截断浮点数以便显示，同时保持计算的完整准确性

Question

下面的 MWE 表明截断fp类型数字不仅仅是为了美观，实际上还会改变数字的值。有没有办法减少显示的小数位数，而不会影响数字在计算中的完整准确性？我浏览了一下，siunitx但什么也没发现。

\documentclass{article}
% RN. 6 April 2017
% TOPIC:
%  Truncating floating point numbers 
%=======================
\usepackage[check-declarations]{expl3}
\usepackage{xparse}
%-----------------------
\ExplSyntaxOn
\NewDocumentCommand\myTruncatingFloatingPoints{mO{3.11998}O{3.0198}}
  {
    \fp_eval:n {trunc(#2,#1)*trunc(#3,#1)}
  }  %  \myTruncatingFloatingPoints

\ExplSyntaxOff
%-----------------------
\begin{document}
    1. Full complement of digits:~\myTruncatingFloatingPoints{5}

    2. Truncated to 2 decimal digits:~  \myTruncatingFloatingPoints{2}
\end{document}

Answer 1

取自如何设置数字的精度（还有更多版本，请查看）：

\documentclass{article}
\usepackage{siunitx}
\begin{document}
\num{0.12368455}

\num[round-mode=places,round-precision=4]{0.12368455}
\end{document}

这就是你要找的东西吗？

Answer 2

这里我\num仅使用漂亮的打印。

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

\begin{document}

\begin{tabular}{ll}
No truncation  & \num{\fpeval{3.11998*3.0198}} \\
Truncation     & \num{\fpeval{trunc(3.11998*3.0198,2)}} \\
Bad truncation & \num{\fpeval{trunc(3.11998,2)*trunc(3.0198,2)}}
\end{tabular}

\end{document}

Answer 3

因为我感觉像是在重新发明轮子……（再次）

已编辑，因此截断到 0 位不会显示小数。

重新编辑以处理负数。

\documentclass{article}
\newcounter{digits}
\newcommand\myTruncatingFloatingPoints[2]{%
  \whole{#1}#2..\relax%
}
\def\whole#1#2.#3.#4\relax{%
    \presentwhole#2\relax\relax%
    \ifx\relax#3\relax.\trunc{#1}00\relax\else%
      \ifnum#1>0\relax.\trunc{#1}#3\relax\fi%
    \fi%
}
\def\presentwhole#1#2\relax{%
  \ifx-#1\ensuremath{-}\the\numexpr0#2\else\the\numexpr0#1#2\fi%
}
\def\trunc#1#2#3#4\relax{%
  \setcounter{digits}{#1}%
  \addtocounter{digits}{-1}%
  \ifnum\thedigits>0\relax%
    #2%
    \trunc{\thedigits}#3#40\relax%
  \else%
    \ifnum#3<5\relax#2\else\the\numexpr#2+1\relax\fi% ROUNDED
%    #2% TRUNCATED
  \fi%%  \fi%
}
\begin{document}
1. \makebox[.8in][r]{9.421715604} to 5 digits: \myTruncatingFloatingPoints{5}{9.421715604}

2. \makebox[.8in][r]{9.421715604} to 3 digits: \myTruncatingFloatingPoints{3}{9.421715604}

3. \makebox[.8in][r]{9.421715604} to 1 digits: \myTruncatingFloatingPoints{1}{9.421715604}

4. \makebox[.8in][r]{9.421715604} to 0 digits: \myTruncatingFloatingPoints{0}{9.421715604}

5. \makebox[.8in][r]{29} to 2 digits: \myTruncatingFloatingPoints{2}{29}

6. \makebox[.8in][r]{.1275} to 3 digits: \myTruncatingFloatingPoints{3}{.1275}

7. \makebox[.8in][r]{.1275} to 4 digits: \myTruncatingFloatingPoints{4}{.1275}

8. \makebox[.8in][r]{$-$.12} to 6 digits: \myTruncatingFloatingPoints{6}{-.12}
\end{document}