Argthtjtr

Please consider the following mwe:

documentclass[tikz, margin=3mm]{standalone}

usetikzlibrary{matrix}



begin{document}

begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   column sep=-pgflinewidth,

                   row sep=-pgflinewidth,

                   every even column/.style={column sep=2pt},

                   every even row/.style={row sep=2pt},% doesn't insert row separation

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}

end{document}

However every even column/.style={column sep=2pt}, yields the expected result, say, introducing additional column separation after each second column, the every even row/.style={row sep=2pt}, doesn't. Did I do something wrong or this is a bug in TikZ v3.1 (I've never tested this before, so I don't know if the same thing happens in v3.0.1)?

edit:

The same thing happens with, for example,

row 2/.style = {row sep=2pt} 

thus it might mean that every even row/.style=... and row <row number> work fine only if one wants to change some properties of cells inside row, for example, the color of nodes borders:

every even row/.style={draw red},

but not if one aims to change row separation (see @AndréC's comment below). However it is interesting, that this is possible to do in every column sep ...

Of course, the (temporary) workaround terminates each second row of the matrix with for example [2pt], but this is annoying when the matrix is huge like here.

The issue is with scopes. In order to make sure that the style code for the current row only applies to the current row, the style code is executed inside of a scope that is local to the current row. This scope is closed prior to ending the row and adding the row separation, so the value is reverted before it is applied. The solution is to make the change to the row sep global. The key row sep stores the value in pgfmatrixrowsep so we globally define this to be the updated value. Because this change is global, it applies to all rows going forward, so we need an every odd row key setting the row sep to zero to switch it back.

documentclass[tikz, margin=3mm]{standalone}

usetikzlibrary{matrix}



begin{document}

begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   column sep=-pgflinewidth,

                   every even column/.style={column sep=2pt},

                   every even row/.style={/utils/exec=gdefpgfmatrixrowsep{2pt}},

                   every odd row/.style={/utils/exec=gdefpgfmatrixrowsep{-pgflinewidth}},

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}

end{document} 

This is more an extended comment than an answer. The comment concerns Hood Chathams proposal to make the dimensions global. Here is what happens:

documentclass[tikz, margin=3mm]{standalone}

usetikzlibrary{matrix}



begin{document}

begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}



begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   column sep=-pgflinewidth,

                   every even column/.style={column sep=2pt},

                   every even row/.style={/utils/exec=gdefpgfmatrixrowsep{2pt}},

                   every odd row/.style={/utils/exec=gdefpgfmatrixrowsep{-pgflinewidth}},

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}



begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}



end{document} 

Let me, however, stress that I really like the answer by Hood Chatham as a whole as it has a very nice explanation for why this happens. I just want add a word of caution.

Here is another proposal.

documentclass[tikz, margin=3mm]{standalone}

usetikzlibrary{matrix}

begin{document}

begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   column sep=-pgflinewidth,

                   row sep=ifoddpgfmatrixcurrentrow%

                   -pgflinewidth%

                   else%

                   2pt%

                   fi,

                   every even column/.style={column sep=2pt},

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}

end{document}

It does produce the desired output. However, it also produces warnings of the type end occurred when ifx on line 21 was incomplete. So I am definitely not claiming this proposal is better than Hood Chathams suggestion.

Of course, a very pragmatic solution would be to use

documentclass[tikz, margin=3mm]{standalone}

usetikzlibrary{matrix}



begin{document}

begin{tikzpicture}

matrix (m) [matrix of math nodes,

            nodes={draw, font=footnotesize, minimum size=1em,

                   anchor=center,inner sep=0pt},

                   column sep=-pgflinewidth,

                   row sep=-pgflinewidth,

                   every even column/.style={column sep=2pt},

                   inner sep=1pt,

                   left delimiter={[},right delimiter={]},

             ]

{

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \[2pt]

alpha  & beta   & gamma  & delta    \

alpha  & beta   & gamma  & delta    \

};

end{tikzpicture}

end{document}