[Tex/LaTex] New math command with alternative definition for inline mode

Question

I'm interested in writing a command which would result in output that varies when called within $...$ tags and when called within, say, displaymath environment.

For instance, assume that I want to define \xnorm command which works as

\newcommand{\xnorm}[1]{\|#1\|}

when called in the inline mode and which works as

\newcommand{\xnorm}[1]{\Big\|#1\Big\|}

when called in the "usual" math mode. How to achieve this?

I know that there exists a control sequence like ifmmode, but it tests whether I am in any type of math mode. Are there any more specific control sequences? Or, is there any other solution?

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Question answered. The goal can be achieved with \mathchoice command. An example code snippet can be found below, in the answers to this post. Thanks all very much.

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Nevertheless, it is worth a warning to others who have ideas similar to mine that math layout generated by \mathchoice can differ with the original one.

For example, try the following two functions:

\newcommand{\xnorma}[1]{\Big\|#1\Big\|}

and

\newcommand{\xnormb}[1]{\mathchoice{\Big\|#1\Big\|}{}{}{}}

(the first argument of \mathchoice provides a definition for "usual" math mode, other arguments do not play role in this example).

Now, simply try

\begin{document}
\begin{displaymath}
\xnorma{x^2}^2 \xnormb{x^2}^2
\end{displaymath}
\end{document}

A difference will be visible.

Answer 1

I don't think you really wants this. However, here it is:

\documentclass{article}
\usepackage{amsmath}

\newcommand{\xnorm}[1]{%
  \mathchoice{\Bigl\|#1\Bigr\|}{\lVert#1\rVert}{\lVert#1\rVert}{\lVert#1\rVert}%
}

\begin{document}
A norm in text mode is $\xnorm{v}$, while in display math it is
\[
\xnorm{v}=\sum_{i=1}^n\lvert v_i\rvert
\]
\end{document}

The result is really ugly (and wrong):

If you change \Bigl and \Bigr with \bigl and \bigr you get a more acceptable result, but I can't see the point of increasing the fences' size.