two-column
I need to write a text just in this format
\documentclass[twocolumn]{article}.
But I need to switch between single column and double column several times in the same page, without jumping to a new page every time I switch the format. Is this possible without using any package like \usepackage{multicol} ?
Here's one possible solution using the twocolumn class option and figure* and two minipages to typeset the listings (since figure* was used, the listings will appear on the top of the next page):
\documentclass[twocolumn]{article}
\usepackage{listings}
\usepackage{lipsum}
\begin{document}
\lipsum[1-6]
\begin{figure*}
\begin{minipage}[t]{0.5\textwidth}
\begin{lstlisting}
if (i<=0) then i := 1;
if (i>=0) then i := 0;
if (i<>0) then i := 0;
\end{lstlisting}
\end{minipage}%
\begin{minipage}[t]{0.5\textwidth}
\begin{lstlisting}
if (i<=0) then i := 1;
if (i>=0) then i := 0;
\end{lstlisting}
\end{minipage}%
\caption{Two side-by-side listings}
\label{fig:listings}
\end{figure*}
\lipsum[1-6]
\end{document}
And here's now another option: instead of using the twocolumn document option, the multicol package is used to produce the text in two-column mode; the multicols environment is ended, the side-by-side listings are typeset using simple minipages (the caption package was used to produce a possible caption using \captionof), and then another multicols environment begins:
\documentclass{article}
\usepackage{multicol}
\usepackage{listings}
\usepackage{caption}
\usepackage{lipsum}
\begin{document}
\begin{multicols}{2}
\lipsum[1-2]
\end{multicols}
\noindent\begin{minipage}{\textwidth}
\begin{minipage}[t]{0.5\textwidth}
\begin{lstlisting}
if (i<=0) then i := 1;
if (i>=0) then i := 0;
if (i<>0) then i := 0;
\end{lstlisting}
\end{minipage}%
\begin{minipage}[t]{0.5\textwidth}
\begin{lstlisting}
if (i<=0) then i := 1;
if (i>=0) then i := 0;
\end{lstlisting}
\end{minipage}%
\captionof{figure}{Two side-by-side listings}
\label{fig:listings}
\end{minipage}
\begin{multicols}{2}
\lipsum[1-2]
\end{multicols}
\end{document}
In this last option, the usual restriction of multicol with floats apply.
With help of the package stfloats you can put figure on the bottom of the same page where is inserted (if there is enough space):
\documentclass[twocolumn]{article}
\usepackage{graphicx}
\usepackage{stfloats} % <---
\begin{document}
\section{Omitted Figures}
\begin{figure*}[b]
\includegraphics{example-image-a}
\end{figure*}
\end{document}
On such a way you can insert more figures on the same page. If for figures you need more pages, than for the figure which will appear on the next page change placement option to [t]
Edit: As I mentioned in my comment below, standard LaTeX enable only 70% of page to contain floats. Other space is reserved for a text. If you have only figures, you need to change this settings, for example as in the next example:
\documentclass[twocolumn]{article}
\usepackage{graphicx}
\usepackage{stfloats}
\renewcommand{\textfraction}{0.09}
\renewcommand{\dbltopfraction}{0.9}
\renewcommand{\bottomfraction}{0.9}
%---------------- show page layout. don't use in a real document!
\usepackage{showframe}
\renewcommand\ShowFrameLinethickness{0.15pt}
\renewcommand*\ShowFrameColor{\color{red}}
%---------------------------------------------------------------%
\begin{document}
\section{Omitted Figures}
\begin{figure*}[b]
\centering
\includegraphics[height=0.35\textheight]{example-image-a}
\caption{First figure in appendix}
\end{figure*}
\begin{figure*}[b]
\centering
\includegraphics[height=0.35\textheight]{example-image-b}
\caption{Second figure in appendix}
\end{figure*}
\end{document}
Of course this works, if the pictures in figure floats are not to height. Their sum had to be smaller than about 70% of text height.
(red lines indicate page layout)
Best Answer
You should look into using the multicol package and its
multicolsenvironment. For instance, the following code produces a page that starts in single-column mode, switches to two-column mode, and finally switches back to single-column mode.