eval.sty

% This is my (Ben Krikler's) attempt to make it easy to update numbers in the text easily
% Perhaps one day this will become a full blown package (with a better name)
% This packages requires that the siunitx package be installed

% Useage:
% %%% Declare a bunch of variables
%\newcommand\x{30}
%\newcommand\y{50}
%
% %%% Declare calculated variables (can be in the body of the document)
%\newcommand{\z}{\x+3*\y}
%\newcommand{\w}{\x/7*\y}
%
%\begin{document}
%\textbackslash{}x=\x
%
%\textbackslash{}y=\y
%
% %%% \decimals prints the number by decimal place rounding (default DP=2, but optional parameter changes this)
%\textbackslash{}z=\z=\decimals{\z}=\decimals[3]{\z}
%
% %%% \sigfigs prints the number by significant figure rounding (default SF=2, but optional parameter changes this)
%\textbackslash{}w=\w=\decimals{\w}=\sigfigs{\w}(def.\ sig.\ figs)=\sigfigs[5]{\w}(5 s.f.)
%\end{document}

\usepackage{xparse}
\ExplSyntaxOn
\DeclareExpandableDocumentCommand{\eval}{m}{\fp_eval:n{#1}}
\ExplSyntaxOff

\newcommand*{\decimals}[2][2]{\num[%
  round-mode=places,% Round output to specified number of places
  round-precision=#1,% Round-precision is 3
  output-decimal-marker={.},% Use , as decimal marker
  round-integer-to-decimal,
  ]{\eval{#2}}\xspace}

%\newcommand*{\int}[1]{\decimals[0]{#1}}

\newcommand*{\sigfigs}[2][2]{\num[%
  round-mode=figures,% Round output to specified number of places
  round-precision=#1,% Round-precision is 3
  output-decimal-marker={.},% Use , as decimal marker
  add-decimal-zero=true,
  ]{\eval{#2}}\xspace}

\newcommand*{\scinot}[2][2]{\num[%
  round-mode=figures,% Round output to specified number of places
  round-precision=#1,% Round-precision is 3
  output-decimal-marker={.},% Use , as decimal marker
  add-decimal-zero=true,
  scientific-notation=true
  ]{\eval{#2}}\xspace}

\newcommand\Var[2]{%
\expandafter\newcommand\csname Var#1\endcsname{#2\xspace}}
\newcommand\VarNum[2]{%
\expandafter\newcommand\csname Var#1\endcsname{\num{#2}\xspace}
\expandafter\newcommand\csname Num#1\endcsname{#2}}
\newcommand\VarUnits[3]{%
\expandafter\newcommand\csname Var#1\endcsname{\SI{#2}{#3}\xspace}}