Table with colored rows alternating every n rows

Question

I'd like to create a table where the background color of the rows changes every n>1 rows. I have seen in wikibooks how alternate rows can have different colors and I have looked at one xcolor documentation I found online at math.washginton.edu but I can't figure out how I can have the rowcolor to change every n rows, say every 5 rows.

Also, would it possible to define some sequence, say {4,9,11..} which will tell LaTeX at which row # to switch colors? That way if my observations are unbalanced then I can still highlight the backgrounds differently. What I mean by this is that if I have 3 rows of data from model-1 and 5 rows of data from Model-2 and 2 rows of data from Model-3 then the first 3 rows of my table can be white, the next 5 can be gray and the next 2 rows can be white again and so on..for improved readability, which I need because I have a ton of numbers, which I guess most tables have. :)

Right now I am laboriously writing \rowcolor{} in front of any row that I'd like colored.

Any help will be greatly appreciated.

Answer 1

This shows one way of adding a counter that increments every row and then having an arbitrary set of tests to switch colours on some condition. This starts off with no background switches to green on row 4 stays green until row 7 when it switches to blue, goes back to no background at row 13, then cycles round again every 20 rows.

\documentclass{article}

\usepackage{colortbl}
\newcounter{myc}
\makeatletter
\def\myrow{}
\CT@everycr{\noalign{%
\global\let\CT@row@color\relax
\stepcounter{myc}%
\ifnum\value{myc}=4
  \gdef\myrow{\rowcolor{green}}
\else\ifnum\value{myc}=7
  \gdef\myrow{\rowcolor{blue}}
\else\ifnum\value{myc}=13
  \gdef\myrow{}
\else\ifnum\value{myc}=20
  \setcounter{myc}{0}
\fi\fi\fi\fi
}\myrow}



\begin{document}
\small

\begin{tabular}{>{[\themyc]}ll}
a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\
a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\
a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\
a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\
a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\a&a\\
\end{tabular}
\end{document}

Answer 2

If you load the xcolor package with the table option you also have access to the rowcolors command.

the syntax is \rowcolors{<starting row index>}{<odd row color>}{<even row color>}

Edit:

if you use the tabu package you can use the \taburowcolors command.

\documentclass[preview]{standalone}
\usepackage{tabu}
\usepackage[table]{xcolor}
\begin{document}

\everyrow{\tabucline-}
\begin{tabu}{|X|X|}
\taburowcolors {green!40..green!40}
test&test\\
test&test\\
test&test\\
test&test\\
\taburowcolors {blue!40..blue!40}
test&test\\
test&test\\
test&test\\
\taburowcolors{white..white}
test&test\\
test&test\\
\end{tabu}

\end{document}

which gives:

Answer 3

With {NiceTabular} of nicematrix, here what you can do if you want n rows of a color followed by n rows of another color.

\documentclass{article}
\usepackage{nicematrix}

\begin{document} 

\begin{NiceTabular}{cc}
\CodeBefore
  \rowlistcolors{1}{red!15,=,=,=, blue!15,=,=,=}
\Body
  1 & one \\
  2 & two \\
  3 & three \\
  4 & four \\
  5 & five \\
  6 & six \\
  7 & seven \\
  8 & eight \\
  9 & nine \\
  10 & ten \\
  11 & eleven \\
  12 & twelve \\
  13 & thirteen \\
  14 & fourteen \\
  15 & fifteen \\
  16 & sixteen \\
\end{NiceTabular}

\end{document}

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