accumulator variable

Question

I know in latex it is possible to use counters which are based on integers. Is there also some way to define a floating point register to perform simple arithmetic operations?

Basically I would need some way to write something like

\newreg{myx}
\setreg{myx}{0.0}
\addreg{myx}{0.5}
\addreg{myx}{-0.25}
\valuereg{myx} % this expands to 0.25

Answer 1

If you need only addition, subtraction and multiplication and numbers are expected in the range +-16380 with four (or less) digits after decimal point then you can use directly dimension registers supported naturally by TeX. For example:

\def\newreg{\csname newdimen\endcsname}
\def\setreg#1#2{#1=#2pt }
\def\addreg#1#2{\advance#1by#2pt }
\def\mulreg#1#2{#1=#2#1}
\def\valuereg#1{\expandafter\ignorept\the#1}
\bgroup\lccode`\?=`\p \lccode`\!=`\t \lowercase{\egroup\def\ignorept#1?!{#1}}

\newreg\myx
\setreg\myx {0.0}
\addreg\myx {0.5}
\addreg\myx {-0.25}
\valuereg\myx  % this expands to 0.25

\bye

But division using dimension registers is somewhat more complicated.

Of course, you can use pgfmath or calc or lua or expl3 or apnum or somewhat similar but with declared needs it seems like using cannon on a sparrow.

Answer 2

Here a simple example with pgfmath, part of pgf/TikZ:

\documentclass{article}
\usepackage{pgfmath}
\begin{document}
  \pgfmathsetmacro\myx{0}
  \pgfmathsetmacro\myx{\myx + 0.5}
  \pgfmathsetmacro\myx{\myx - 0.25}
  \myx
\end{document}

The "register" is a simple macro (\myx) here. Also, an "addto" macro can be defined:

\documentclass{article}
\usepackage{pgfmath}

% #1: macro token, e.g. \myx
% #2: math expression
\newcommand*{\pgfmathaddtomacro}[2]{%
  \pgfmathsetmacro#1{#1+(#2)}
}

\begin{document}
  \pgfmathsetmacro\myx{0}
  \pgfmathaddtomacro\myx{0.5}
  \pgfmathaddtomacro\myx{-0.25}
  \myx
\end{document}

Answer 3

You can use the powerful floating point module of expl3. I use global assignments, like for LaTeX counters, but it would be possible to do local assignments.

\documentclass{article}
\usepackage{xparse,xfp}

\ExplSyntaxOn
\NewDocumentCommand{\newreg}{m}
 {
  \fp_new:c { g_nicmus_#1_fp }
 }
\NewDocumentCommand{\setreg}{mm}
 {
  \fp_gset:cn { g_nicmus_#1_fp } { #2 }
 }
\NewDocumentCommand{\addreg}{mm}
 {
  \fp_gset:cn { g_nicmus_#1_fp } { \fp_use:c { g_nicmus_#1_fp } + (#2) }
 }
\NewExpandableDocumentCommand{\valuereg}{m}
 {
  \fp_use:c { g_nicmus_#1_fp }
 }
\ExplSyntaxOff

\newreg{myx}

\begin{document}

\setreg{myx}{0.0}
\addreg{myx}{0.5}
\addreg{myx}{-0.25}
\valuereg{myx} % this expands to 0.25

\addreg{myx}{sqrt(3)+pi}

\valuereg{myx} % this expands to 5.12364346115867

\fpeval{round(\valuereg{myx},2)} % this expands to 5.12

\end{document}