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I am using the algorithm and algpseudocode packages. When I define an algorithm it ends with a horizontal line. I would like to have some text below this horizontal line explaining some special case scenarios. I've seen this used in various books. How do I put text below this horizontal line?

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Are you talking about a caption? Within the algorithm environment, just provide a \caption{my text here}. –  Sean Allred Apr 6 at 18:13
    
That puts the text the top of the algorithm beside the algorithm name, I want it under the line at the end of the algorithm. –  sonicboom Apr 6 at 18:25

2 Answers 2

I've defined a new command \algcomment to be used just after the algorithm environment.

\newcommand{\algcomment}[1]{%
    \vspace{-\baselineskip}%
    \noindent%
    {\footnotesize #1\par}%
    \vspace{\baselineskip}%
    }

The only limitation is that you cannot allow the algorithm to float.

MWE:

\documentclass{article}

\usepackage{algorithm}
\usepackage{algpseudocode}

\usepackage{lipsum} % just for the example

\newcommand{\algcomment}[1]{%
    \vspace{-\baselineskip}%
    \noindent%
    {\footnotesize #1\par}%
    \vspace{\baselineskip}%
    }

\begin{document}
\lipsum[1-2]
\begin{algorithm}[H]
    \begin{algorithmic}[1]
        \For{$x<10$}
            \State $x\gets x+1$
        \EndFor
    \end{algorithmic}
    \caption{My algorithm}
\end{algorithm}
\algcomment{This algorithm talks about}
\lipsum[1-2]
\end{document} 

Output:

enter image description here

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This is another option, removing the limitation in Karl Koeller's answer (the environment can freely float if desired, and \algcomment can be used inside the algorithm environment):

\documentclass{article}s
\usepackage{etoolbox}
\usepackage{algorithm}
\usepackage{algpseudocode}
\usepackage{lipsum} % just for the example

\makeatletter
\AfterEndEnvironment{algorithm}{\let\@algcomment\relax}
\AtEndEnvironment{algorithm}{\kern2pt\hrule\relax\vskip3pt\@algcomment}
\let\@algcomment\relax
\newcommand\algcomment[1]{\def\@algcomment{\footnotesize#1}}

\renewcommand\fs@ruled{\def\@fs@cfont{\bfseries}\let\@fs@capt\floatc@ruled
  \def\@fs@pre{\hrule height.8pt depth0pt \kern2pt}%
  \def\@fs@post{}%
  \def\@fs@mid{\kern2pt\hrule\kern2pt}%
  \let\@fs@iftopcapt\iftrue}
\makeatother

\begin{document}

Some test text.
\begin{algorithm}
  \algcomment{This algorithm talks about a really special procedure which will be described elsewhere in this document}
  \begin{algorithmic}[1]
    \For{$x<10$}
    \State $x\gets x+1$
    \EndFor
  \end{algorithmic}
  \caption{My algorithm}
\end{algorithm}
\lipsum[4]

\end{document}

enter image description here

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