[Tex/LaTex] How to write a macro which enters math mode when called in text mode but when called in mathmode doesn’t break

macrosmath-mode

I'm using the package braket for my physics notes, and this speeds things up a great deal but I'm constantly writing \ket{\psi} or \bra{\psi}, and because this specific pair of bra and ket appears so often, this gets repetitive and I would like a macro to speed this up. However since the braket package works only in math mode, my attempt \newcommand{\ketp}{$\ket{\psi}}$ work when I'm writing in text mode, but when I'm writing in mathmode it breaks (unsurprisingly).

Is there any way I can get this command to work in both mathmode and textmode without having to define two different macros, one for each?

Thanks in advance for any help.

P.S. I'm certain this is a very simple question but I could find little to no explanation of this feature.

Best Answer

You can use

\newcommand{\ketp}{\ensuremath{\ket{\psi}}}

However it isn't clear that it's always a good idea to do this. TeX markup is based on a design that has a clear distinction between text and math commands, the assumption is that the author always knows which mode is in use, and typically similar commands for use in text and math have different names, compare \^ and \hat for example.

That said, \ensuremath was added to LaTeX to do exactly what you ask, so if you want to use it, it's there...

Related Solutions

macros – Write a Macro Called with \begin and \end

You can define an environment just as you would define a macro. Only instead of saying \newcommand\mycmd[1]{...#1...} you must write

\newenvironment{myenv}[1]{Begin...#1...}{End}

Here the last two parameters say what should be put before and after the contents.

This means the first try would be to write

\newenvironment{scaleblock}[1]{\begin{block}{\scalebox{0.4}{#1}}
    \scalebox{0.4}{\vbox{}%
  {}}
\end{block}}

But this does not reflect the intended nesting of parameters and you actually only give the begin part of your environment, while the end part is the newline character in the fourth line.

So in your case it is probably better to first collect everything inside the environments body and then use it. This can be done with the environ package. It provides the command \NewEnviron where you can use \BODY to access those content. Assuming block is the one from beamer, you can do the following:

\usepackage{environ}
\NewEnviron{scaleblock}[1]{\begin{block}{\scalebox{0.8}{#1}}
  \scalebox{0.8}{\vbox{\BODY}}
\end{block}}

Math Mode – How to Make a Macro Require Math Mode?

You can use \ifmmode to check if you are in math mode, and trigger an error if not. An attempt to use it outside of math mode yields:

Attempt to use \MyMathModeMacro outside of math mode.

See my preamble documentation for explanation.

Type H for immediate help.

l.16 \MyMathModeMacro % <--- This will produce an error ?

Notes:

As David Carlisle pointed out, need to use \DeclareRobustCommand instead of \newcommand to avoid premature expansion. See \ifmmode doesn't seem to work correctly inside an array environment.
If you want to allow the macro to be used in or outside of math mode, you can use \ensuremath to typeset the content in math mode. But you should refer to When not to use \ensuremath for math macro? before doing this.

Code:

\documentclass{article}
\makeatletter
\DeclareRobustCommand{\MyMathModeMacro}{%
    \ifmmode
         E = m c^2
    \else
        \GenericError{\space\space\space\space}
            {Attempt to use \@backslashchar MyMathModeMacro outside of math mode}
            {See my preamble documentation for explanation.}
            {Need to use either use inline or display math.}%
    \fi
}
\makeatother

\begin{document}
$\MyMathModeMacro$

\MyMathModeMacro% <--- This will produce an error
\end{document}