[Tex/LaTex] 3 Algorithms in the same column with algorithmic

algorithmsfloats

I have 3 algorithms that I want to be displayed in the same column of a 2-column document. They are all slight variations of each other, so it is important that they be displayed right next to each other for easy comparison.

The problem is that I can't figure out a way to place all three in the column. I've tried putting them inside a figure environment, but then I get the error "Not in outer par mode." Using another algorithm package is not an option for me.

Best Answer

The main problem here is using floats in a twocolumn document. Although it is possible, your needs do not conform to their purpose. Instead, the caption package removes the float-y-ness of floats, allowing you to specify a caption in the text where it is called.

The following MWE uses algorithmicx's algpseudocode to display an algorithm with slight variations across the three. The main interface for non-floating captions using caption is

\captionof{<float env>}{<caption>}

Since I loaded algorithm, <float env> can be algorithm to obtain the appropriate caption.

enter image description here

\documentclass[twocolumn]{article}
\usepackage[margin=1in]{geometry}% http://ctan.org/pkg/geometry
\usepackage{algorithm,algpseudocode,lipsum}% http://ctan.org/pkg/{algorithms,algorithmicx,lipsum}
\usepackage{caption}% http://ctan.org/pkg/caption
\begin{document}
\lipsum[87]
\captionof{algorithm}{Euclid’s algorithm A}\label{euclidA}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_1\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile1}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}

\captionof{algorithm}{Euclid’s algorithm B}\label{euclidB}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_2\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile2}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}

\captionof{algorithm}{Euclid’s algorithm C}\label{euclidC}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_3\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile3}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}

\lipsum[1-5]
\end{document}

geometry, lipsum were just used for layout and dummy text.

There's also the stand-alone capt-of package which does something similar, as well as ccaption.

Keeping things together in a float-like environment is possible. Wrap the set of algorithms in a figure (yes, a figure, or table) environment while still using \captionof{algorithm}, or just place them all in an algorithm environment with a \caption for each. The following shows the former:

enter image description here

\documentclass[twocolumn]{article}
\usepackage[margin=1in]{geometry}% http://ctan.org/pkg/geometry
\usepackage{algorithm,algpseudocode,lipsum}% http://ctan.org/pkg/{algorithms,algorithmicx,lipsum}
\usepackage{caption}% http://ctan.org/pkg/caption
\begin{document}
\lipsum[1-2]
\begin{figure}[t]
\captionof{algorithm}{Euclid’s algorithm A}\label{euclidA}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_1\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile1}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}

\captionof{algorithm}{Euclid’s algorithm B}\label{euclidB}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_2\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile2}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}

\captionof{algorithm}{Euclid’s algorithm C}\label{euclidC}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{GCD of $a$ and $b$}
    \State $r_3\gets a\bmod b$
    \While{$r\not=0$}\Comment{$r=0\mapsto\mbox{answer}$}
    \State $a\gets b$
    \State $b\gets r$
    \State $r\gets a\bmod b$
    \EndWhile\label{euclidendwhile3}
  \State \textbf{return} $b$\Comment{The GCD is $b$}
  \EndProcedure
\end{algorithmic}
\end{figure}

\lipsum[1-5]
\end{document}

You may have to adjust the space between the algorithmic and \captionof to suit your needs.

Some not-so-optimal approaches

One attempt at keeping algorithms together would be to use the same float specifier for both floats:

%...
\begin{algorithm}[t]% First algorithm
  %...
\end{algorithm}
\begin{algorithm}[t]% Second algorithm
  %...
\end{algorithm}
%...

LaTeX will ensure that they follow in sequence since the floats are managed by a list that is flushed in a FIFO (first-in-first-out) manner. However, this is very unreliable since float placement is dependent on the number of floats already on the page, as well as the remaining space on a page. So, algorithms could be broken fairly easily in this way.

Another attempt would be to use the H float specifier supplied by the float package. This float specifier virtually removed the float-ness of a float and places it in the text where the environment is called:

%...
\begin{algorithm}[H]% First algorithm
  %...
\end{algorithm}
\begin{algorithm}[H]% Second algorithm
  %...
\end{algorithm}
%...

