\titleformat* Problem

Question

I used this and trying to put underline as shown here

This works alone perfectly but whenever I try to add the line

\titleformat*{\section}{\bfseries\sectionfont}{\thesubsection}{1em}{#1}[{\titlerule[0.8pt]}]

instead of

\titleformat*{\section}{\bfseries\sectionfont} it says

Missing \begin{document}. ...ion}{\bfseries\sectionfont}{\thesubsection

\documentclass[12pt,a4paper]{article}

\usepackage[explicit]{titlesec} % control over title section
\usepackage{fontspec}       % change to system font
\usepackage{anyfontsize}    % changing font size any
\usepackage{graphicx}   % import figure
\usepackage{setspace}   % line gap => \setstretch
\usepackage{parskip}    % linegap => \linespread
\usepackage{gensymb}    % degree symbol
\usepackage{ragged2e}   % Text alignment
\usepackage{xcolor} % changing color
\usepackage{sectsty}    % underline section

\definecolor{Nblack}{rgb}{0,0,0}
\newfontfamily\sectionfont[Color=Nblack,Scale=1.34]{Arial}
\newfontfamily\subsectionfont[Color=Nblack,Scale=1.34]{Garton Medium}
\newfontfamily\subsubsectionfont[Color=Nblack,Scale=1.34]{Garton Medium}

\titleformat{\section}{\bfseries\sectionfont}{\thesubsection}{1em}{#1}[{\titlerule[0.8pt]}]
\titleformat{\subsection}{\subsectionfont}
\titleformat{\subsubsection}{\itshape\subsubsectionfont}

\begin{document}
\section{Introduction}
\end{document}

or if I remove [explicit] and add * after \titleformat it gives

Illegal parameter number in definition of \ttlf@section. ...esubsection}{1em}{#1}[{\titlerule[0.8pt]}]

Also, like to have First Word Big caps and others small caps

Answer 1

The syntax of \titleformat and \titleformat* is very different; the latter just takes two arguments, the former is more complex.

One rarely needs explicit: the last mandatory argument to \titleformat can end with a one-parameter macro, as in the example below.

Beware also that titlesec and sectsty are incompatible with each other. Since you need the former for its advanced features, don't load sectsty.

\documentclass[12pt,a4paper]{article}

\usepackage{fontspec}       % change to system font
\usepackage{titlesec} % control over title section

\ExplSyntaxOn
\NewDocumentCommand{\firstword}{m}
 {
  \seq_set_split:Nnn \l_tmpa_seq {~} {#1}
  \seq_pop_left:NN \l_tmpa_seq \l_tmpa_tl
  \seq_put_left:Nn \l_tmpa_seq {\MakeUppercase{\l_tmpa_tl}}
  \seq_use:Nn \l_tmpa_seq {~}
 }
\ExplSyntaxOff

\newfontfamily\sectionfont{TeX Gyre Heros}
\newfontfamily\subsectionfont{TeX Gyre Heros}

\titleformat{\section}
  {\bfseries\scshape\sectionfont}
  {\thesection}
  {1em}
  {}
  [{\titlerule[0.8pt]}]
\titleformat*{\subsection}{\subsectionfont}
\titleformat*{\subsubsection}{\itshape\subsectionfont}

\begin{document}
\section{Introduction and something else}
\subsection{Subsection}
\subsubsection{Subsubsection}
\end{document}

I used fonts I have on my system (and I recommend you not to use Garton).

The \firstword macro splits the argument at spaces, isolates the first item, encloses it into \MakeUppercase and finally reinstates it for printing all items separated by spaces.