Separate styles for section headers in table of contents and body

Question

I'm looking to have different 'styles' (not sure if this is the right word) for my section headers depending on whether they're listed in the TOC or the body. That's probably not clear, so let me demonstrate.

In the body, I'd like to see:

Article I. Intro 
....
Article II. Methods
....

But in the TOC I'd like to see

I. Intro .......... 5
II. Methods ........10

without the "Article" bit.

Right now, I'm using this to set the section headers:

\renewcommand{\thesection}{Article \Roman{section}.}

I should mention I'm using the article class.

Answer 1

The titlesec package was used to add "Article" before the section number and the dot after the number. The tocloft package was used to add the dot after the number in the table of contents entries and also to increase the space reserved for the number:

\documentclass{article}
\usepackage{titlesec}
\usepackage{tocloft}

\renewcommand\thesection{\Roman{section}}
\titleformat{\section}
  {\normalfont\Large\bfseries}{Article~\thesection.}{1em}{}
\renewcommand\cftsecaftersnum{.}
\addtolength\cftsecnumwidth{1em}

\begin{document}

\tableofcontents
\section{Introduction}
\section{Methods}

\end{document}

Answer 2

You could use the method described on p. 26f. of "The LaTeX Companion" to generate the following output:

\documentclass{article}
\renewcommand{\thesection}{\Roman{section}}
\makeatletter
%% The "\@seccntformat" command is an auxiliary command
%% (see pp. 26f. of 'The LaTeX Companion,' 2nd. ed.)
\def\@seccntformat#1{\@ifundefined{#1@cntformat}%
  {\csname the#1\endcsname\ }%   default
  {\csname #1@cntformat\endcsname}% enable individual control
}
%% Put everything together by defining the macros 
%% '\...@seccntformat', where ... can be 'section', 
%% 'subsection', etc.
\newcommand{\section@cntformat}{Article \thesection. }
\makeatother

\begin{document}
\tableofcontents
\section{Intro}
\section{Methods}
\end{document}

---

Addendum: You've probably noticed that this method does not insert a period (full stop) after the Roman section number in the table of contents. (I'm not a fan of providing punctuation marks needlessly...) If you do require the dots following the Roman section numbers in the ToC, be sure load the tocloft package and issue the command \renewcommand\cftsecaftersnum{.} (in the preamble, of course). And, if you need the line of dots between the ToC-style section headers and the associated page numbers, you should issue the command \renewcommand\cftsecleader{\cftdotfill{\cftdotsep}} after loading the tocloft package.