Although this works pretty well, you lose the mobility offered by a float, so algorithms will appear where you put them. Moreover, this approach is not perfect around page boundaries, where the two algorithms could be separated. Moreover, the effects could be worse if using the H float specifier for some algorithms and other (regular) float specifiers for other non-stick-together algorithms. Here is a small example that illustrates this problematic instance:

\documentclass{article}
\usepackage{algorithm}% http://ctan.org/pkg/algorithm
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\usepackage{float}% http://ctan.org/pkg/float
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\begin{document}

\listofalgorithms

\section{First section}

\lipsum[1-2]

\begin{algorithm}[H]
  \caption{First algorithm}\label{first-alg}%
  \begin{algorithmic}[1]
    \Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
      \State $r\gets a\bmod b$
      \While{$r\not=0$}\Comment{We have the answer if r is 0}
        \algstore{euclidA}
  \end{algorithmic}
\end{algorithm}
\begin{algorithm}[H]
  \caption{Second algorithm}\label{second-alg}
  \begin{algorithmic}[1]
        \algrestore{euclidA}
        \State $a\gets b$
        \State $b\gets r$
        \algstore{euclidB}
  \end{algorithmic}
\end{algorithm}

\lipsum[2]

\section{Second section}

\lipsum[3]

\begin{algorithm}[t]
  \caption{Third algorithm}\label{third-alg}
  \begin{algorithmic}[1]
        \algrestore{euclidB}
        \State $r\gets a\bmod b$
      \EndWhile\label{euclidendwhile}
      \State \textbf{return} $b$\Comment{The gcd is b}
    \EndProcedure
  \end{algorithmic}
\end{algorithm}

\lipsum[4]

\end{document}

The example has been chosen specifically to break the two algorithms that are supposed to stick together across the first page boundary, while the third algorithm is left to float. As such, the sequence of algorithms displayed in the output is jumbled: Algorithm 1 is followed by Algorithm 3, which is followed by Algorithm 2:

Not only that, as can be seen from the output (click to enlarge), the order in the "List of Algorithms" is also incorrect.

A slightly manual (but functional) approach

The idea is to use an existing float (like figure) as a float wrapper and define some commands that mimic the captioning and typesetting behaviour of the algorithm package. It is perfectly fine to use a figure as a float wrapper, just as long as there is no "offical \caption" used inside (which will typeset a figure caption, and would eventually make its way into the LoF).

I've decided to use the ruled float style of algorithm. Additionally, I've chosen algpseudocode (from the algorithmicx bundle) for typesetting the algorithm within an algorithmic environment. Similar functionality is provided by the algorithmic package (also from the algorithms bundle), so the example is transferable^*.

In order to mimic the captioning style of ruled, a new macro \algcaption[<LoA>]{<caption>} should be used. This typesets a thick 0.8pt rule (\toprule), followed by the caption (flush left), followed by another rule (\midrule), taken directly from the float package. The algorithmic environment should be finished off with a \bottomrule, making the approach very much tabular-like.

\documentclass{article}
\usepackage{algorithm}% http://ctan.org/pkg/algorithm
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{ifmtarg}% http://ctan.org/pkg/ifmtarg
\makeatletter
\newcommand{\listofalgorithms}{\listof{algorithm}{List of Algorithms}}%
\newcommand{\toprule}{\hrule height.8pt depth0pt \kern2pt} % Caption top horizontal rule+skip
\newcommand{\midrule}{\kern2pt\hrule\kern2pt} % Caption bottom (or mid) horizontal rule+skip
\newcommand{\bottomrule}{\kern2pt\hrule\relax}% Algorithm bottom rule
\newcommand{\algcaption}[2][]{%
  \refstepcounter{algorithm}%
  \@ifmtarg{#1}
    {\addcontentsline{loa}{figure}{\protect\numberline{\thealgorithm}{\ignorespaces #2}}}
    {\addcontentsline{loa}{figure}{\protect\numberline{\thealgorithm}{\ignorespaces #1}}}%
  \toprule
  \textbf{\fname@algorithm~\thealgorithm}\ #2\par % Caption
  \midrule
}
\makeatother
\begin{document}

\listofalgorithms

\section{First section}

\lipsum[1-3]

\begin{figure}[ht]
  \algcaption{First algorithm}\label{first-alg}%
  \begin{algorithmic}[1]
    \Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
      \State $r\gets a\bmod b$
      \While{$r\not=0$}\Comment{We have the answer if r is 0}
        \algstore{euclidA}
  \end{algorithmic}
  \bottomrule
  
