Use external file for database equation

Question

I created an external reference file with many mathematical formulas. very similar to this: foonotes in external page the equations.tex is so structured :

\documentclass[10pt]{article}

\usepackage{amssymb,amsmath}
%\usepackage{catchfilebetweentags}
\usepackage{stackengine}  %% barbII
\usepackage{cancel}
\usepackage{amssymb}
\usepackage{amsmath}
\usepackage[overload]{empheq}
\newcommand{\for}{\text{for }}

\begin{document}
\begin{enumerate}
 \item \underline{\textit{Principio di conservazione della massa }}: 
%<*eq001>
\begin{center}
    \begin{equation}\label{eq001}
    \textbf{m} = \Intlim{V_{m}}\rho(x,t)\de{V} 
    \end{equation}
\end{center}
%</eq001>

%<*eq002>
\begin{center}
    \begin{equation}\label{eq002}
    \Dtot{m}{t} =\Dtot{}{t} \Intlim{V_{m}}\rho(x,t)\de{V}  = 0 
    \end{equation}
\end{center}
%</eq002>

\item \underline{\textit{Principio di bilancio della quantità di moto}} (Eulero, 1752):

%<*eq003>
\begin{center}
    \begin{equation}\label{eq003}
    \textbf{q}  = \Intlim{V_{m}}\rho\textbf{u}\de{V}  
    \end{equation}
\end{center}
%</eq003>
\end{enumerate}
\end{document}

In the preamble of the main.tex i inserted this:

\usepackage{catchfilebetweentags}
\newcommand{\loadeq}[1]{%
    \ExecuteMetaData[equations.tex]{eq#1}%
}

I call the various formulas in chapter01.tex with the loadeq {equation number} command :

bla bla bla bla bla bla bla 

\loadeq{003}

the problem is born if:

• I want to align two equations; \loadeq{003} = \loadeq{003}
• I want to align the equation and a text; ciao = \loadeq{003}
• I want to align a particular symbol with the equation. $\vec{\Pi}=$ \loadeq{003}

It would be interesting to understand how to be able to concatenate different \loadeq{arg}

Answer 1

You should structure your equations. No need of catchfilebetweentags.

In the following, if you call \useeq*{<number>}, both sides of the saved equation are typeset; with \useeq{<number>}, only the right-hand side. In case of need, this could be easily augmented with \useeq[<side>]{<number>} where <side> is either l or r to print the left-hand side or the right-hand side.

\begin{filecontents}{antonio-equations}
\EQ{001}{
  \mathbf{p} = \Intlim{V_{m}}\rho(x,t)\de{V}
}
\EQ{002}{
  \Dtot{m}{t} = \Dtot{}{t} \Intlim{V_{m}}\rho(x,t)\de{V}
}
\EQ{003}{
  \mathbf{q} = \Intlim{V_{m}}\rho\mathbf{u}\de{V}
}
\end{filecontents}

\documentclass{article}
\usepackage{amsmath}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\EQ}{mm}
 {
  \prop_new:c { g_antonio_eq_#1_prop }
  \seq_set_split:Nnn \l__antonio_eq_temp_seq { = } { #2 }
  \prop_gput:cnx { g_antonio_eq_#1_prop }
   { l } { \seq_item:Nn \l__antonio_eq_temp_seq { 1 } }
  \prop_gput:cnx { g_antonio_eq_#1_prop }
   { r } { \seq_item:Nn \l__antonio_eq_temp_seq { 2 } }
 }
\NewDocumentCommand{\useeq}{sm}
 {
  \IfBooleanT { #1 }
   {
    \prop_item:cn { g_antonio_eq_#2_prop } { l } =
   }
  \prop_item:cn { g_antonio_eq_#2_prop } { r }
 }
\seq_new:N \l__antonio_eq_temp_seq
\ExplSyntaxOff

\newcommand{\de}[1]{\mathop{}\!d#1}
\newcommand{\Dtot}[2]{\frac{\de{#1}}{\de{#2}}}
\newcommand{\Intlim}[1]{\int\limits_{#1}}

\input{antonio-equations} % load the equations

\begin{document}

\begin{enumerate}
\item \textit{Principio di conservazione della massa}: 
\begin{gather}
\useeq*{001} \label{eq001}
\\
\useeq*{002}=0 \label{eq002}
\end{gather}

\item \textit{Principio di bilancio della quantità di moto} (Eulero, 1752):
\begin{equation}
\useeq*{003} \label{eq003}
\end{equation}

\item Combiniamo
\begin{align}
  \vec{\Pi} &= \useeq{001}
  \\
  x &= \useeq{003}
\end{align}

\end{enumerate}

\end{document}

Please, don't underline text in italic: double emphasis is too much and underlining is a frowned upon device among typographers.

Note also that you should never surround equation by center. And never should be taken in its literal sense. Also consecutive equation environments are to be avoided: amsmath offers a wealth of display environments.