TeX \newcommand and D.R.Y. (don't repeat yourself)

Question

My goal is to create a list within a varwidth, and alternate the colors of each item. At the time, the best I could do is this:

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage[
  a4paper,
  margin=15mm,
  bindingoffset=2mm,
  heightrounded,
]{geometry}

\usepackage{amsmath}
\usepackage{microtype}
\usepackage{xcolor}
\usepackage{varwidth}
\usepackage{enumitem}
\usepackage{hyperref}
\usepackage{enumitem}

\newif\ifodd
\oddtrue
\newcommand\odd{\oddtrue\item}
\newcommand\pair{\oddfalse\item}

\definecolor{lightgray}{HTML}{EFEFEF}

\begin{document}


\begingroup\setlength{\fboxsep}{6pt}
\colorbox{white}{%
  \begin{varwidth}[t]{\textwidth}
  TRIGONOMETRIA
  \begin{itemize}[leftmargin=*,label={\ifodd\color{black}\else\color{blue}\fi\textbullet}]

  \odd $\sin^{2} \alpha + \cos^{2} \alpha = 1$

  \pair \textcolor{blue}{$\tan \alpha = \dfrac{\sin \alpha}{\cos \alpha}$}

  \odd $\sin \alpha + \beta = \sin \alpha \cos \beta + \sin \beta \cos \alpha$

  \pair \textcolor{blue}{$\cos \alpha + \beta = \cos \alpha \cos \beta - \sin \alpha \sin \beta$}

  \odd $\tan \alpha + \beta = \dfrac{\tan \alpha + \tan \beta}{1 - \tan \alpha \tan \beta}$

  \pair \textcolor{blue}{$\sin \alpha - \beta = \sin \alpha \cos \beta - \sin \beta \cos \alpha$}

  \odd $\cos \alpha - \beta = \cos \alpha \cos \beta + \sin \alpha \sin \beta$

  \pair \textcolor{blue}{$\tan \alpha - \beta = \dfrac{\tan \alpha - \tan \beta}{1 + \tan \alpha \tan \beta}$}

  \odd $\sec \alpha = \dfrac{1}{\cos \alpha}$

  \pair \textcolor{blue}{$\dfrac{\sin^{2} \alpha}{\cos^{2} \alpha} + 1 = \tan^{2} \alpha + 1 = \sec^{2} \alpha$}

  \odd $\sin(\arctan z) = \cfrac{z}{\sqrt{z^{2} + 1}}$

  \pair \textcolor{blue}{$\cos(\arctan z) = \cfrac{1}{\sqrt{z^{2} + 1}}$}

  \odd $\cos 2\alpha =\begin{cases}
               \cos^{2} \alpha - \sin^{2} \alpha\\
               2 \cos^{2} \alpha - 1\\
               1 - \sin^{2} \alpha
            \end{cases}
   $

    \odd $\cos 2\alpha =\begin{cases}
               \cos^{2} \alpha - \sin^{2} \alpha\\
               2 \cos^{2} \alpha - 1\\
               1 - \sin^{2} \alpha
            \end{cases}
   $

  \end{itemize}
  \end{varwidth}%
}\endgroup

\end{document}

I would know if there is a way to define a command so I should not re-write each time

 \pair \textcolor{blue}{$...$}

Thanks in advance

score 11 · Answer 1 · edited Jun 10 '20 at 12:32

You could simply use the builtin \ifodd and check the value of the item. A better solution is to define your own list type with

\newlist{altcolorslist}

and simply use

\begin{altcolorslist}
    \item ...
\begin{altcolorslist}

similar to Andrew Swann's answer.

References:

ifnum x is even DO THIS \else DO THIS \fi
To redefine a macro that has optional parameters, you have to use \LetLtxMacro from the letltxmacro package . A detailed description of \LetLtxMacro can be found at this question at When to use \LetLtxMacro?.

