[Tex/LaTex] How to redefine one-letter commands

macros

I want to change the function of some commands consisting of only one letter, such as \e or \i, and make them abide by MY rules. (Mouhahaha!) I have done this before with other commands like \in or \=, but it seems that one-letter commands are special for some mysterious reason.

In particular, I'd like to make a new command \i to use it in math-mode, but this produces an error.

Here is just a part of my document:

\documentclass[12pt]{article}

%   % symbols
\let\i\undefined
\newcommand{\i}{\mathrm{i}}


% begin languages
\usepackage[LGR,T1]{fontenc}


\begin{document}

$\i$ $i$ i

\end{document}

I get the error

Command \i invalid in math mode

The .pdf file is, nevertheless, produced and \i appears as the greek letter beta. I tried other ways (which now I cannot recall) that had \i to appear as a dotless i instead of beta.

And why don't I get the same error with \e that also has a predefined meaning?

Best Answer

There is nothing special about one letter commands you can (re)define them the same as multiletter commands. The point is that most of them are already defined and unless you are certain that you are not breaking latex, redefining latex's internals is not normally recommended.

You did not show any reproducible code to show the error that you showed in your previous question. Your actual redefinition redefined \i and it would have worked in math mode at that point. However \i is (normally) an encoding-specific command and probably some package you loaded reset those commands at \begin{document} so over-writing your definition.

But the solution is not to ask how to redefine latex's inner workings without breaking it, just use a different name.

Related Solutions

[Tex/LaTex] \newcommand gives errors in math mode (with or without arguments)

Here is an alternative to your current situation - using an optional argument to specify the derivative order. This way you don't have to define a macro for "each" derivative:

\deriv[<order>]{<func>}{<var>}

Here's a minimal example:

enter image description here

\documentclass{article}
\newcommand{\deriv}[3][]{% \deriv[<order>]{<func>}{<var>}
  \ensuremath{\frac{\partial^{#1} {#2}}{\partial {#3}^{#1}}}}
\begin{document}
In text mode there is~\deriv{y}{x} and~\deriv[2]{y}{x}. In display mode there is
\[
  \deriv{y}{x}\ \textrm{and}\ \deriv[2]{y}{x}\rlap{.}
\]
\end{document}

The default <order> is empty, implying the first order partial derivative. If you want the default to be 2, modify the definition to read

\newcommand{\deriv}[3][2]{...}

Technically it is possible to use a macro with numbers in them, but the usage is much less intuitive than adding something like an optional argument (as given above). Here's an implementation that now allows you to use \nameuse{deriv2}{y}{x}:

\expandafter\def\csname deriv2\endcsname#1#2{%
  \ensuremath{\frac{\partial^2 {#1}}{\partial {#2}^2}}}
\makeatletter
\newcommand{\nameuse}[1]{\@nameuse{#1}}%
\makeatother

The optional argument beats this hands down.

[Tex/LaTex] How to redefine \ref as \eqref

If you really want to do this, in amsmath.sty, \eqref is defined as

\renewcommand{\eqref}[1]{\textup{\tagform@{\ref{#1}}}}

so you could say

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\let\origref\ref
\renewcommand{\ref}[1]{\textup{\tagform@{\origref{#1}}}}
\makeatother

\begin{document}

\section{Test}\label{a}
Section~\ref{a}? Equation~\ref{b}
\begin{equation}\label{b}
a=b
\end{equation}

\end{document}

enter image description here

Notice the inconsistencies that the redefinition introduces; you are cross-referencing section (1), but your document has section 1; a similar problem will appear for floats, and other objects in your document.

Adendum

Perhaps, if you are interested in redefining the cross-reference format for a particular type of object, the cleveref package could be a better option; a little example in which I changed the formatting for cross-referencing figures, leaving all other cross.references unalatered:

\documentclass{article}
\usepackage{amsmath}
\usepackage{cleveref}

\crefformat{figure}{figure~(#2#1#3)}
\Crefformat{figure}{Figure~(#2#1#3)}

\begin{document}

\section{Test}\label{a}
Section~\ref{a}. Equation~\ref{b}. \Cref{c}? or \cref{c}?
\begin{equation}\label{b}
a=b
\end{equation}
\begin{figure}
\centering
A
\caption{test figure}
\label{c}
\end{figure}

\end{document}

enter image description here

Best Answer

Related Solutions

[Tex/LaTex] \newcommand gives errors in math mode (with or without arguments)

[Tex/LaTex] How to redefine \ref as \eqref

Adendum

Related Question