LaTeX 可选参数

问题描述

指南中的示例：

\newcommand{\example}[2][YYY]{Mandatory arg: #2;
                                 Optional arg: #1.}

This defines \example to be a command with two arguments, 
referred to as #1 and #2 in the {<deFinition>}--nothing new so far. 
But by adding a second optional argument to this \newcommand 
(the [YYY]) the first argument (#1) of the newly defined 
command \example is made optional with its default value being YYY.

Thus the usage of \example is either:

   \example{BBB}
which prints:
Mandatory arg: BBB; Optional arg: YYY.
or:
   \example[XXX]{AAA}
which prints:
Mandatory arg: AAA; Optional arg: XXX.

解决方法

如何在 LaTeX 中创建带有可选参数的命令？就像是：

\newcommand{\sec}[2][]{
    \section*{#1
        \ifsecondargument
            and #2
        \fi}
    }
}

然后，我可以这样称呼它

\sec{Hello}
%Output: Hello
\sec{Hello}{Hi}
%Output: Hello and Hi