newcommand 中的 iffalse 问题

Question

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsthm,amsfonts}
\usepackage{xifthen}


\newcounter{example}[subsection]

\newcommand{\challenge}[3]{\refstepcounter{example}\par\medskip\noindent \textbf{Challenge~\theexample.}
    #1
    \vskip0.2cm
    \noindent\textbf{Solution}: #2
    \vskip0.2cm
    \ifthenelse{\equal{#3}{}}{}{\noindent\textbf{Notes}: #3}}


\begin{document}
\challenge{
    Write a note.
}{
    We simply write a note.
}{
    A note.
}

\challenge{
    Do not write any notes.
}{
    We simply leave the last argument empty.
}{}

\challenge{
    Demonstrate that notes can cause problems
}{
    We simply put a matrix to the notes.
}{
    Let $\varepsilon > 0$
%       and 
%       $$
%       A = \begin{matrix}
%       1
%       \end{matrix}
%       $$
%       be a matrix
        .
}
\end{document}

其结果是

当我取消注释矩阵部分时，出现错误：

第 46 行：未定义控制序列。} ：不完整 \iffalse；第 46 行之后的所有文本都被忽略。

第 46 行是第三个挑战的第三个参数的结束 } 的位置。

当我将矩阵等放入前两个参数中的任意一个或两个中时，我没有遇到任何问题。

Answer 1

Ifthenelse 完全展开它的第一个参数，所以脆弱的命令必须\protected

You could use

{
    Let $\varepsilon > 0$
       and 
       $$
       A = \protect\begin{matrix}
       1
       \protect\end{matrix}
       $$
       be a matrix
        .
}

但更好的方法是使用更好的空参数测试

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsthm,amsfonts}
\usepackage{xifthen}


\newcounter{example}[subsection]

\newcommand{\challenge}[3]{\refstepcounter{example}\par\medskip\noindent \textbf{Challenge~\theexample.}
    #1
    \vskip0.2cm
    \noindent\textbf{Solution}: #2
    \vskip0.2cm
    \if\relax\detokenize{#3}\relax\else\noindent\textbf{Notes}: #3\fi}


\begin{document}
\challenge{
    Write a note.
}{
    We simply write a note.
}{
    A note.
}

\challenge{
    Do not write any notes.
}{
    We simply leave the last argument empty.
}{}

\challenge{
    Demonstrate that notes can cause problems
}{
    We simply put a matrix to the notes.
}{
    Let $\varepsilon > 0$
       and 
       $$
       A = \begin{matrix}
       1
       \end{matrix}
       $$
       be a matrix
        .
}
\end{document}

Answer 2

我使用了一个token寄存器来预先扩展内容#3以便使\if...那里的嵌套内容静音。

不过，我建议使用类似xparse带有可选参数的东西（请参阅本文末尾的另一个答案）

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsthm,amsfonts}
\usepackage{xifthen}


\newcounter{example}[subsection]
\newtoks\daviddoesnotliketoksasname
\newcommand{\challenge}[3]{%
  \refstepcounter{example}\par\medskip% \par?
  \noindent%
  \textbf{Challenge~\theexample.}
  #1%
  \vskip0.2cm
  \noindent\textbf{Solution}: #2%
  \vskip0.2cm
  \daviddoesnotliketoksasname={#3}%
  \ifthenelse{\equal{\the\daviddoesnotliketoksasname}{}}{}{\noindent\textbf{Notes}: \the\toks}%
}


\begin{document}
\challenge{
    Write a note.
}{
    We simply write a note.
}{
    A note.
}

\challenge{
    Do not write any notes.
}{
    We simply leave the last argument empty.
}{}

\challenge{
    Demonstrate that notes can cause problems
}{
    We simply put a matrix to the notes.
}{%
  Let $\varepsilon > 0$
  and 
  \[
  A = \begin{matrix}
    1
  \end{matrix}
  \]
  be a matrix.
}
\end{document}

xparse使用版本更新

\documentclass[12pt,a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage{amsmath,amsthm,amsfonts}
\usepackage{xparse}
%\usepackage{xifthen}


\newcounter{example}[subsection]

\NewDocumentCommand{\challenge}{+m+m+o}{%
  \refstepcounter{example}\par\medskip% \par?
  \noindent%
  \textbf{Challenge~\theexample.}
  #1%
  \vskip0.2cm
  \noindent\textbf{Solution}: #2%
  \vskip0.2cm
  \IfValueT{#3}{%
    \noindent\textbf{Notes}: #3%
  }%
}


\begin{document}
\challenge{%
    Write a note.
}{%
    We simply write a note.
}[
    A note.
]

\challenge{%
    Do not write any notes.
}{%
    We simply leave the last argument empty.
}

\challenge{%
    Demonstrate that notes can cause problems
}{
    We simply put a matrix to the notes.
}[%
  Let $\varepsilon > 0$
  and 
  \[
  A = \begin{matrix}
    1 & 5  \\
    3 & 4 
  \end{matrix}
  \]
  be a matrix.
]
\end{document}