[Tex/LaTex] algorithm2e Command \algorithm already defined

Question

I want to draw vertical line by using \SetAlgoLined, but LaTeX told me Undefined control sequence.

Then I add \usepackage{algorithm2e} to the end of my preamble. Then it said Command\algorithmalready defined.

\usepackage{graphicx}  
\usepackage{xthesis}  
\usepackage{xtocinc}  
\usepackage{mystyle}  
\usepackage{url}  
\usepackage{subfigure}  
\usepackage{booktabs}  
\usepackage{multirow}  
\usepackage[printonlyused]{acronym}  
\usepackage{algorithm} 
\usepackage{algorithmic}  
\usepackage{float}  
\usepackage{epstopdf}  
\usepackage{amssymb,amsmath}  
\usepackage{graphicx,epsfig}  
\usepackage{multicol}  
\usepackage{ifthen}  
\usepackage{algorithm2e}  

I'm using Window 7 + TexWorks. I have another paper works perfect with algorithm2e and \SetAlgoLined:

\usepackage{cite} 
\usepackage{url}  
\usepackage{ifthen} 
\usepackage{multicol}  
\usepackage[utf8]{inputenc} 
\usepackage{graphicx}
\usepackage{graphicx,epsfig}
\usepackage{amssymb,amsmath}
\usepackage{subfigure}
\usepackage{epstopdf}
\usepackage{float}
\usepackage{algorithm2e}
\usepackage{multirow}  

Just had no idea why it happened? Please help.

Answer 1

Since the algorithm and algorithm2e packages both define an algorithm environment, simply loading both with no extra precaution will cause a name clash like you experienced.

However, you can still use both packages (if you really need both), but you need to pass the option algo2e to algorithm:

\usepackage[algo2e]{algorithm2e} 

This option changes the name of the environment algorithm from the algorithm2e package into algorithm2e and so avoids the conflict with the package which already define an algorithm environment; the option also changes the command name for the list of algorithms to \listofalgorithmes.

A complete example:

\documentclass{article}
\usepackage{algorithm} 
\usepackage{algorithmic}  
\usepackage[algo2e]{algorithm2e} 

\begin{document}

\begin{algorithm}%>- from algorithm package
test
\end{algorithm}

\begin{algorithm2e}%>- from algorithm2e package
test
\end{algorithm2e}

\end{document}