How can I color/highlight the diagonal of a matrix

Question

I have a table such as the one below

\begin{tiny}\begin{tabular}{|l|c|c|c|}
\hline
&\textbf{ciao}&\textbf{ciao2}&\textbf{ciao3}\\\hline
\textbf{ciao}&0.815&0.913&0.278\\\hline
\textbf{ciao2}&0.906&0.632&0.547\\\hline
\textbf{ciao3}&0.127&0.098&0.958\\\hline
\end{tabular}
\end{tiny}

How can I highlight the diagonal, the first row and the first column with different colours?

Answer 1

Easy with colortbl (loaded via xcolor):

\documentclass{article}
\usepackage[table]{xcolor}

\begin{document}

\begin{tabular}{|>{\columncolor{red!20}}l|c|c|c|}
\hline
\rowcolor{blue!20}%
&\textbf{ciao}&\textbf{ciao2}&\textbf{ciao3}\\\hline
\textbf{ciao}&\cellcolor{green}0.815&0.913&0.278\\\hline
\textbf{ciao2}&0.906&\cellcolor{green}0.632&0.547\\\hline
\textbf{ciao3}&0.127&0.098&\cellcolor{green}0.958\\\hline
\end{tabular}

\end{document}

For getting automatically what you want you just need to use the following code; it's necessary to change the name of the environment, though.

\documentclass{article}
\usepackage[table]{xcolor}
\usepackage{xparse,environ}

\ExplSyntaxOn
\NewEnviron{matlabtabular}[1]
 {
  \tl_set:Nx \l_angelino_matlab_preamble_tl 
   {
    \tl_tail:n { #1 }
   }
  \tl_put_left:Nn \l_angelino_matlab_preamble_tl { | >{\columncolor{red!20}} }
  \angelino_matlabtab:VV \l_angelino_matlab_preamble_tl \BODY
 }

\cs_new_protected:Nn \angelino_matlabtab:nn
 {
  \seq_set_split:Nnn \l_angelino_matlabtab_rows_seq { \hline } { #2 }
  % the first item is empty
  \seq_pop_left:NN \l_angelino_matlabtab_rows_seq \l_angelino_matlabtab_body_tl
  % the last item is empty
  \seq_pop_right:NN \l_angelino_matlabtab_rows_seq \l_tmpa_tl
  % separate the first row
  \seq_pop_left:NN \l_angelino_matlabtab_rows_seq \l_angelino_matlabtab_body_tl
  % fix the first row
  \tl_put_left:Nn \l_angelino_matlabtab_body_tl { \hline\rowcolor{blue!20} }
  \tl_put_right:Nn \l_angelino_matlabtab_body_tl { \hline }
  \seq_map_inline:Nn \l_angelino_matlabtab_rows_seq
   {
    \int_incr:N \l_angelino_matlab_currentrow_int
    \int_zero:N \l_angelino_matlab_currentcol_int
    \seq_set_split:Nnn \l_angelino_matlabtab_row_seq { & } { ##1 }
    \seq_pop_left:NN \l_angelino_matlabtab_row_seq \l_angelino_matlabtab_first_tl
    \tl_put_right:NV \l_angelino_matlabtab_body_tl \l_angelino_matlabtab_first_tl
    \seq_map_inline:Nn \l_angelino_matlabtab_row_seq
     {
      \int_incr:N \l_angelino_matlab_currentcol_int
      \int_compare:nTF { \l_angelino_matlab_currentcol_int == \l_angelino_matlab_currentrow_int }
       {
        \tl_put_right:Nn \l_angelino_matlabtab_body_tl { & \cellcolor{green} ####1 }
       }
       {
        \tl_put_right:Nn \l_angelino_matlabtab_body_tl { & ####1 }
       }
     }
    \tl_put_right:Nn \l_angelino_matlabtab_body_tl { \hline }
   }
  \begin{tabular}{#1}
  \l_angelino_matlabtab_body_tl
  \end{tabular}
 }
\cs_generate_variant:Nn \angelino_matlabtab:nn { VV }
\int_new:N \l_angelino_matlab_currentcol_int
\int_new:N \l_angelino_matlab_currentrow_int
\seq_new:N \l_angelino_matlabtab_rows_seq
\seq_new:N \l_angelino_matlabtab_row_seq
\tl_new:N \l_angelino_matlabtab_body_tl
\tl_new:N \l_angelino_matlabtab_first_tl
\tl_new:N \l_angelino_matlab_preamble_tl
\ExplSyntaxOff

\begin{document}

\begin{matlabtabular}{|l|c|c|c|}
\hline
&\textbf{ciao}&\textbf{ciao2}&\textbf{ciao3}\\\hline
\textbf{ciao}&0.815&0.913&0.278\\\hline
\textbf{ciao2}&0.906&0.632&0.547\\\hline
\textbf{ciao3}&0.127&0.098&0.958\\\hline
\end{matlabtabular}

\end{document}

The output is the same as before.

Note that this makes some assumptions on the environment's contents. The argument should begin with | that's changed into |>{\columncolor{red!20}}. It is also assumed that every row ends with \\\hline.

Answer 2

For comparison, here is how to do this in ConTeXt. I think that ConTeXt provides a much nicer separation of content and presentation for tables.

\definecolor[lightblue]  [b=1,r=0.8,g=0.8]
\definecolor[lightred]   [r=1,b=0.8,g=0.8]
\definecolor[lightgreen] [g=1,r=0.8,b=0.8]

\startsetups diagonal
  \setupTABLE[offset=0.5em]
  \dorecurse{20}% Assuming there are no more than 20 rows
    {\setupTABLE[\recurselevel][\recurselevel][background=color,backgroundcolor=lightgreen]}
  \setupTABLE[column][first][background=color, backgroundcolor=lightred, style=bold]
  \setupTABLE[row][first][background=color, backgroundcolor=lightblue, style=bold]
\stopsetups

\starttext

\startTABLE[setups=diagonal]
  \NC       \NC ciao  \NC ciao2 \NC ciao3 \NC \NR
  \NC ciao  \NC 0.815 \NC 0.913 \NC 0.287 \NC \NR
  \NC ciao2 \NC 0.906 \NC 0.632 \NC 0.547 \NC \NR
  \NC ciao3 \NC 0.127 \NC 0.098 \NC 0.958 \NC \NR
\stopTABLE

\stoptext

Answer 3

Here is a way to do that in an automatic way by using the environment {NiceTabular} of nicematrix.

Moreover, you will have a perfect output: the rules won't seem to disappear in some PDF viewers at somme levels of zoom.

\documentclass{article}
\usepackage{xcolor}
\usepackage{nicematrix}
\usepackage{pgffor}
\NewDocumentEnvironment { matlabtabular } { O { } m ! O { } }
  { 
    \begin{NiceTabular}[#1, #3, hvlines] { #2 }
      \CodeBefore

         \columncolor{red!15}{1}
         \rowcolor{blue!15}{1}
         \foreach \x in { 2,...,\arabic{iRow} } { \cellcolor{green!15}{\x-\x} } 
      \Body
  }
  { \end{NiceTabular} }
\begin{document}
\begin{matlabtabular}{lccc}
&\textbf{ciao}&\textbf{ciao2}&\textbf{ciao3}\
\textbf{ciao}&0.815&0.913&0.278\
\textbf{ciao2}&0.906&0.632&0.547\
\textbf{ciao3}&0.127&0.098&0.958\
\end{matlabtabular}
\end{document}

You need several compilations because nicematrix uses PGF/Tikz nodes under the hood.