Writing a negative number in text mode: $-$9 or $-9$?

Question

If we want to produce -9, typing -9 in an input file is not correct because - is intentionally designed for hyphenation so its glyph is bad to represent the minus sign.

To produce the minus sign, we use $-$ instead of -. My question is, should a number be enclosed in $ together with the - if we want to render a negative number?

In other words, which one is recommended in text mode: $-$9 or $-9$?

In other words, which one is recommended out of:

Option A

$-$9 is a negative number.

or

Option B

$-9$ is a negative number.

?

Answer 1

$-9$

Minus nine is a number, so you should type the whole number in a single mode, in this case the math mode. If you can find a minus sign in text font, typing the sign and nine both in text mode is acceptable depend on the context.

Answer 2

Option C

\documentclass{article}

\usepackage{textcomp}

\begin{document}
\textminus 9
\end{document}

This produces a legit minus sign.

Answer 3

Although it's a bit overkill, you could use the \num macro of the siunitx package:

\documentclass{article}

\usepackage{siunitx}

\begin{document}

\Huge

In mathmode: \(-9\)

With \textsf{siunitx}: \num{-9}

\end{document}

Answer 4

I'd also say Option B or Option C since the minus sign and the number are consistently formatted in these two cases.

If you prefer Option C and your editor supports Unicode (such as Emacs) you can facilitate entering negative numbers by loading the inputenc or even better the inputenx package and associate \textminus with the Unicode minus sign U+2212:

\documentclass{article}

\usepackage{textcomp}

\usepackage[utf8]{inputenc}

\DeclareUnicodeCharacter{2212}{\textminus}

\begin{document}

\noindent
number \textminus9\\
number −9

\end{document}