Capturing number of decimals in a macro argument

Question

Unfortunately, in some of our documents we need to print both SI units as well as Imperial units. We have some macros defined as follows:

\documentclass{article}
\usepackage{fp}

\def\GPM#1{#1 gpm\FPmul\temp{#1}{3.785}
\FPround\temp{\temp}{2}(\temp\ lpm) }

\def\GALLONS#1{#1 gallons \FPmul\temp{#1}{3.785}
\FPround\temp{\temp}{2}(\temp\ litres)}

\def\LPM#1{#1 lpm\FPdiv\temp{#1}{3.785}
\FPround\temp{\temp}{2}(\temp\ gpm)}

\begin{document}
\noindent
\GALLONS{200.00}\\
\GPM{500.00}\\
\LPM{1892.50}
\end{document}

How can one modify the macros, so that if you type a number, the conversion macro picks up the number of decimals automatically and sets the conversion accuracy to the same number of decimals, i.e., typing \GALLONS{150.000} should typeset 567.750 litres.

Answer 1

Here is an example for gallons.

\documentclass{article}
\usepackage{fp}

\makeatletter
\newcounter{NoDecim@ls}
\def\getDecim@ls#1{%
  \setcounter{NoDecim@ls}{0}%
  \getDecim@ls@i#1!!\@nil}
\def\getDecim@ls@i#1#2\@nil{%
  \ifx!#1\else\stepcounter{NoDecim@ls}\getDecim@ls@i#2\@nil\fi}

\def\GALLONS#1{\Gallons@i#1..\@nil}
\def\Gallons@i#1.#2.#3\@nil{%
  \FPmul\temp{#1}{3.785} #1%
  \ifx\relax#2\relax\else%
    \FPmul\tempD{#2}{3.785}%
    \FPadd\temp\temp\tempD%
    .#2%
  \fi
  \getDecim@ls{#2}%
  \FPround\temp\temp{\theNoDecim@ls}
  gallons (\temp\ litres)}
\makeatother

\begin{document}

\GALLONS{200.00}\\

\GALLONS{150.000} should typeset 567.750 litres.

\GALLONS{150.00} should typeset 567.75 litres.

\GALLONS{150} should typeset 568 litres.
\end{document}

Answer 2

\documentclass{article}
\usepackage{xstring}
\usepackage{fp}
\newcount\acc
\begin{document}

\def\gallons#1{\IfDecimal{#1}%
  {\StrLen{#1}[\templen]%
   \StrPosition{#1}{.}[\temppos]%
   \ifnum\temppos=0%
     \def\theacc{0}%
   \else
     \acc=\numexpr\templen-\temppos
     \edef\theacc{\the\acc}%
   \fi
   \FPupn\result{3.785 #1 * \theacc{} round}%
   \result
  }%
  {NaN} litres}

\gallons{?}      % NaN litres

\gallons{150}    % 568 litres

\gallons{150.0}  % 567.8 litres

\gallons{150.00} % 567.75 litres

\gallons{150.000}% 567.750 litres

\end{document}