Index shouldn't start new page

Question

I am writing an article in LaTeX 2e with \documentclass[twocolumn]{article}, including an index. The document prelude includes \usepackage{makeidx} and the \makeindex command. At the point where I want the index to print I say \printindex. The indexing operations all work perfectly. However, when the document is formatted, a page break is inserted both before and after the index. I want to eliminate both page breaks. I assume that \printindex calls \include, which performs \clearpage, though I don't really know that. How can I eliminate the two page breaks?

Answer 1

The theindex environment is defined in article.cls as follows:

\newenvironment{theindex}
               {\if@twocolumn
                  \@restonecolfalse
                \else
                  \@restonecoltrue
                \fi
                \twocolumn[\section*{\indexname}]%
                \@mkboth{\MakeUppercase\indexname}%
                        {\MakeUppercase\indexname}%
                \thispagestyle{plain}\parindent\z@
                \parskip\z@ \@plus .3\p@\relax
                \columnseprule \z@
                \columnsep 35\p@
                \let\item\@idxitem}
               {\if@restonecol\onecolumn\else\clearpage\fi}

The line

\twocolumn[\section*{\indexname}]

forces a new page (\twocolumn does it) and the line

\if@restonecol\onecolumn\else\clearpage\fi

also forces a new page (\onecolumn and \clearpage handle this).

To avoid this behaviour you can redefine the theindex environment; since you are using the twocolumn class option, this redefinition can be done by adding the following lines to the preamble of your document:

\makeatletter
\renewenvironment{theindex}
               {\section*{\indexname}%
                \@mkboth{\MakeUppercase\indexname}%
                        {\MakeUppercase\indexname}%
                \thispagestyle{plain}\parindent\z@
                \parskip\z@ \@plus .3\p@\relax
                \columnseprule \z@
                \columnsep 35\p@
                \let\item\@idxitem}
               {}
\makeatother

A little working example:

\documentclass[twocolumn]{article}
\usepackage{makeidx}

\makeatletter
\renewenvironment{theindex}
               {\section*{\indexname}%
                \@mkboth{\MakeUppercase\indexname}%
                        {\MakeUppercase\indexname}%
                \thispagestyle{plain}\parindent\z@
                \parskip\z@ \@plus .3\p@\relax
                \columnseprule \z@
                \columnsep 35\p@
                \let\item\@idxitem}
               {}
\makeatother

\makeindex

\begin{document}

A\index{A}B\index{B}
\printindex
some text

\end{document}

which produces

Answer 2

The answer of Gonzalo Medina is great for documents which use the twocolumn class-option (as requested by the OP; already gave +1 for Gonzalo Medina's answer).

For "overkill" (http://tex.blogoverflow.com/2011/10/overkill-is-a-good-thing/; and because this is way too long for a comment and comments cannot be formated like this) it is possible to handle this for one- and two-column documents (I had this problem for a one-column document and was able to solve it because of Gonzalo Medina's answer):

\documentclass{article}
%\documentclass[twocolumn]{article}
% both work
\usepackage{makeidx}

\makeatletter

% * define a `\twoidxcolumn` based on `\twocolumn`:
\def\twoidxcolumn{%
%\clearpage
\global\columnwidth\textwidth
\global\advance\columnwidth-\columnsep
\global\divide\columnwidth\tw@
\global\hsize\columnwidth
\global\linewidth\columnwidth
\global\@twocolumntrue
\global\@firstcolumntrue
\col@number \tw@
%\@ifnextchar [\@topnewpage
\@floatplacement
}


% * define a `\oneidxcolumn` based on `\onecolumn`:
\def\oneidxcolumn{%
%\clearpage
\global\columnwidth\textwidth
\global\hsize\columnwidth
\global\linewidth\columnwidth
\global\@twocolumnfalse
\col@number \@ne
\@floatplacement}


% * replace `\twocolumn` in `theindex` by `\twoidxcolumn`, remove the `[` `]`, 
% and replace `\if@restonecol\onecolumn\else\clearpage\fi` by 
% `\if@restonecol\oneidxcolumn\else\relax\fi`:
\renewenvironment{theindex}
  {\if@twocolumn
     \@restonecolfalse
   \else
     \@restonecoltrue
   \fi
   \twoidxcolumn \section*{\indexname}%
   \@mkboth{\MakeUppercase\indexname}%
           {\MakeUppercase\indexname}%
   \thispagestyle{plain}\parindent\z@
   \parskip\z@ \@plus .3\p@\relax
   \columnseprule \z@
   \columnsep 35\p@
   \let\item\@idxitem}
  {\if@restonecol\oneidxcolumn\else\relax\fi}
\makeatother

\makeindex

\begin{document}

A\index{A} B\index{B} Lorem ipsum dolor sit amet, consectetuer adipiscing elit. 
Ut purus elit, vestibulum ut, placerat ac, adipiscing vitae, felis. 
\par\printindex 
% Note the `\par` before `\printindex`! Without this, the paragraph
% before `\printindex` will be in twocolumn mode, too. 
% (An empty line before `\printindex` works, too, of course.)
Curabitur dictum gravida mauris. Nam arcu libero, nonummy eget, consectetuer id, 
vulputate a, magna. Donec vehicula augue eu neque. Pellentesque habitant morbi 
tristique.

\end{document}