\lccode to be activate in latex command

Question

Why does \lccode`~=`, with an active , work only in an environment definition but not in a LaTeXcommand? We are using an active , to produce bullets:

Here is an MWE:

\documentclass{book}
\usepackage{color}
\usepackage{xparse}
\makeatletter
\def\keybullet{\textbullet}
\newenvironment{keywords}{\bgroup\sloppy   \begingroup\lccode~=, \lowercase{\endgroup\def~}{${}\hbox{\ \keybullet\ }{}$}   \catcode`,=\active \noindent\textbf{Keywords}\hskip1em}{\egroup\par\addvspace{\baselineskip}}
\newcommand{\Keywords}[1]{\sloppy   \begingroup\lccode~=, \lowercase{\endgroup\def~}{${}\hbox{\ \keybullet\ }{}$}   \catcode`,=\active \noindent\textbf{Keywords}\hskip1em#1\par\addvspace{\baselineskip}}
\makeatother
\begin{document}
\begin{keywords} Harmonic balance, Multi-degrees-of-freedom systems, Fundamental harmonic, Higher harmonics, Energy method \end{keywords}
\Keywords{Harmonic balance, Multi-degrees-of-freedom systems, Fundamental harmonic, Higher harmonics, Energy method}
\end{document}

Answer 1

You need to absorb the argument after the setting of the category code, otherwise the commas would be tokenized when the category code has not yet been changed.

\documentclass{book}
\newcommand{\keybullet}{\unskip\space\textbullet\space\ignorespaces}
\newcommand{\activecommabullet}{%
  \begingroup\lccode~=,\lowercase{\endgroup\let~\keybullet}%
}
\newenvironment{keywords}
 {%
  \par\sloppy\activecommabullet\catcode`,=\active
  \noindent\textbf{Keywords}\quad
 }
 {\par\addvspace{\baselineskip}}
\newcommand{\Keywords}{%
  \par\sloppy
  \activecommabullet
  \catcode`,=\active
  \noindent\textbf{Keywords}\quad
  \Keywordsaux
}
\newcommand{\Keywordsaux}[1]{#1\par\addvspace{\baselineskip}}
\begin{document}
\begin{keywords}
Harmonic balance, Multi-degrees-of-freedom systems, Fundamental harmonic, 
Higher harmonics, Energy method
\end{keywords}
\Keywords{Harmonic balance, Multi-degrees-of-freedom systems, 
  Fundamental harmonic, Higher harmonics, Energy method}
\end{document}

A simpler implementation with expl3, that needs no category code change.

\documentclass{book}
%\usepackage{xparse} % not needed for LaTeX 2020-10-01 or later
\NewDocumentCommand{\keybullet}{}{ \textbullet\ }
\ExplSyntaxOn
\NewDocumentEnvironment{keywords}{b}
 {
  \rajesh_keywords:n { #1 }
 }
 {}
\NewDocumentCommand{\Keywords}{m}
 {
  \rajesh_keywords:n { #1 }
 }
\cs_new_protected:Nn \rajesh_keywords:n
 {
  \par\sloppy\noindent\textbf{Keywords}\quad
  \clist_set:Nn \l_tmpa_clist { #1 }
  \clist_use:Nn \l_tmpa_clist { \keybullet }
  \par\addvspace{\baselineskip}
 }
\ExplSyntaxOff
\begin{document}
\begin{keywords}
Harmonic balance, Multi-degrees-of-freedom systems, Fundamental harmonic, 
Higher harmonics, Energy method
\end{keywords}
\Keywords{Harmonic balance, Multi-degrees-of-freedom systems, 
  Fundamental harmonic, Higher harmonics, Energy method}
\end{document}