# Mark a pseudocode block and insert comments near it

I am currently working on a LaTeX beamer presentation where I have to explain an algorithm. So I wrote this algorithm with

\usepackage{algorithm,algpseudocode}


This is the current algorithm:

\begin{frame}
\begin{algorithm}[H]
\begin{algorithmic}
\Function{tarjan}{Node* node}
\State $node.visited \gets$ \textbf{true}
\State $node.index \gets indexCounter$
\State $s.push(node)$
\ForAll{$successor$ in $node.successors$}
\If{$!node.visited$}
\Call{tarjan}{successor}
\EndIf
\State $node.lowlink \gets$ \Call{min}{$node.lowlink, successor.lowlink$}
\EndFor

\If{$node.lowlink == node.index$}
\Repeat
\State $successor \gets stack.pop()$
\Until{$successor == node$}
\EndIf
\EndFunction
\end{algorithmic}
\label{alg:seq2}
\end{algorithm}
\end{frame}


It looks like that:

Now I would like to mark a block and write a comment to it:

I made the box with Gimp. How do I get this result with LaTeX?

(Notes / hints for a better usage of the algorithmic environment are also welcome. It's the first time I use it.)

-

The idea is based on the tikzmark macro and actually it is an adaptation from Background coloring with overlay specification in algorithm2e + beamer package with the difference that you require an annotation.

The code is:

\documentclass{beamer}
\usepackage{algorithm,algpseudocode}
\usepackage{tikz}
\usetikzlibrary{calc}
\usepackage{xcolor}
\makeatletter

% to change colors
\newcommand{\fillcol}{green!20}
\newcommand{\bordercol}{black}

% code from Andrew Stacey (with small adjustment to the border color)
% http://tex.stackexchange.com/questions/51582/background-coloring-with-overlay-specification-in-algorithm2e-beamer-package
\newcounter{jumping}
\resetcounteronoverlays{jumping}

\def\jump@setbb#1#2#3{%
\@ifundefined{jump@#1@maxbb}{%
\expandafter\gdef\csname jump@#1@maxbb\endcsname{#3}%
}{%
\csname jump@#1@maxbb\endcsname
\pgf@xa=\pgf@x
\pgf@ya=\pgf@y
#3
\pgfmathsetlength\pgf@x{max(\pgf@x,\pgf@xa)}%
\pgfmathsetlength\pgf@y{max(\pgf@y,\pgf@ya)}%
\expandafter\xdef\csname jump@#1@maxbb\endcsname{\noexpand\pgfpoint{\the\pgf@x}{\the\pgf@y}}%
}
\@ifundefined{jump@#1@minbb}{%
\expandafter\gdef\csname jump@#1@minbb\endcsname{#2}%
}{%
\csname jump@#1@minbb\endcsname
\pgf@xa=\pgf@x
\pgf@ya=\pgf@y
#2
\pgfmathsetlength\pgf@x{min(\pgf@x,\pgf@xa)}%
\pgfmathsetlength\pgf@y{min(\pgf@y,\pgf@ya)}%
\expandafter\xdef\csname jump@#1@minbb\endcsname{\noexpand\pgfpoint{\the\pgf@x}{\the\pgf@y}}%
}
}

\tikzset{%
remember picture with id/.style={%
remember picture,
overlay,
draw=\bordercol,
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}%
}
},
onslide/.code args={<#1>#2}{%
\only<#1>{\pgfkeysalso{#2}}%
},
alt/.code args={<#1>#2#3}{%
\alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}}%
},
stop jumping/.style={
execute at end picture={%
\stepcounter{jumping}%
\immediate\write\pgfutil@auxout{%
\noexpand\jump@setbb{\the\value{jumping}}{\noexpand\pgfpoint{\the\pgf@picminx}{\the\pgf@picminy}}{\noexpand\pgfpoint{\the\pgf@picmaxx}{\the\pgf@picmaxy}}
},
\csname jump@\the\value{jumping}@maxbb\endcsname
\path (\the\pgf@x,\the\pgf@y);
\csname jump@\the\value{jumping}@minbb\endcsname
\path (\the\pgf@x,\the\pgf@y);
},
}
}

\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,\pgfpointorigin\@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}%
}%
}
\newcommand\tikzmark[2][]{%
\tikz[remember picture with id=#2] #1;}
\makeatother

\resetcounteronoverlays{algocf}

\newcommand<>{\boxto}[1]{%
\only#2{\tikz[remember picture with id=#1]
\draw[line width=1pt,fill=\fillcol,rectangle,rounded corners]
(pic cs:#1) ++(5.2,-.1) rectangle (-0.4,0)
;\tikz\node [anchor=base] (#1){};}% <= insertion to store the anchor to be used as based for the annotation
}

\begin{document}
\begin{frame}
\begin{algorithm}[H]
\begin{algorithmic}
\Function{tarjan}{Node* node}
\State $node.visited \gets$ \textbf{true}
\State $node.index \gets indexCounter$
\State $s.push(node)$
\ForAll{$successor$ in $node.successors$}
\If{$!node.visited$}
\Call{tarjan}{successor}
\EndIf
\State $node.lowlink \gets$ \Call{min}{$node.lowlink, successor.lowlink$}
\EndFor

\boxto<1->{a}\If{$node.lowlink == node.index$}
\Repeat
\State $successor \gets stack.pop()$
\Until{$successor == node$}
\EndIf\tikzmark{a}
\EndFunction
\end{algorithmic}
\label{alg:seq2}
\end{algorithm}

% To insert the annotation
\begin{tikzpicture}[remember picture,overlay]
\coordinate (aa) at ($(a)+(8.5,3)$); % <= adjust this parameter to move the position of the annotation
\node[rectangle,draw, gray,text width=3cm,align=left,right] at (aa) {The root node was found and the whole SCC gets poped from stack};
\end{tikzpicture}
\end{frame}

\end{document}


Explanation

Thanks to the command \boxto and \tikzmark you define the delimeters to the zone to be highlighted. Notice that you need to compile twice. Subsequently, inside the tikzpicture it is possible to use the anchor a as base to define the rectangle for the annotation, inserted as a node.

If you want to create some animation, for example display the note after having highlighted the algorithm, you can slightly modify the previous code into:

% To insert the annotation later
\begin{tikzpicture}[remember picture,overlay]
\coordinate (aa) at ($(a)+(8.5,3)$); % <= adjust this parameter to move the position of the annotation
\node<2->[rectangle,draw, gray,text width=3cm,align=left,right] at (aa) {The root node was found and the whole SCC gets poped from stack};
\end{tikzpicture}
\end{frame}

-
Thanks for this fast and great answer! –  moose May 23 '12 at 18:12
Glad of having help you. However this solution is suitable just in that case: if the statement is more longer there's no space to insert the annotation ;) Moreover to highlight different parts you should adjust properly the dimensions in \boxto. –  Claudio Fiandrino May 23 '12 at 18:35
Do you have any idea why I have to compile my document now at least twice (the first time latexmk always breaks with an error)? –  moose May 23 '12 at 19:19
This is due to the fact that the first time the position of the anchors have to be computed and the second time are ready to be used to highlight. –  Claudio Fiandrino May 23 '12 at 19:23
@Misaki: That error means that your current distribution does not have Beamer installed at the moment. You have to install it either manually or via a packet manager. I would recommend anyway to switch to TeXLive which install all packages. The probability to get such messages will be almost zero after having installed TeXLive full. –  Claudio Fiandrino Jul 16 at 10:39