Looping through array like structure in LaTeX

Question

Suppose I have the following latex file:

\documentclass{article}
\usepackage{amsmath}

\begin{equation}
  a+b
\end{equation}

\begin{equation}
  a^2+b^2
\end{equation}

\begin{equation}
  a^2+b^2=c^2
\end{equation}

\end{document}

I would like to avoid rewriting the equation environment for every new equation. Is it possible to do the following in LaTeX?

• Assign the equations to a variable
• Loop through the equations such that I only explicitly write the equation environment once

If so, how? I saw that TikZ does supply a for-each type command, but it seems like an overkill to import all of TikZ just for that, since I am not generating any diagrams.

Answer 1

Using consecutive equation environments is wrong, more so if you separate them with blank lines. Use the appropriate amsmath environments such as align or gather.

I'm not sure that a syntax such as

\printequations{a+b=c \\ a^2+b^2 \\ a^2+b^2=c^2}

is really better than

\begin{gather}
a+b=c \\
a^2+b^2 \\
a^2+b^2=c^2
\end{gather}

but you can obtain what you want in a simple way:

\documentclass{article}
\usepackage{amsmath}

\newcommand{\printequations}[1]{%
  \begin{gather}#1\end{gather}%
}

\begin{document}

\printequations{a+b=c \\ a^2+b^2 \\ a^2+b^2=c^2}

\end{document}

If you prefer a different syntax,

\documentclass{article}
\usepackage{amsmath}

%\usepackage{xparse}% uncomment for LaTeX prior to 2020-10-01

\ExplSyntaxOn
\NewDocumentCommand{\printequations}{m}
 {
  \begin{gather}
  \seq_set_split:Nnn \l_tmpa_seq {} { #1 }
  \seq_use:Nn \l_tmpa_seq { \\ }
  \end{gather}
 }
\ExplSyntaxOff

\begin{document}

\printequations{{a+b}{a^2+b^2}{a^2+b^2=c^2}}

\end{document}

will produce the same as before.

If your aim is to define a set of equations and use them one after the other, you can do

\documentclass{article}
\usepackage{amsmath}

%\usepackage{xparse}% uncomment for LaTeX prior to 2020-10-01

\ExplSyntaxOn

\seq_new:N \l_egreg_equations_list_seq

\NewDocumentCommand{\defineequations}{m}
 {
  \seq_set_split:Nnn \l_egreg_equations_list_seq {\\} { #1 }
 }
\NewDocumentCommand{\nextequation}{}
 {
  \seq_pop_left:NN \l_egreg_equations_list_seq \l_tmpa_tl
  \begin{equation}
  \l_tmpa_tl
  \end{equation}
 }

\ExplSyntaxOff

\begin{document}

\defineequations{a+b \\ a^2+b^2 \\ a^2+b^2=c^2}

Some text before the first equation
\nextequation
Some other text between the first and the second equation
\nextequation
Some other text between the second and third equation
\nextequation
Some final text

\end{document}

If you want to index your equations here is a possibility. The kay before the = is an arbitrary string.

\documentclass{article}
\usepackage{amsmath}

%\usepackage{xparse}% uncomment for LaTeX prior to 2020-10-01

\ExplSyntaxOn
\NewDocumentCommand{\defineequations}{m}
 {
  \prop_set_from_keyval:Nn \l_egreg_equations_list_prop { #1 }
 }
\NewDocumentCommand{\useequation}{m}
 {
  \begin{equation}
  \prop_item:Nn \l_egreg_equations_list_prop { #1 }
  \end{equation}
 }

\ExplSyntaxOff

\begin{document}

\defineequations{
 1={a+b},
 2={a^2+b^2},
 3={a^2+b^2=c^2}
}
Some text before the first equation
\useequation{2}
Some other text between the first and the second equation
\useequation{1}
Some other text between the second and third equation
\useequation{3}
Some final text

\end{document}

Answer 2

\@tfor is part of the LaTeX format:

\@tfor\Macroname:=Tokenlist\do{stuff to be done}

In order to use @ in the macro name \@tfor, you have to wrap its use within \makeatletter and \makeatother (unless the definition is in a file loaded with \usepackage). The % signs avoid spurious spaces.

\documentclass{article}
\makeatletter
\newcommand\manyequations[1]{%
  \@tfor\Equation:=#1\do{%
    \begin{equation}
      \Equation
    \end{equation}%
  }%
}
\makeatother
\begin{document}
\manyequations{{a+b}{a^2+b^2}{a^2+b^2=c^2}}
\end{document}

Answer 3

There are several looping macros available including \@for in the format but it's simpler (and more efficient actually) just to use a custom macro that is directly executed, as all looping macros essentially end up being syntactic sugar around such an execution.

\documentclass{article}
\usepackage{amsmath}


\begin{document}


\section{testing\ldots}
\newcommand\mydo[1]{Some text\begin{equation}#1\end{equation}}

\mydo{a+b}\mydo{a^2+b^2}\mydo{a^2+b^2=c^2}

\end{document}