Code:

\documentclass{article}
\usepackage{amsmath}
\usepackage{xcolor}
\usepackage{varwidth}
\usepackage{enumitem}
\newcommand*{\Item}{\item\ifodd\value{enumi}\color{black}\else\color{blue}\fi}
\begin{document}
\begin{varwidth}[t]{\textwidth}
  TRIGONOMETRIA
  \begin{enumerate}[leftmargin=*,label={\textcolor{black}{\textbullet}}]
\Item  $\sin^{2} \alpha + \cos^{2} \alpha = 1$
\Item {$\tan \alpha = \dfrac{\sin \alpha}{\cos \alpha}$}
\Item $\sin \alpha + \beta = \sin \alpha \cos \beta + \sin \beta \cos \alpha$
\Item {$\cos \alpha + \beta = \cos \alpha \cos \beta - \sin \alpha \sin \beta$}
\Item $\tan \alpha + \beta = \dfrac{\tan \alpha + \tan \beta}{1 - \tan \alpha \tan \beta}$
\Item {$\sin \alpha - \beta = \sin \alpha \cos \beta - \sin \beta \cos \alpha$}
\Item $\cos \alpha - \beta = \cos \alpha \cos \beta + \sin \alpha \sin \beta$
\Item {$\tan \alpha - \beta = \dfrac{\tan \alpha - \tan \beta}{1 + \tan \alpha \tan \beta}$}
\Item $\sec \alpha = \dfrac{1}{\cos \alpha}$
\Item {$\dfrac{\sin^{2} \alpha}{\cos^{2} \alpha} + 1 = \tan^{2} \alpha + 1 = \sec^{2} \alpha$}
\Item $\sin(\arctan z) = \cfrac{z}{\sqrt{z^{2} + 1}}$
\Item {$\cos(\arctan z) = \cfrac{1}{\sqrt{z^{2} + 1}}$}
\Item $\cos 2\alpha =\begin{cases}
               \cos^{2} \alpha - \sin^{2} \alpha\
               2 \cos^{2} \alpha - 1\
               1 - \sin^{2} \alpha
            \end{cases}
   $
\Item $\cos 2\alpha =\begin{cases}
           \cos^{2} \alpha - \sin^{2} \alpha\\
           2 \cos^{2} \alpha - 1\\
           1 - \sin^{2} \alpha
        \end{cases}

$
\end{enumerate}
\end{varwidth}%
\end{document}

Code: Custom List Version

\documentclass{article}
\usepackage{letltxmacro}
\usepackage{amsmath}
\usepackage{xcolor}
\usepackage{varwidth}
\usepackage{enumitem}
\LetLtxMacro{\OldItem}{\item}
\newcommand{\MyItem}{\OldItem\ifodd\value{altcolorslisti}\color{black}\else\color{blue}\fi}
\newlist{altcolorslist}{enumerate}{1}
\setlist[altcolorslist]{
    leftmargin=,label={\textcolor{black}{\textbullet}},
    before={\let\item\MyItem},
    after={\let\item\OldItem},
}
\begin{document}
\begin{varwidth}[t]{\textwidth}
  TRIGONOMETRIA
  \begin{altcolorslist}
\item $\sin^{2} \alpha + \cos^{2} \alpha = 1$
\item $\tan \alpha = \dfrac{\sin \alpha}{\cos \alpha}$
\item $\sin \alpha + \beta = \sin \alpha \cos \beta + \sin \beta \cos \alpha$
\item $\cos \alpha + \beta = \cos \alpha \cos \beta - \sin \alpha \sin \beta$
\end{altcolorslist}
\end{varwidth}%
\end{document}

So I can not avoid writing \textcolor{blue}{my formula} to print in blue my formula? — Salvo Matteini, Apr 07 '17 at 08:34
@SalvoMatteini: Yes, you can now do that. Sorry that I did not notice the \textcolor{blue}.. — Peter Grill, Apr 07 '17 at 08:56

score 11 · Accepted Answer · answered Apr 07 '17 at 09:12

Here is a version where the item command retains its name and its optional argument.

