如何进行条件换行

Question

我想知道如何进行条件换行。每当出现错误消息“此处没有行结束”时，我都想使用命令\\或\newline命令禁用换行，否则启用它。

正如所建议的，我现在用一个最小的示例 tex 文件来解释我的问题。

我使用两个宏，它们允许我“吃掉”问题或答案，同时允许我在源中将它们保留在一起。例如，当我进行考试时，我会吃掉答案，只给出问题。

此示例导致以下错误消息：

LaTeX Error: There's no line here to end.  

原因是我\\ [1ex]在宏的开头有命令，\ans。

如果我\\ [1ex]从宏中删除该命令\ans，错误就会消失。但不幸的是，第一个问题的答案并没有从新行开始。

我还想\\ [1ex]在不需要时禁用该命令。例如，在第二个问题中，无论是否存在此命令，行为都是相同的。事实上，在某些情况下，此命令可能会创建不必要的额外空间。

\documentclass[12pt,a4paper]{article}
\newcommand{\quest}[1]{#1}
\newcommand{\eatquest}[0]{\renewcommand{\quest}[1]{}}
\newcommand{\ans}[1]{\\ [1ex] Answer: #1}
\newcommand{\eatans}[0]{\renewcommand{\ans}[1]{}}
\begin{document}
%\eatquest
%\eatans
\begin{enumerate}
\item
\quest{What is the effect of bad zeros?  How can it be overcome?
}
\ans{
 This results in poor performance of the system.  The bad zeros
  cannot be changed by feedback.  The only way to change them is to
  redesign the system (not controller) itself.
}
\item 
\quest{This question is on 2-DOF pole placement controller
  design. Answer the following: 
\begin{enumerate}
\item In the class, a method was proposed to handle unstable 2-DOF
  controllers.  This method also has unstable pole-zero cancellation.
  Why was this acceptable? 
\end{enumerate}
}
\ans{A short answer is given now:
\begin{enumerate}
\item The pole and zero come for the same, ``identical'', source.  In
  fact, one can even say that this is NOT a case of pole-zero
  cancellation. 
\end{enumerate}
}
\end{enumerate}
\end{document}

Answer 1

使用\par\addvspace{1ex}而不是\\[1ex]

\documentclass[12pt,a4paper]{article}
\newcommand{\quest}[1]{#1}
\newcommand{\eatquest}[0]{\renewcommand{\quest}[1]{}}
\newcommand{\ans}[1]{\par\addvspace{1ex}Answer: #1}
\newcommand{\eatans}[0]{\renewcommand{\ans}[1]{}}
\begin{document}
%\eatquest
%\eatans
\begin{enumerate}
\item
\quest{What is the effect of bad zeros?  How can it be overcome?
}
\ans{
 This results in poor performance of the system.  The bad zeros
  cannot be changed by feedback.  The only way to change them is to
  redesign the system (not controller) itself.
}
\item 
\quest{This question is on 2-DOF pole placement controller
  design. Answer the following: 
\begin{enumerate}
\item In the class, a method was proposed to handle unstable 2-DOF
  controllers.  This method also has unstable pole-zero cancellation.
  Why was this acceptable? 
\end{enumerate}
}
\ans{A short answer is given now:
\begin{enumerate}
\item The pole and zero come for the same, ``identical'', source.  In
  fact, one can even say that this is NOT a case of pole-zero
  cancellation. 
\end{enumerate}
}
\end{enumerate}
\end{document}