Is it possible to write all mathematical formulas in a separate file and add them in main file on-demand?

Question

Consider this scenario: I have a report to write and I want to write all mathematical formulas in a separate file and then add them in the main report file on demand (i.e. include/import/add/call a certain formula in specific part of my report).

Please note that I'm not talking about referencing or citation (\ref or \cir) to that formula. What I want, is to have the formula itself in the generated pdf. Somehow like a function call in programming languages!

Is this possible?

Answer 1

You can place (almost) arbitrary LaTeX code in macros for later use. So you can use these to store your equations. Here is a simple example showing some possibilities:

\documentclass{article}

%% Can be in external file which can be inserted with \input
\newcommand{\myeqintf}{f(x) = \int_a^x g(t)\,dt}
\newcommand{\myeqintt}{\begin{displaymath} f(x) = \int_a^x g(t)\,dt \end{displaymath}}
\newcommand{\myeqint}{\begin{equation} f(x) = \int_a^x g(t)\,dt \label{eq:int}\end{equation}}
\newcommand{\myeqintl}[1]{\begin{equation} f(x) = \int_a^x g(t)\,dt \label{#1}\end{equation}}
%%%%%%%%%%%%%%%%%%%

\begin{document}
An equation \( \myeqintf \) inline.  The same equation displayed
\begin{displaymath}
  \myeqintf .
\end{displaymath}
A stored display.
\myeqintt
A stored display with fixed label
\myeqint
don't reuse!!  A stored display with specified label
\myeqintl{eq:intl}

\end{document}

As you see there are a number of issues you will have to think about. Is it just the body of the formula that should be saved or a whole displayed environment? Should equation numbers be included? What about punctuation? (The breqn package could help with the latter issue.) Also this will make it hard to tweak the formulae for use in different situations.

Answer 2

Your comments indicate that you'd like to have some kind of database of equations. An easy approach would be to have a separate file equations.tex, say, which defines two macros

\saveequation{<ID>}{<equation code>}
\useequation{<ID>}

and also contains the equations defined with \saveequation and maybe also calls some often needed related packages. Adapting Harish's example this could look as follows:

\documentclass{article}

% this is the separate file for the sake of this example
% created with {filecontents}:
\usepackage{filecontents}
\begin{filecontents}{equations.tex}
\makeatletter
% \saveequation{<ID>}{<equation>}
\newcommand\saveequation[2]{%
  \@namedef{equation@#1}{#2}%
}

% \useequation{<ID>}
\newcommand\useequation[1]{%
  \@nameuse{equation@#1}%
}
\makeatother
\RequirePackage{amsmath,bm}
\saveequation{massenergy}{E = mc^{2}}
\saveequation{curlE}{\nabla \times \bm{E} = 0}
\end{filecontents}

% input the file:    
\input{equations}

\begin{document}
% usage:
This is Einstein's relation: $\useequation{massenergy}$.
This is the curl of electric field:
\begin{equation}
 \useequation{curlE}\label{eq:curlE}
\end{equation}
and without number:
\[
 \useequation{curlE}
\]

\end{document}

Answer 3

You could also use the catchfilebetweentags package. This package enables you to use a separate file to store you equations and then refer to them using tags that you specify.

This blog post, Loading equations from an external file, explains this quite well but for the lazy I'll include an example:

First you put your equations in a .tex file, delimiting them using tags (<*eq01>):

%<*eq01>
\begin{equation}
{x = \frac{ { - b \pm \sqrt {b^2 - 4ac} } }{2a}}
\end{equation}
%</eq01>

Then in you main file you do something like this:

\documentclass[11pt]{article}

\usepackage{amssymb,amsmath}
\usepackage{catchfilebetweentags}

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

% Begin document
\begin{document}

Look at equation eq01!
\loadeq{01}

\end{document}

And vola!

Example derived from the aforementioned blog post.

Answer 4

I heard that equations will be stored in separate files -- each equation in a separate file and you want to build a database of these files!: This is possible.

You can save an equation as a tex file, for example massenergy.tex:

E = mc^{2}

and curl of electric field to be saved as curlE.tex

\nabla \times \bm{E} = 0

Note that we have not used any math delimiters ($ or \begin{equation} or \[...\]) in these files so that they can be used inside variety of math environments. Now we have two .tex files: viz; massenergy.tex: and curlE.tex. For the time being, I assume that they are stored in the same folder as our main .tex file (you can store them elswhere and call them by providing the correct path).

Now we use them like this:

\documentclass{article}
\usepackage{filecontents}
\begin{filecontents}{massenergy.tex}
  E = mc^{2}
\end{filecontents}
\begin{filecontents}{curlE.tex}
  \nabla \times \bm{E} = 0
\end{filecontents}

\usepackage{amsmath,bm}
\begin{document}
This is Einstein's relation : $\input{massenergy}$.
This is the curl of electric field:
\begin{equation}
\input{curlE}
\end{equation}
and without number:
\[
\input{curlE}
\]


\end{document}

I have used filecontents so that one need not create the equation files separately while testing this example.

Answer 5

One more variation ...using a modification of the example provided by @Andrew Swann. Using the environment version scontents and the key write-env=file.ext you can write all the equations in separate files, but, I prefer to keep them in memory :)

\documentclass{article}
\usepackage[store-env=equations,store-cmd=equations]{scontents}
% In memory :)
\Scontents{f(x) = \int_a^x g(t)\,dt}
\begin{scontents}
\begin{displaymath}
f(x) = \int_a^x g(t)\,dt
\end{displaymath}
\end{scontents}
\Scontents{\begin{equation} f(x) = \int_a^x g(t)\,dt \label{eq:03}\end{equation}}
\Scontents{\begin{equation} f(x) = \int_a^x g(t)\,dt \label{eq:04}\end{equation}}

\begin{document}
An equation \( \getstored[1]{equations} \) inline.  The same equation displayed
\begin{displaymath}
  \getstored[1]{equations} .
\end{displaymath}
A stored display.
\getstored[2]{equations}
A stored display with fixed label
\getstored[3]{equations}
don't reuse!!  A stored display with specified label
\getstored[4]{equations}

\end{document}

Answer 6

If you wanted to, you could create a separate file for each formula, and then include each formula using \input{filename}. For example, if you have a file called euler.tex that is just

$e^{i \pi} + 1 = 0$

you would then just use $\input{euler}$. But this is a really bad idea. Use \newcommands. Or better yet, \newcommand your common subexpressions, but build the actual equations from those in each instance.

But actually, the above approach can be useful if you are generating your equations programmatically (e.g., as output from some computer algebra system).

Answer 7

Once having such an external file, you most likely will have a problem to collaborate and share your sources - you will always have to include in the collaboration the equations file (which might be located somewhere else in your directory tree). In other words you will have a problem of non-standard code. In this blog post I offered a solution to this issue in the form of using code snippet manager. This way, you could set a file called my_equations.yasnippet which will hold all your equations, and instead of typing them by hand you could simply hit eq_foo_bar TAB and this will be expanded to the right equation.

There is one downside to this approach: If you want to change something in eq_foo_bar you could only do it for future documents. Naturally, a change in the snippets database file will not effect existing files.