class.cls我创建了一个名为如下的类文件：
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{class}
\LoadClass{report}
\RequirePackage{indentfirst}
\DeclareOption{withinchap}{%
\newcounter{exercounter}[chapter]%
\setcounter{exercounter}{0}}
\DeclareOption{withinsec}{%
\newcounter{exercounter}[section]%
\setcounter{exercounter}{0}}
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{report}}
\ExecuteOptions{withinchap}
\ProcessOptions\relax
\newenvironment{exercise}%
{\stepcounter{exercounter}\par\textbf{\theexercounter.}}%
{\ignorespacesafterend}
我想要实现的是设置一对类选项withinchap(default) 和withinsec。如果使用withinchap(default) 选项，则exercounter在开始新章节时将练习计数器重置为零。如果使用withinsec选项，则在开始新部分时将练习计数器exercounter重置为零。但是当我withinsec在文件中使用选项时test.tex，如下所示：
\documentclass[withinsec]{class}
\usepackage{showframe}
\begin{document}
\chapter{AAAAA}
\section{aaaaa}
\begin{exercise}
This is an exercise.
\end{exercise}
\begin{exercise}
This is another exercise.
\end{exercise}
\begin{exercise}
This is the third exercise.
\end{exercise}
\section{bbbbb}
\begin{exercise}
This is the fourth exercise in chapter one.
\end{exercise}
\begin{exercise}
This is the fifth exerecise in chapter one.
\end{exercise}
\end{document}
在 Windows 终端中运行后pdflatex test，我收到一条错误消息
! LaTeX Error: Command \c@exercounter already defined.
Or name \end... illegal, see p.192 of the manual.
See the LaTeX manual or LaTeX Companion for explanation.
Type H <return> for immediate help.
...
l.18 \ProcessOptions\relax
?
! Emergency stop.
...
l.18 \ProcessOptions\relax
! ==> Fatal error occurred, no output PDF file produced!
Transcript written on test.log.
Type H <return> for immediate help.
...
l.18 \ProcessOptions\relax
?
我想知道错误的原因。
答案1
你是重复\newcounter{exercounter}多次。您可以改用\counterwithin。
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{class}
\LoadClass{report}
\RequirePackage{indentfirst}
\newcounter{exercounter}
\DeclareOption{withinchap}{\counterwithin{exercounter}{chapter}}
\DeclareOption{withinsec}{\counterwithin{exercounter}{section}}
%\DeclareOption*{\PassOptionsToClass{\CurrentOption}{report}}
\ExecuteOptions{withinchap}
\ProcessOptions\relax
\newenvironment{exercise}%
{\stepcounter{exercounter}\par\textbf{\theexercounter.}}%
{\ignorespacesafterend}
withinchap使用选项输出
withinsec使用选项输出
请注意，类代码中的注释行没有任何作用，因为该类report已经被加载。
为了向其传递选项，您需要延迟代码。
\NeedsTeXFormat{LaTeX2e}
\ProvidesClass{class}
\newcounter{exercounter}
\DeclareOption{withinchap}{\def\exercounter@within{chapter}}
\DeclareOption{withinsec}{\def\exercounter@within{section}}
\DeclareOption*{\PassOptionsToClass{\CurrentOption}{report}}
\ExecuteOptions{withinchap}
\ProcessOptions\relax
\LoadClass{report} % should go after processing options
\RequirePackage{indentfirst}
\newcounter{exercounter}
\counterwithin{exercounter}{\exercounter@within}
\newenvironment{exercise}%
{\stepcounter{exercounter}\par\textbf{\theexercounter.}}%
{\ignorespacesafterend}
如果您只想要锻炼号码，请使用\counterwithin*。