\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage{amsmath}
\usepackage{microtype}
\usepackage{xcolor}
\usepackage{varwidth}
\usepackage{etoolbox}
\usepackage{enumitem}
\usepackage{hyperref}

\definecolor{lightgray}{HTML}{EFEFEF}

\newtoggle{odditem}
\def\myitem{\iftoggle{odditem}%
{\color{black}\togglefalse{odditem}}%
{\color{blue}\toggletrue{odditem}}%
\olditem}

\newenvironment{myitemize}[1]{%
  \begin{varwidth}[t]{\textwidth}
  #1
  \begin{itemize}[leftmargin=*]%
  \toggletrue{odditem}%
  \let\olditem\item%
  \let\item\myitem}{\end{itemize}\end{varwidth}}

\begin{document}

\begin{myitemize}{TRIGONOMETRIA}
  \item $\sin^{2} \alpha + \cos^{2} \alpha = 1$
  \item $\tan \alpha = \dfrac{\sin \alpha}{\cos \alpha}$
  \item $\sin \alpha + \beta = \sin \alpha \cos \beta + \sin \beta \cos \alpha$
  \item[$*$] $\cos \alpha + \beta = \cos \alpha \cos \beta - \sin \alpha
    \sin \beta$
  \item $\tan \alpha + \beta = \dfrac{\tan \alpha + \tan \beta}{1 -
    \tan \alpha \tan \beta}$
\end{myitemize}

\end{document}

score 10 · Answer 3 · answered Apr 07 '17 at 10:28

In my opinion, a tabular is handier.

Please, note that \ifodd is a TeX primitive: you're trying (quite successfully) to break LaTeX by doing \newif\ifodd.

_{Linguistic note. The English “pair” means “coppia”; the English term for “pari” is “even”.}

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[italian]{babel}
\usepackage[
  a4paper,
  margin=15mm,
  bindingoffset=2mm,
  heightrounded,
]{geometry}

\usepackage{amsmath}
\usepackage{microtype}
\usepackage[table]{xcolor}
\usepackage{enumitem}
\usepackage{array}
\usepackage{booktabs}
\usepackage{hyperref}

\newenvironment{formulas}
  {\setcounter{formulas}{0}%
   \begin{tabular}{@{}>{\changecolor\textbullet\ }l@{}}}
  {\end{tabular}}
\newcounter{formulas}
\newcommand{\changecolor}{%
  \stepcounter{formulas}%
  \ifodd\value{formulas}\color{black}\else\color{blue!70!green}\fi
}

\begin{document}

\begin{formulas}
\toprule
\multicolumn{1}{@{}l@{}}{TRIGONOMETRIA} \\
\midrule
$\sin^{2} \alpha + \cos^{2} \alpha = 1$
\\ \addlinespace
$\tan \alpha = \dfrac{\sin \alpha}{\cos \alpha}$
\\ \addlinespace
$\sin \alpha + \beta = \sin \alpha \cos \beta + \sin \beta \cos \alpha$
\\ \addlinespace
$\cos \alpha + \beta = \cos \alpha \cos \beta - \sin \alpha \sin \beta$
\\ \addlinespace
$\tan \alpha + \beta = \dfrac{\tan \alpha + \tan \beta}{1 - \tan \alpha \tan \beta}$
\\ \addlinespace
$\sin \alpha - \beta = \sin \alpha \cos \beta - \sin \beta \cos \alpha$
\\ \addlinespace
$\cos \alpha - \beta = \cos \alpha \cos \beta + \sin \alpha \sin \beta$
\\ \addlinespace
$\tan \alpha - \beta = \dfrac{\tan \alpha - \tan \beta}{1 + \tan \alpha \tan \beta}$
\\ \addlinespace
$\sec \alpha = \dfrac{1}{\cos \alpha}$
\\ \addlinespace
$\dfrac{\sin^{2} \alpha}{\cos^{2} \alpha} + 1 = \tan^{2} \alpha + 1 = \sec^{2} \alpha$
\\ \addlinespace
$\sin(\arctan z) = \cfrac{z}{\sqrt{z^{2} + 1}}$
\\ \addlinespace
$\cos(\arctan z) = \cfrac{1}{\sqrt{z^{2} + 1}}$
\\ \addlinespace
$\cos 2\alpha =\begin{cases}
               \cos^{2} \alpha - \sin^{2} \alpha\\
               2 \cos^{2} \alpha - 1\\
               1 - \sin^{2} \alpha
            \end{cases}$
\\ \addlinespace
$\cos 2\alpha =\begin{cases}
               \cos^{2} \alpha - \sin^{2} \alpha\\
               2 \cos^{2} \alpha - 1\\
               1 - \sin^{2} \alpha
            \end{cases}$
\\
\bottomrule
\end{formulas}

\end{document}

TeX \newcommand and D.R.Y. (don't repeat yourself)

3 Answers3

References:

Code:

Code: Custom List Version