"Missing { inserted." error when using \NewDocumentCommand command as a subscript

Question

I'd like to create a custom command with an optional parameter, which could be used in math mode as a sub- or superscript. However, the following code:

\documentclass[pdftex,11pt]{report}

\usepackage{xparse}

\NewDocumentCommand{\MyCmd}{o}{\IfNoValueTF{#1}{X}{X_\mathrm{{#1}}}}

\begin{document}

$\MyCmd$

$Q_{\MyCmd}$

$Q_{\MyCmd[Y]}$

$Q_\MyCmd$  % <-- fails

$Q_\MyCmd[Y]$  % <-- fails

\end{document}

produces the following errors:
Missing { inserted. $Q_\MyCmd
Missing } inserted. $Q_\MyCmd$
Missing { inserted. $Q_\MyCmd
Missing } inserted. $Q_\MyCmd[Y]$

I.e., it seems I need to wrap my command call in braces whenever I'd like to use it in a sub- or superscipt.

How to properly define such a command so that it wouldn't be necessary to insert extra braces?

Welcome to tex.sx. See How to avoid using curly braces when placing a \DeclareMathOperator command in subscript or superscript, while preserving spacing? (potential duplicate) — barbara beeton, Oct 13 '19 at 20:39
@barbarabeeton the situation is rather different for DeclareMathOperator, as egreg comments in his answer there. You can add extra braces to the definition, but for mathop that's bad as it makes it a mathord, but here X and X_{..} are mathord anyway. — David Carlisle, Oct 17 '19 at 11:13

David Carlisle · Answer 1 · 2019-10-17T13:05:51.317

It is possible to define the command so that braces are not needed by relying on the low level detail that _ scans for an argument in a way that is not at all like the scanning for a macro argument, so you just need a \bgroup before looking for the optional argument:

\documentclass[pdftex,11pt]{report}

\usepackage{xparse}

\NewDocumentCommand\MyCmd{}{\bgroup\MyCmdx}
\NewDocumentCommand{\MyCmdx}{o}{\IfNoValueTF{#1}{X}{X_\mathrm{#1}}\egroup}

\begin{document}

$\MyCmd$

$Q_{\MyCmd}$

$Q_{\MyCmd[Y]}$

$Q_\MyCmd$  % <-- fails

$Q_\MyCmd[Y]$  % <-- fails

\end{document}

However this will fail if used in conjunction with one of the many packages that redefine _ to be active character taking a normal argument.

The fact that X_\mathrm{b} or X_\frac12 work is an accident of the implementation and really it is just a mis-feature of the language that they do not give errors. The documented supported markup would be X_{\mathrm{b}} or X_{\frac{1}{2}}.

So the response to a missing { inserted error should be to add the missing { not to re-code the macro so the error is not given:-)

score 0 · Accepted Answer · answered Oct 17 '19 at 10:58

0

Based on the link provided barbara beeton:

How to avoid using curly braces when placing a \DeclareMathOperator command in subscript or superscript, while preserving spacing?
Double subscript error with \newcommand?

in my case the answer would be:

There is no such way to avoid using braces. On the other hand, it is a good "coding" practice to always include them when using sub- and superscripts (as it will save me from head scratching when something goes wrong).

answered Oct 17 '19 at 10:58

Paweł Kłeczek

111

It is possible to write the command so braces are not needed but I would never do that, the fact that X_\frac12 with no braces gives no error and a subscript half is an accident of the implementation and a mis-feature in latex, not something to be emulated. (I would write X_{\mathrm{#1}}` for similar reasons. – David Carlisle Oct 17 '19 at 11:10

"Missing { inserted." error when using \NewDocumentCommand command as a subscript

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