A command which produces index if optional argument is not empty

Question

I would like to define a command which creates an index with a certain name, when i pass the name, and doesn't create any index if there is no name passed.

My first idea was to write it like that:

\newcommand{\kw}[1][]{\emph{#2}\index{#1}}

but the compiler gets stuch when I pass an empty optional argument. How can I build an if-then structure like the following?

\newcommand{\kw}[1][]{
 if(#1 is not empty)
  \emph{#2}\index{#1}}
 else
  \emph{#2}
}

Answer 1

If you don't want to use xparse (why not?), you can use some traditional tricks, for example:

\newcommand{\kw}[2][]{%
  \emph{#2}%
  \if&#1&\else
    \index{#1}%
  \fi
}

xparse's \IfValueT is safer in some cases. A safer implementation without LaTeX3 is:

\usepackage{etoolbox}
\newcommand{\kw}[2][]{%
  \emph{#2}%
  \ifstrempty{#1}{}{\index{#1}}%
}

Answer 2

You can use the xparse package from the LaTeX3 project.

\usepackage{xparse}
\NewDocumentCommand\kw{o}{%
  \IfNoValueTF{#1}{%
    % #1 is empty.
  }{%
    % #1 has a value.
  }%
}

To recreate your example command:

\NewDocumentCommand\kw{om}{%
  \IfNoValueTF{#1}{%
    \emph{#2}%
  }{%
    \emph{#2}\index{#1}%
  }%
}