Can't use numexpr in horizontal mode

Question

I am trying to learn to work with numexpr and am having some trouble with the basics, and etex_man has not been detailed enough to help in this case.

Here is an MWE:

\documentclass{article}
\begin{document}
Hi \numexpr 6 + 1 \relax
\end{document}

The error when I call pdflatex from TexLive 2018 is "You can't use `\numexpr' in horizontal mode. Hi \numexpr." Similar error if I switch to vertical or math mode.

So clearly I am missing something basic. Could someone enlighten me?

Thanks in advance

Answer 1

\numexpr is an additional item that can be used in contexts where TeX is looking for a <number>.

It basically is an “unnamed counter register” as far as its syntax is concerned and is not allowed “bare”.

It essentially works like \countX (where \countX stands for an “unreachable” count register number) after this unnamed register is loaded with the value of the expression.

So just like you can't say Hi \count232 to print the value stored in \count232, you cannot say

Hi \numexpr 6+1\relax

Just consider \count232 and \numexpr 6+1\relax as referring to an abstract number, which exists independently of its representation. You need to access a representation of this abstract number:

Hi \number\count232

Hi \romannumeral\count232

Hi \number\numexpr 6+1\relax

Hi \romannumeral\numexpr 6+1\relax

Bare bones TeX only provides \number (for the decimal representation) and \romannumeral (for lowercase Roman numeral representation). Add-on packages may provide other representations.

Answer 2

You need \number or \the in front of the expression: \numexpr works like a TeX \count. Alternatively, you could use the xfp package to provide a thin wrapper

\documentclass{article}
\usepackage{xfp}
\begin{document}

\number\numexpr 1 + 2 * 3 \relax

\the\numexpr 1 + 2 * 3 \relax

\inteval{1 + 2 * 3}

\end{document}