Siunitx，保存变量并用FP进行计算

Question

我正在尝试存储变量及其相关信息，对变量号进行计算，并使用 siunitx 正确显示。当仅将数字保存为变量时，这种方法效果很好，但尝试将变量保存为 fpeval 的输出会产生错误。似乎它实际上并没有存储 fpeval 的输出，而是公式本身，尽管我不太确定。在下面的代码中，我注释掉了产生错误的行。

\documentclass{article}
\usepackage{amsmath}
\usepackage[nomessages]{fp}% http://ctan.org/pkg/fp
\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{\varSet}{mmmm}
 {
  \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 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { name } { #4 }
 }

 \NewDocumentCommand{\varChange}{mm}
 {
  \prop_gput:cnn { g_giacomo_var_#2_prop } { value } { #1}
 }

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



% syntactic sugar
\DeclareExpandableDocumentCommand{\varGet}{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


\varSet{Area}{1}{\metre\squared}{A_{rmin}}
\varSet{Width}{200}{\metre}{W_{sq}}
\varSet{Length}{10}{\metre}{L_{sq}}

\begin{document}
Input Length = $\varPrint*{Length}$\\
Input Width = $\varPrint*{Width}$\\
Calculated area: \fpeval{\varGet{Width}*\varGet{Length}}\\

%\varSet{Area}{\fpeval{\varGet{Width}*\varGet{Length}}}{\metre\squared}{A_{rmin}}
Saved Area: $\varPrint*{Area}$

\end{document}

Answer 1

如果您计划在的“值”参数中执行操作\varSet，则需要\prop_gput:cnx在相应的行中使用；当值明确时，这也只会进行无用的扩展，但无需担心。

\NewDocumentCommand{\varSet}{mmmm}
 {
  \prop_gclear_new:c { g_giacomo_var_#1_prop }
  \prop_gput:cnx { g_giacomo_var_#1_prop } { value } { #2 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { unit } { #3 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { name } { #4 }
 }

这样，l3fp在将值存储到属性列表之前，第三个参数就完全展开了（使用兼容的语法是可能的）。

\documentclass{article}
\usepackage{amsmath}
\usepackage[nomessages]{fp}% http://ctan.org/pkg/fp
\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{\varSet}{mmmm}
 {
  \prop_gclear_new:c { g_giacomo_var_#1_prop }
  \prop_gput:cnx { g_giacomo_var_#1_prop } { value } { #2 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { unit } { #3 }
  \prop_gput:cnn { g_giacomo_var_#1_prop } { name } { #4 }
 }

\NewDocumentCommand{\varChange}{mm}
 {
  \prop_gput:cnn { g_giacomo_var_#2_prop } { value } { #1}
 }

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



% syntactic sugar
\DeclareExpandableDocumentCommand{\varGet}{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


\varSet{Area}{1}{\metre\squared}{A_{rmin}}
\varSet{Width}{200}{\metre}{W_{sq}}
\varSet{Length}{10}{\metre}{L_{sq}}

\begin{document}

Input Length = $\varPrint*{Length}$

Input Width = $\varPrint*{Width}$

Calculated area: \fpeval{\varGet{Width}*\varGet{Length}}

\varSet{Area}{\fpeval{\varGet{Width}*\varGet{Length}}}{\metre\squared}{A_{rmin}}
Saved Area: $\varPrint*{Area}$

\end{document}