Frame for pieces of algorithms

Question

I would like to put frames around pieces of algorithms like in the following picture. If the background of the rectangle can be changed, it would be nice.

Here is the LaTeX code used for the algorithm.

\documentclass[10pt,a4paper]{article}
\usepackage[ruled]{algorithm2e}

\begin{document}

\begin{algorithm}[H]
    \caption{Le crible d'Eratosthène}

    \KwData{la liste $L$ des naturels compris de $2$ à $N$.}
    \KwResult{la liste $P$ des nombres premiers compris entre $2$ et $N$.}
    \BlankLine
    \For{$i = 2$ \KwTo $N$}{
        \If{$i$ n'est pas barré dans la liste $L$}{
            Barrer dans la liste $L$ les mutltiples $ki$ où $k \geq 2$.
        }
    }
    \Return{la liste $P$ des nombres non barrés.}
\end{algorithm} 

\end{document}

Answer 1

A job for \tikzmark; the \MyBox command allows to draw the colored frame; the syntax is

\MyBox[<vertical offset>]{<mark>}{<mark>}{<color>}

An example:

\documentclass[10pt,a4paper]{article}
\usepackage[ruled]{algorithm2e}
\usepackage{tikz}
\usetikzlibrary{calc,fit}

% code by Andrew Stacey
% http://tex.stackexchange.com/a/50054/3954    
\makeatletter
\tikzset{%
  remember picture with id/.style={%
    remember picture,
    overlay,
    save picture id=#1,
  },
  save picture id/.code={%
    \edef\pgf@temp{#1}%
    \immediate\write\pgfutil@auxout{%
      \noexpand\savepointas{\pgf@temp}{\pgfpictureid}}%
  },
  if picture id/.code args={#1#2#3}{%
    \@ifundefined{save@pt@#1}{%
      \pgfkeysalso{#3}%
    }{
      \pgfkeysalso{#2}%
    }
  }
}

\def\savepointas#1#2{%
  \expandafter\gdef\csname save@pt@#1\endcsname{#2}%
}

\def\tmk@labeldef#1,#2\@nil{%
  \def\tmk@label{#1}%
  \def\tmk@def{#2}%
}

\tikzdeclarecoordinatesystem{pic}{%
  \pgfutil@in@,{#1}%
  \ifpgfutil@in@%
    \tmk@labeldef#1\@nil
  \else
    \tmk@labeldef#1,(0pt,0pt)\@nil
  \fi
  \@ifundefined{save@pt@\tmk@label}{%
    \tikz@scan@one@point\pgfutil@firstofone\tmk@def
  }{%
  \pgfsys@getposition{\csname save@pt@\tmk@label\endcsname}\save@orig@pic%
  \pgfsys@getposition{\pgfpictureid}\save@this@pic%
  \pgf@process{\pgfpointorigin\save@this@pic}%
  \pgf@xa=\pgf@x
  \pgf@ya=\pgf@y
  \pgf@process{\pgfpointorigin\save@orig@pic}%
  \advance\pgf@x by -\pgf@xa
  \advance\pgf@y by -\pgf@ya
  }%
}
\newcommand\tikzmark[2][]{%
\tikz[remember picture with id=#2] #1;}
\makeatother
% end of code by Andrew Stacey

\newcommand\MyBox[4][-1ex]{%
  \tikz[remember picture,overlay,pin distance=0cm]
  {\draw[draw=#4,line width=1pt,fill=#4!20,rectangle,rounded corners]
( $ (pic cs:#2) + (-1ex,2ex) $ ) rectangle ( $ (pic cs:#3) + (1ex,#1) $ );
}
}

\begin{document}

\MyBox{starta}{enda}{blue}
\MyBox[-3ex]{startb}{endb}{red}
\begin{algorithm}[H]
    \caption{Le crible d'Eratosthène}

    \tikzmark{starta}\KwData{la liste $L$ des naturels compris de $2$ à $N$.}
    \KwResult{la liste $P$ des nombres premiers compris entre $2$ et $N$.\tikzmark{enda}}
    \BlankLine
    \For{$i = 2$ \KwTo $N$}{
        \tikzmark{startb}\If{$i$ n'est pas barré dans la liste $L$}{
            Barrer dans la liste $L$ les mutltiples $ki$ où $k \geq 2$.\tikzmark{endb}
        }
    }
    \Return{la liste $P$ des nombres non barrés.}
\end{algorithm} 

\end{document}