Math font change for subscript only

Question

Goal: I want a macro that changes the math font of only subscript and superscript. In TeX terms, I think this means I want to change only S, S' styles, which are typeset using scriptfont.

Graphically, I'd want

\bfscript{v_i}

to be displayed as $v_{\mathbf{i}}$.

Tries: I've been trying to do it the TeX way; that is, something like

\def\bfscript#1{\hbox{$\scriptfont0=\tenbf #1$}}

but this fails miserably, as scriptfont and the likes seem completely ignored by LaTeX. It seems that LaTeX only allows to make distinctions on the fontsize, with \DeclareMathSizes.

Nota: I was first interested in this as a real-life case, but I'm not even sure I'll need it any longer; I'm just really wondering what the answer is.

Answer 1

LaTeX loads math fonts and assigns them on demand so you need to have an initial math setup to ensure that there is a font to be used as scriptfont. Also you want fam 1 not 0 in the usual setup.

I don't recommend that you do this:

\documentclass{article}

\begin{document}
\showoutput

\def\bfscript#1{%
\setbox0\hbox{$\scriptstyle\mathbf{\xdef\tmp{\the\scriptfont\fam}}$}%
\hbox{$\scriptfont1=\tmp #1$}}

$\bfscript{v_i}$

$v_{\mathbf{i}}$

\end{document}