# How do counts differ from numexpr

To do basic arithmetic on numbers, one can utilize TeX counts,

``````\newcount\c
\c=2
\typeout{\the\c}
``````

LaTeX counters

``````\newcounter{c}
\setcounter{c}{2}
\typeout{\thec}
``````

or the `\numexpr` macro.

``````\edef\c{\numexpr 2 + 5 \relax}
\typeout{\the\c}
``````

Apart from their syntax, how do they differ? Which use cases are they most suitable for? Are there differences in performance? Which should be preferred for basic integer arithmetic as demonstrated in the examples above?

TeX has a fixed number (256 in classic TeX 32768 in etex and xetex and 65536 in luatex) of registers which store integer values.

`\newcount\c` allocates the name `\c` to one of these registers, and classic TeX primitives like `\advance` operate on them, note that addition here involves assignment back to the register so it is not an expandable operation.

LaTeX's `\newcounter` can be viewed as a syntactic wrapper around `\newcount`. `\newcounter{abc}` allocates the name `\c@abc` to a primitive TeX register allocated with `\newcount`. However as is often the case the extra layer of abstraction gives some useful features, notably that subsidiary macros are defined such as `\theabc` which defines the print form, and the counter is placed on reset lists so that for example incrementing `section` automatically sets `subsection` to zero. A similar list is used to preserve counters for the `\include` mechanism.

TeX primitive operations may be local or global

``````\c=2 { \advance\c 1 } \the\c
``````

will produce 2 as the increment will be lost at the `}`

``````\c=2 { \global\advance\c 1 } \the\c
``````

will produce 3 as the global assignment will be seen at all grouping levels.

LaTeX counter assignments are always global which reflects their top level use as for example a `figure` counter, the caption is typically inside a box which forms a group but you want the counter to have global document scope.

This means you often see primitive register use for local "scratch" arithmetic and latex counters for top level structural document counters.

`\numexpr` is an e-tex extension which gives (more or less) the same arithmetic operations as the `\advance`, `\multiply` etc primitives, but is classified as an expandable operation and does not automatically assign the value back to a register.

Because it is expandable it works in an `\edef` as shown in your (corrected)

``````\edef\c{\the\numexpr 2 + 5 \relax}

\show\c
``````

example.

• Thanks for the detailed answer. So LaTex counters wrap TeX counts, but numexpr does not build up on counts and counts are not built up on numexpr? Am I understanding correctly that `\numexpr` saves the assignment, so multiple operations condensed into a single numexpr-statement should likely be faster than count arithmetic? – XZS Mar 31 '15 at 19:53
• @XZS I doubt there is any measurable speed difference, the same underlying arithmetic functions are used. It's mainly one of author convenience, usually you end up assigning to a count anyway, but can use infix arithmetic `\count0=\numexpr2*\count4 + 5\relax` instead of `\count0=\count4 \multiply\count0 by 2 \advance\count0 by 5` – David Carlisle Mar 31 '15 at 19:56
• There is one other (very puzzling) difference: `\numexpr` rounds division instead of truncating it. (See e.g. this question.) – Alan Nov 12 at 14:08
• @Alan major design flaw in etex, what can I say it's not my fault :-) – David Carlisle Nov 12 at 14:21