

Answer 2

Parenthesis you can dear :-)

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}
\matrix (m) [matrix of nodes, % <-- added matrix name
             row sep=3mm, 
             column sep=3mm,
            ]
{
a11 & a12 & a13 & a14 & a15 \
a21 & a22 & a23 & a24 & a25 \
a31 & a32 & a33 & a34 & a35 \
a41 & a42 & a43 & a44 & a45 \
a51 & a52 & a53 & a54 & a55 \
a61 & a62 & a63 & a64 & a65 \
a71 & a72 & a73 & a74 & a75 \
};
\draw[thick, blue, rounded corners=2mm]
    ([xshift=+2mm] m-1-4.north west) -| (m-2-4.south west) -- ++ (+2mm,0)
    ([xshift=-2mm] m-1-5.north east) -| (m-2-5.south east) -- ++ (-2mm,0)
%
    ([xshift=+2mm] m-5-1.north west) -| (m-7-1.south west) -- ++ (+2mm,0)
    ([xshift=-2mm] m-5-3.north east) -| (m-7-3.south east) -- ++ (-2mm,0);
\end{tikzpicture}
\end{document}

Answer 3

To get original amsmath-brackets, I worked with some tricks due to length-measurement of the calc-library:

\documentclass[border=5mm, varwidth]{standalone}
\usepackage{amsmath}
\usepackage{tikz}
\usetikzlibrary{matrix}
\usetikzlibrary{calc}
\pgfkeys{/tikz/savevalue/.code 2 args={\global\edef#1{#2}}}
%\pgfmathsetlengthmacro{\InnerSep}{1pt} % optional 1/2
\begin{document}
\section{amsmath-Brackets inside a TikZ-matrix}
\begin{tikzpicture}[]
\matrixmatrix of math nodes, nodes in empty cells,
row sep=5mm, column sep=3mm,
nodes={draw=none, % Test
%inner sep=\InnerSep % optional 2/2
},
% Maybe useful:
%every odd row/.style={anchor=base west},% <--- don't know, how
%every even row/.style={anchor=base west},% to make this shorter
{
a11 a2& a12 & a13 & a14 & a15 \
a21 & a22 & a23 & a24 & a25 \
a31 & a32 & a33 & a34 & a35 \
a41 & a42 & a43 & a44 & a45 \
a51 & a52 & a53 & a54 & a55 \
a61 & a62 & a63 & a64 & a65 \
a71 & a72 & a73 & a74 & a75 \
};
% Brackets around A
\coordinateAul at (m-5-1.north west);
\coordinateAll at (m-7-1.south west);
\coordinateAur at (m-5-3.north east);
\coordinateAlr at (m-7-3.south east);
\path let \p1=($(Aul)-(All)$) in 
\pgfextra{  \pgfmathsetlengthmacro{\templength}{veclen(\x1,\y1)}   }
[savevalue={\u}{\templength}];
\pgfmathsetlengthmacro{\uHalf}{0.5*\u}
\node[] at ($(Aul)!0.5!(All)$) {$
\left( \rule[-\uHalf]{0mm}{\u}  \right.$};
\node[] at ($(Aur)!0.5!(Alr)$) {$
\left. \rule[-\uHalf]{0mm}{\u} \right)$};
% Brackets around B
\coordinateBul at (m-1-4.north west);
\coordinateBll at (m-2-4.south west);
\coordinateBur at (m-1-5.north east);
\coordinateBlr at (m-2-5.south east);
\path let \p1=($(Bul)-(Bll)$) in 
\pgfextra{  \pgfmathsetlengthmacro{\templength}{veclen(\x1,\y1)}   }
[savevalue={\u}{\templength}];
\pgfmathsetlengthmacro{\uHalf}{0.5\u1}
\node[blue] at ($(Bul)!0.5!(Bll)$) {$
\left( \rule[-\uHalf]{0mm}{\u}  \right.$};
\node[blue] at ($(Bur)!0.5!(Blr)$) {$
\left. \rule[-\uHalf]{0mm}{\u} \right)$};
\end{tikzpicture}
\section{For Comparison: Brackets at amsmath-Matrix}
$\begin{pmatrix}
a51 & a52 & a53 \[5mm]
a61 & a62 & a63  \[5mm]
a71 & a72 & a73
\end{pmatrix}$
\end{document}

Answer 4

The matrix library of TikZ already comes with a tool to put delimiters around a \matrix. We can re-use that to put delimiteres around any nodes.

