设置变量和 siunitx 指数到前缀产生错误“仅前缀”

Question

我正在尝试将 SI 单位存储在变量中，以便我可以对数字进行数学运算，然后根据我们学校的工程师科学符号使用正确的前缀和指数显示它。我偶然发现了一个可以轻松存储这些的代码，但由于某种原因，当我将 SIsetup exponent-to-prefix 设置为 true 时，它​​会失败。有什么办法可以解决这个问题吗？

\documentclass{article}
\usepackage[per-mode=fraction]{siunitx} % also loads xparse and expl3
\sisetup
  {
    exponent-to-prefix = true        ,
    round-mode         = figures     ,
    round-precision    = 3           ,
    scientific-notation = engineering,
  }


\ExplSyntaxOn
\NewDocumentCommand{\setvariable}{mmm}
 {
  \prop_gclear_new:c { g_giacomo_var_#1_prop }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { value } { #2 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { unit } { #3 }
 }

\NewDocumentCommand{\printvariable}{sm}
 {
  \IfBooleanTF{#1}
   {
    \num { \__giacomo_get:nn { #2 } { value } }
   }
   {
    \SI { \__giacomo_get:nn { #2 } { value } }
        { \__giacomo_get:nn { #2 } { unit } }
   }
 }

% syntactic sugar
\DeclareExpandableDocumentCommand{\getvalue}{m}
 {
  \__giacomo_get:nn { #1 } { value }
 }
\cs_new:Npn \__giacomo_get:nn #1 #2
 {
  \prop_item:cn { g_giacomo_var_#1_prop } { #2 }
 }
\cs_set_eq:NN \fpeval \fp_eval:n
\ExplSyntaxOff

\setvariable{Length}{800000}{\metre}

\begin{document}

\SI{800000}{\metre}

\printvariable{Length}

\end{document}

Answer 1

当exponent-to-prefix设置为 true 时，代码会进入无限循环。

解决方法：在看到第二个参数之前将\SI其完全展开。

\documentclass{article}
\usepackage[per-mode=fraction]{siunitx} % also loads xparse and expl3
\sisetup
  {
    exponent-to-prefix = true        ,
    round-mode         = figures     ,
    round-precision    = 3           ,
    scientific-notation = engineering,
  }


\ExplSyntaxOn
\NewDocumentCommand{\setvariable}{mmm}
 {
  \prop_gclear_new:c { g_giacomo_var_#1_prop }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { value } { #2 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { unit } { #3 }
 }

\NewDocumentCommand{\printvariable}{sm}
 {
  \IfBooleanTF{#1}
   {
    \num { \__giacomo_get:nn { #2 } { value } }
   }
   {
    \exp_args:Nnx \SI { \__giacomo_get:nn { #2 } { value } }
        { \__giacomo_get:nn { #2 } { unit } }
   }
 }

% syntactic sugar
\DeclareExpandableDocumentCommand{\getvalue}{m}
 {
  \__giacomo_get:nn { #1 } { value }
 }
\cs_new:Npn \__giacomo_get:nn #1 #2
 {
  \prop_item:cn { g_giacomo_var_#1_prop } { #2 }
 }
\cs_set_eq:NN \fpeval \fp_eval:n

\ExplSyntaxOff

\setvariable{Length}{800000}{\metre}

\begin{document}

\SI{800000}{\metre}

\printvariable{Length}

\printvariable*{Length}

\end{document}