  \bigskip

  \algcaption{Second algorithm}\label{second-alg}
  \begin{algorithmic}[1]
        \algrestore{euclidA}
        \State $a\gets b$
        \State $b\gets r$
        \algstore{euclidB}
  \end{algorithmic}
  \bottomrule
\end{figure}

\lipsum[2]

\section{Second section}

\lipsum[3]

\begin{algorithm}[t]
  \caption{Third algorithm}\label{third-alg}
  \begin{algorithmic}[1]
        \algrestore{euclidB}
        \State $r\gets a\bmod b$
      \EndWhile\label{euclidendwhile}
      \State \textbf{return} $b$\Comment{The gcd is b}
    \EndProcedure
  \end{algorithmic}
\end{algorithm}

\lipsum[4]

\end{document}

Algorithms meant to stick together this way are contained within the same wrapper float (figure in this case) and are separated using \bigskip (which roughly match the space between other floats). Using float to create an algorithm float (rather than using the algorithm package) is merely so one can tap into things like a \listofalgorithms as well as an algorithm counter.

Additional packages used in the examples include lipsum (not required) for dummy text and the ifmtarg package (required) to check for an empty argument to \algcaption.

^* The algorithmicx bundle provides more advanced features, including the possibility to store/restore algorithmic settings (that were used in the above examples), which is not offered by that of the algorithms bundle.

[Tex/LaTex] Formatting algorithms to gain place in the document

This a a slight modification of How to put algorithm and figure(s) side by side?

Here is a minimal example showing how to pair algorithms side-by-side:

enter image description here

\documentclass{llncs}% http://www.springer.com/computer/lncs/lncs+authors?SGWID=0-40209-0-0-0
\usepackage[margin=1in]{geometry}% http://ctan.org/pkg/geometry
\usepackage{lipsum}% http://ctan.org/pkg/lipsum
\usepackage{algorithm}% http://ctan.org/pkg/algorithm
\usepackage{algpseudocode}% http://ctan.org/pkg/algorithmicx
\usepackage[compatibility=false]{caption}% http://ctan.org/pkg/caption
\begin{document}
\lipsum[1]

\medskip

\noindent\begin{minipage}{.5\textwidth}
\captionof{algorithm}{Euclid’s algorithm}\label{algo1}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
    \State $r\gets a\bmod b$
    \While{$r\not=0$}\Comment{We have the answer if r is 0}
      \State $a\gets b$
      \State $b\gets r$
      \State $r\gets a\bmod b$
    \EndWhile
    \State \textbf{return} $b$\Comment{The gcd is b}
  \EndProcedure
\end{algorithmic}
\end{minipage}%
\begin{minipage}{.5\textwidth}
\captionof{algorithm}{Euclid’s algorithm}\label{algo2}
\begin{algorithmic}[1]
  \Procedure{Euclid}{$a,b$}\Comment{The g.c.d. of a and b}
    \State $r\gets a\bmod b$
    \While{$r\not=0$}\Comment{We have the answer if r is 0}
      \State $a\gets b$
      \State $b\gets r$
      \State $r\gets a\bmod b$
    \EndWhile
    \State \textbf{return} $b$\Comment{The gcd is b}
  \EndProcedure
\end{algorithmic}
\end{minipage}

\medskip

On the left is Algorithm~\ref{algo1}. On the right is Algorithm~\ref{algo2}.

\lipsum[2]

\end{document}

The idea behind this approach is to have two minipages spanning the entire \linewidth. These minipages then house the non-floating algorithmic environments, as well as the non-floating \captionof caption. If need be, you can wrap the entire double minipage inside a floating algorithm environment.

The use of geometry was just to gain some stock real estate (may not be required in your instance), while lipsum provided some dummy text, Lorem ipsum style.

caption provides the means to have a caption outside of a float via \captionof. However, it requires the compatibility=false option to work, since llncs already redefines \caption - detected by caption. algorithm is still required, since it provides the algorithm counter and "List of Algorithms" capability.

Best Answer

Related Solutions

[Tex/LaTex] How to format multiple algorithms so that they appear together

Some not-so-optimal approaches

A slightly manual (but functional) approach

[Tex/LaTex] Formatting algorithms to gain place in the document

Related Question