Xparse t argument type (underscore with optional parameter)

Question

I am not understanding how to use the t type parameter with xparse's \NewDocumentCommand.

I am trying to define a macro that accepts one optional parameter, but the optional parameter is specified as a subscript using an underscore. That is I want to define something that behaves like

\def\MyMacro_#1{A_{\textcolor{red}{#1}}}

but with the subscript being optional. I need #1 to be parameter not simply a subscript added outside of the macro (i.e., \MyMacro must be the one that invokes the actual subscript)

The MWE below yields:

but the desired result is:

with the color and subscript applied by \MyMacro.

Notes:

• The documentation states that t is

  An optional ⟨token⟩, which will result in a value \BooleanTrue if ⟨token⟩ is present and \BooleanFalse otherwise. Given as t⟨token⟩.

  but no examples are given.

Code:

\documentclass{article}

\usepackage{amsmath}
\usepackage{xparse}
\usepackage{xcolor}

%\newcommand*{\MyMacro}[1][]{}% Ensure we are not overwriting anything
%\def\MyMacro_#1{A_{\testcolor{red}{#1}}}% <-- Want this behavior but with the _{#1} being optional

\NewDocumentCommand{\MyMacro}{t_}{%
    \IfBooleanTF{#1}{%
        A_{\textcolor{red}{#1}}
    }{%
        \mathbf{A}
    }%
}

\begin{document}

With a subscript: $\MyMacro_{\pi}$ 

Without any subscript: $\MyMacro$ 

\end{document}

Answer 1

The t argument specifier is a generalization of s that is the same as t*.

I don't think it's the right approach, but if you really want to go this way:

\NewDocumentCommand{\MyMacro}{t_}{A\IfBooleanT{#1}{\MyMacroAux}}
\NewDocumentCommand{\MyMacroAux}{m}{_{\textcolor{red}{#1}}}

Full example:

\documentclass{article}

\usepackage{amsmath}
\usepackage{xparse}
\usepackage{xcolor}

\NewDocumentCommand{\MyMacro}{t_}{A\IfBooleanT{#1}{\MyMacroAux}}
\NewDocumentCommand{\MyMacroAux}{m}{_{\textcolor{red}{#1}}}

\begin{document}

With a subscript: $\MyMacro_{\pi}$

Without any subscript: $\MyMacro$

\end{document}

In case the first part also depends on the presence of _:

\NewDocumentCommand{\MyMacro}{t_}{%
  \IfBooleanTF{#1}
    {A\MyMacroAux}
    {\mathbf{A}}%
}
\NewDocumentCommand{\MyMacroAux}{m}{_{\textcolor{red}{#1}}}

Answer 2

Looks like an e-type argument to me

\documentclass{article}

\usepackage{amsmath}
\usepackage{xparse}
\usepackage{xcolor}

\NewDocumentCommand\MyMacro{e_}{%
    \IfNoValueTF{#1}{%
        \mathbf{A}
    }{%
        A_{\textcolor{red}{#1}}
    }%
}

\begin{document}

With a subscript: $\MyMacro_{\pi}$ 

Without any subscript: $\MyMacro$ 

\end{document}

Answer 3

Is this what is meant?

The t_ check works, but \textcolor{red}{#1} will try to typeset _ then, so {\pi} never enters the subscript.

In my opinion, the {\pi} is an 2nd optional argument. Either use g or the G{} type to grab this.

\documentclass{article}

\usepackage{amsmath}
\usepackage{xparse}
\usepackage{xcolor}

%\newcommand*{\MyMacro}[1][]{}% Ensure we are not overwriting anything
%\def\MyMacro_#1{A_{\testcolor{red}{#1}}}% <-- Want this behavior but with the _{#1} being optional

\NewDocumentCommand{\MyMacro}{t_G{}}{%
  \IfBooleanTF{#1}{%
    A_{\textcolor{red}{#2}}
  }{%
    \mathbf{A}
  }%
}

\begin{document}

With a subscript: $\MyMacro_{\pi}$ 

Without any subscript: $\MyMacro$ 

\end{document}