---

My first approach was to define a new macro

\newcommand*\delimAroundTikZCells[5][]{%
  \path[every delim around tikz cells/.try,#1,local bounding box=datc]
    (#2.north west)(#3.south east) [freeze local bounding box=datc]
    (datc) [late options={left delimiter=#4, right delimiter=#5}];}

that can be used as such (when the matrix is named a):

\delimAroundTikZCells{a-1-4}{a-2-5}()
\delimAroundTikZCells{a-5-1}{a-7-3}()

This commands puts a rectangular node around the range provided by #2 and #3 (this is similar to the fit key from the fit library) and then adds to delimiters #4 (left) and #5 (right) around this new pseudo-node.

---

Though, we can do this also as an option to the matrix it self with my second solution, the keys are

• lr delim = <left><left options> and <right><right options> around <top left> to <bottom left> and similarly
• ab delim.

The options are optionally. This also works without explicitly naming the matrix.

There are a lot of { and } involved because the \tikzlastnode macro gets overwritten by the way the late options work.

---

Since the nodes have some inner xseps (default .3333em) I've added a delim xshift and delim yshift key that can be used to correct the placement of delimiters.

Due to how these delimiters are set these options (as all styles like every [left|right|above|below] delimiter) need to be defined at an encompassing group, that could be the tikzpicture it self, a scope, \scoped (though this needs ampersand replacemnt) or simply

\path[delim xshift = .5em, lr delims = green] node[
  matrix of nodes,
  lr delim = ( and ) around 2-2 to 4-4] {
  <matrix content>
};

I've added a key delim options as a way to set options that apply to all delimiters that come after it and are added with the keys lr delim and ab delim.

---

Internally, the delimiters are real math delimiters done via

\left( <a box with the same height as the node> \right.

I've also added a more tabular/array like version of the TikZ \matrix for comparison.

Code

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{matrix}
\tikzset{
  delim options/.style={append after command={[#1]}},
  lr delims/.style={
    every delim around tikz cells/.append style={
      every left delimiter/.append style={#1},
      every right delimiter/.append style={#1}}},
  ab delims/.style={
    every delim around tikz cells/.append style={
      every above delimiter/.append style={#1},
      every below delimiter/.append style={#1}}},
  delim xshift/.style={
    every delim around tikz cells/.append style={
      every left delimiter/.append style={xshift={#1}},
      every right delimiter/.append style={xshift={-(#1)}}}},
  delim yshift/.style={
    every delim around tikz cells/.append style={
      every above delimiter/.append style={yshift={-(#1)}},
      every below delimiter/.append style={yshift={#1}}}},
  lr delim/.style args={#1#2 and #3#4 around #5 to #6}{% Left Right
    append after command={
      {{[local bounding box=@] (\tikzlastnode-#5.north west)(\tikzlastnode-#6.south east)}
      (@)[every delim around tikz cells/.try,
          every left delimiter/.append style={#2},
          every right delimiter/.append style={#4},
          late options={left delimiter={#1},right delimiter={#3}}]}[]}},
  ab delim/.style args={#1#2 and #3#4 around #5 to #6}{% Above Below
    append after command={
      {{[local bounding box=@] (\tikzlastnode-#5.north west)(\tikzlastnode-#6.south east)}
      (@)[every delim around tikz cells/.try,
          every above delimiter/.append style={#2},
          every below delimiter/.append style={#4},
          late options={above delimiter={#1},below delimiter={#3}}]}[]}},
}
\begin{document}
\begin{tikzpicture}[
  delim yshift=.8ex,
  delim xshift=.5em,
  lr delims=blue]
\matrix[
  matrix of nodes,
  row sep=5mm,
  column sep=3mm,
  lr delim=( and ) around 1-4 to 2-5,
  lr delim=( and ) around 5-1 to 7-3,
  ab delim={[ red and ] green around 1-2 to 4-3},
] (a) { a11 & a12 & a13 & a14 & a15 \ a21 & a22 & a23 & a24 & a25 \
        a31 & a32 & a33 & a34 & a35 \ a41 & a42 & a43 & a44 & a45 \
        a51 & a52 & a53 & a54 & a55 \ a61 & a62 & a63 & a64 & a65 \
        a71 & a72 & a73 & a74 & a75 \ };
\end{tikzpicture}
\begin{tikzpicture}
\matrix[
  matrix of nodes,
  row sep=0mm, column sep=2\tabcolsep,
  inner sep=0pt, execute at begin node=\strut,
  delim options={delim xshift=.1em, lr delims=blue},
  lr delim=( and ) around 1-4 to 2-5,
  lr delim=( and ) around 5-1 to 7-3] (a) {
  a11 & a12 & a13 & a14 & a15 \ a21 & a22 & a23 & a24 & a25 \
  a31 & a32 & a33 & a34 & a35 \ a41 & a42 & a43 & a44 & a45 \
  a51 & a52 & a53 & a54 & a55 \ a61 & a62 & a63 & a64 & a65 \
  a71 & a72 & a73 & a74 & a75 \ };
\end{tikzpicture}
\end{document}

Output