Numbering many equations automatically plus allowing to add new equations

Question

I'm facing a problem with my document that has many equations. The problem with the labelling issue if I decide to modify or add new equations. Basically so far, I'm using \label{eq4.1} where 4 for the chapter four and 1 for the first equation. With this approach, if I decide later to add new equations before eq4.1, then I have to change all subsequent equations to reflect this change. I'm looking for a better and flexible approach to cope with this problem.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
x &= 2^{100}, \label{eq4.1} \\
y &= 2^{100}, \label{eq4.2} \\
z &= 2^{100}. \label{eq4.3}
\end{align}

These are \eqref{eq4.1} and \eqref{eq4.2} and \eqref{eq4.3}. 

\end{document}

Answer 1

The purpose of the \label–\ref mechanism is to free the user from the burden of renaming cross references as soon as some of the numbers change.

Your projected scheme is completely equivalent to not adding any \label command and doing cross references just by the assigned number. Yes, you have the benefit of the \ref command, but your code would be completely equivalent with

\DeclareRobustCommand{\eqref}[1]{\eqrefaux#1\eqrefaux}
\def\eqrefaux eq#1\eqrefaux{\textup{(#1)}}

in the preamble and typing

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align}
x &= 2^{100}, \\
y &= 2^{100}, \\
z &= 2^{100}.
\end{align}

These are \eqref{eq4.1} and \eqref{eq4.2} and \eqref{eq4.3}. 

\end{document}

The macros just remove the eq string. Note that this wouldn't need two passes, because it doesn't exploit \label. But it's wrong, deadly wrong.

If you add an equation somewhere, you need to keep track of where it was and to find all cross references with a higher number, start from the one with the largest number, adding one to it and go down step by step. A nightmare! And a sure source of errors: suppose your chapters are stored in different files: you need to check each one of them and fix the cross references.

And, maybe, your supervisor will email you, a week later, “Dear CroCo, that equation you added last week should be removed”. Did you think to this? By Murphy's law, this will happen with utmost certainty.

There is already a flexible mechanism: using \label and \ref properly!

Add meaningful names to your equations,

\documentclass{article}
\usepackage{amsmath}
\usepackage{showkeys} % remove for final versions

\begin{document}

\begin{align}
x &= 2^{100}, \label{eq-x100}\\
y &= 2^{100}, \label{eq-y100}\\
z &= 2^{100}. \label{eq-z100}
\end{align}

These are \eqref{eq-x100} and \eqref{eq-y100} and \eqref{eq-z100}. 

\end{document}

You won't have to worry about adding or removing equations or other numbered objects, because LaTeX will take care of this. Even warning you if you have cross-referenced an equation (or chapter, section, whatever) you have meanwhile removed.

With showkeys you have the labels shown in the PDF, so you can easily skim through it to find the exact name when you want to do \ref. If the names are good enough, you'll probably remember them.

Answer 2

In general the labelling of the equations is based on their description rather than the order in which they appear. In a technical writing it is very common to add/delete the equations, figures, and tables. A general scheme followed in my lab is:

1. Use fg for figure, tb for table, and eq for equations. For example, in your case you might want to use \label{eq:defx}.

2. If there are several equations and you wish to separate them out as per their sections, for convenient recall of the label names, you might want to use something like \label{eq:bbt:defx} for an equation which defines x under a section say Big Bang Theory.

3. When there is a set of equations which can be referred as a single equation (similar to your example), you might want to use \nonumber to suppress the equation number.

   \documentclass{article}
   \usepackage{amsmath}
   
   \begin{document}
   \begin{align}
   x &= 2^{100}, \nonumber \\
   y &= 2^{100}, \nonumber \\
   z &= 2^{100}. \label{eq:defxyz}
   \end{align}
   
   These are defined in Eqs.~\ref{eq:defxyz}. 
   \end{document}
   

Edit: This does not answer the question but is a suggestion. It might take time for you to re-label all the equations, but in the long-run you would probably be saving lot of time---considering that you are already into adding/deleting equations.

Answer 3

Caution: Stupid code ahead:

I show a way to store the equations first in a expl3 seq - variable and write the equations with labels to a separate file. Adding new equations before or after writing or interchanging the order is easy using \AddEquation.

However, the file is rewritten each time, so design has to be revised ;-)

\documentclass{article}
\usepackage{amsmath}

\usepackage{xparse}

\ExplSyntaxOn
\seq_new:N \l__croco_equation_seq
\iow_new:N \l__croco_equation_file


\NewDocumentCommand{\AddEquation}{v}{%
  \seq_put_right:Nn \l__croco_equation_seq {#1}
}

\NewDocumentCommand{\WriteEquations}{O{eq4}}{%
  \int_zero:N \l_tmpa_int
  \iow_open:Nn \l__croco_equation_file {\jobname.equ}
  \iow_now:Nn \l__croco_equation_file {\begin{align}}
    \seq_map_inline:Nn \l__croco_equation_seq {%
      \int_incr:N \l_tmpa_int
      \iow_now:Nx \l__croco_equation_file {##1\space \string\label{#1.\int_use:N \l_tmpa_int} \string\\}
    }
    \iow_now:Nn \l__croco_equation_file {\end{align}}
  \iow_close:N \l__croco_equation_file 
  \seq_gclear:N \l__croco_equation_seq
}

\ExplSyntaxOff


\begin{document}

\AddEquation{x &= 2^{100}}
\AddEquation{y &= 2^{100}}
\AddEquation{z &= 2^{100}}

\WriteEquations

\InputIfFileExists{\jobname.equ}{}{}


\AddEquation{x &= 2^{100}}
\AddEquation{y &= 2^{100}}
\AddEquation{z &= 2^{100}}

\WriteEquations[eqother]

\InputIfFileExists{\jobname.equ}{}{}



These are \eqref{eq4.1} and \eqref{eq4.2} and \eqref{eq4.3}. 

and those are are \eqref{eqother.1} and \eqref{eqother.2} and \eqref{eqother.3}. 



\end{document}