2

There are switches for amsmath options leqno and reqno.

Then, how to define switches for turning on and off the option fleqn after amsmath is loaded?

I looked into the source of amsmath.sty and define the switches as

\makeatletter
\newcommand*{\fleqnon}{\setboolean{@fleqn}{true}}
\newcommand*{\fleqnoff}{\setboolean{@fleqn}{false}}
\makeatother

These two switches work only when amsmath is loaded with the option fleqn.

MWE:

\documentclass[b6paper]{scrartcl}

%\usepackage[fleqn]{amsmath}  % works with the option `fleqn`
\usepackage{amsmath}          % fails without the option `fleqn`
\usepackage{ifthen}

\makeatletter
% https://tex.stackexchange.com/a/212099
\newcommand*{\leqnomode}{\tagsleft@true\let\veqno=\@@leqno}
\newcommand*{\reqnomode}{\tagsleft@false\let\veqno=\@@eqno}
% based on my guess on `amsmath.sty`
\newcommand*{\fleqnon}{\setboolean{@fleqn}{true}}
\newcommand*{\fleqnoff}{\setboolean{@fleqn}{false}}
\makeatother

\begin{document}

\fleqnon

\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f
\end{align}

\fleqnoff

\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f \tag{e}
\end{align}

\fleqnon
\leqnomode

\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f \tag{a}
\end{align}

\fleqnoff
\begin{align}
  f(x) &= ax^2 + bx + c \\
  g(x) &= dx^2 + ex + f
\end{align}

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

\reqnomode

\begin{equation}
  -\Delta\phi=4\pi k\rho.\tag{2}
\end{equation}

\end{document}

output

Eli4ph
  • 805
  • Assuming that there are reasons for doing this, why don't you simply load amsmath with the fleqn option, given that it works? – egreg Oct 28 '19 at 12:26
  • the package only defines flush left or centred layouts when the package is loaded, so you can not switch mid document without copying chunks of the package code for the other case, there is no simple switch. – David Carlisle Oct 28 '19 at 12:37
  • @egreg Hmm, from my side there are several reasons for this question. Firstly, I am not sure whether the version based on my guess is correct. Secondly, there are switches for leqno and reqno, so it may be helpful to have one for fleqn. – Eli4ph Oct 28 '19 at 12:38
  • 1
    @Eli4ph there are not really supported switches for leqno either. I think the answer that you link to is misleading, they work sometimes but not in general. I may fix that later.... – David Carlisle Oct 28 '19 at 12:50
  • @DavidCarlisle The motivation behind is to achieve an enumeration of a list of display equations similar to inline math inside enumerate environment. Many answers on tex.sx use fleqn option. But I want other math not changed. That's why I want such switches. – Eli4ph Oct 28 '19 at 12:58
  • The package is designed for the memory requirements of 1980s.... There simply wasn't room to define all four versions and have a switch to enable any of them mid-document. It wouldn't be that hard these days to go through the package and every time it has a top level test such as \iftagsleft@\if@fleqn modify it so that the false branch is not skipped but instead saved in a macro that you could locally execute to change the choice locally, but that is not what the package does now. – David Carlisle Oct 28 '19 at 13:02
  • 1
    Not exactly switches, but nccmath defines fleqn and ceqn environments, which work somewhat like subequations. – Bernard Oct 28 '19 at 13:29

1 Answers1

7

The comments in amsmath.dtx say:

%    The code for calculating the appropriate placement of equation
%    tags in the \env{align} environments is quite complicated and
%    varies wildly depending on the settings of the |tagsleft@| and
%    |@fleqn| switches.  To minimize memory and hash space usage, we
%    only define the variant appropriate for the current setting of
%    those switches.
%

this means that of the four possible combinations of fleqn and leqno options being used or not used, definitions are made for only one combination, chosen at package loading time. The other three possibilities are skipped over and the TeX definitions for that combination are not made.

So if you change either of these switches mid document without copying code from the package sources and implementing the appropriate definitions, then the layout will basically be arbitrary and typically wrong, as you show.

David Carlisle
  • 757,742