Background coloring in algorithm2e pseudocode in beamer

Question

I tried to highlight parts of my pseudocode in algorithm2e with the following answer to a similar question: https://tex.stackexchange.com/a/51590/287916. I copied the code exactly the same and run it, but it doesn't compile with the following error: ! Package tikz Error: Cannot parse this coordinate.See the tikz package documentation for explanation.Type H for immediate help.... \end{frame}

Can anybody help me highlighting "if p<x" in one color and the corresponding "else" part in another one with a box around it?

\usepackage[edges]{forest}
\usepackage{caption}
\usepackage[linesnumbered,noend]{algorithm2e}
\newcommand\mycommfont[1]{\tiny\ttfamily\textcolor{blue}{#1}}
\SetCommentSty{mycommfont}
\SetKw{Or}{or}
\SetKw{And}{and}
\SetKw{Break}{break}
\SetKw{True}{true}
\SetKw{False}{false}
\SetKw{myfor}{for loop}
\begin{document}
%\begin{frame}[fragile]
%\begin{forest}
%  for tree={circle, draw, 
%    minimum size=1.5em,
%    inner sep=1pt, 
%    align=center, top color=white
%  },
%  for leaves={rectangle},
%  my label/.style={label={[font=\scriptsize]south:#1}},
%  [, top color=gray, bottom color=gray, minimum size=4mm
%    [E
%      [ILIG, my label=3]
%      [U
%        [CH, my label=1]
%        [LE, my label=5]]]
%    [FLOG, my label=2]
%    [GEL
%      [BE, my label=7]
%      [D, my label=8]]
%    [MIT, my label=4]
%    [ZU, my label=6]]
%\end{forest}
%\end{frame}
%
%\begin{frame}{TIKZ ARRAYBOX}
%   \begin{tikzpicture}
%    \pgftransparencygroup
%    \nodes{p,a,t,t,e,r,n}
%    \endpgftransparencygroup
%    \pgfsetstrokeopacity{0.5}
%    \pgfsetfillopacity{0.5}
%    \pgftransparencygroup
%    \nodes{?,?,?,?,?,?}
%    \endpgftransparencygroup
%    \pgfsetstrokeopacity{.75}
%    \pgfsetfillopacity{.75}
%    \pgftransparencygroup
%    \brckt{1}{6}{0}{size=6}
%    \endpgftransparencygroup
%    \pgfsetfillopacity{0.5}
%    \pgfsetstrokeopacity{0}
%    \pgftransparencygroup
%    \brckt{7}{12}{0}{free space}
%    \endpgftransparencygroup
%    \pgfsetstrokeopacity{0.5}
%    \pgftransparencygroup
%    \brckt{1}{12}{1}{capacity=12}
%    \endpgftransparencygroup
%  \end{tikzpicture}
%

%

%  \begin{tikzpicture}[font=\ttfamily,
%array/.style={matrix of nodes,nodes={draw, minimum size=7mm, fill=green!30},column sep=-\pgflinewidth, row sep=0.5mm, nodes in empty cells,
%row 1/.style={nodes={draw=none, fill=none, minimum size=5mm}},
%row 1 column 1/.style={nodes={draw}}}]
%
%\matrix[array] (array) {
%0 & 1 & 2 & 3 & 4 & 5 & 6 & 7 & 8 & 9\
%  &   &   &   &   &   &   &   &   &  \};
%\node[draw, fill=gray, minimum size=4mm] at (array-2-9) (box) {};
%
%\begin{scope}[on background layer]
%\fill[green!10] (array-1-1.north west) rectangle (array-1-10.south east);
%\end{scope}
%
%\draw[<->]([yshift=-3mm]array-2-1.south west) -- node[below] {Array length is 10} ([yshift=-3mm]array-2-10.south east);
%
%\draw (array-1-1.north)--++(90:3mm) node [above] (first) {First index};
%\draw (array-1-10.east)--++(0:3mm) node [right]{Indices};
%\node [align=center, anchor=south] at (array-2-9.north west|-first.south) (8) {Element\ (at index 8)};
%\draw (8)--(box);
%%
%\end{tikzpicture}
%\end{frame}
\begin{frame}
\scalebox{0.5}{
    \begin{algorithm}[H]
\SetInd{0.3em}{0.8em}
\DontPrintSemicolon
\KwIn{Compact Suffix Trie T für Text X, Pattern P}
\KwOut{Startindex eines Matchs in X oder "KEIN MATCH"}
$p \gets P.length()$ ;

$j \gets 0$         ;

$v \gets T.root()$;
\Repeat{failed \Or T.isExternal(v)}{
    $failed \gets$ \True ;

    \ForEach{Kindknoten w von v}{
        $i \gets start(w)$;
        \If{P[j] = X[i]}
        {
            $x \gets end(w)-i+1$;
            \eIf(\tcp[h]{HIGHLIGHT FROM HERE IN ONE COLOR}){$p \leq x$}{
                \eIf{$P[j..j+p-1] = X[i..i+p-1]$}{
                    \Return{i-j}

                }{
                    \Return{"KEIN MATCH"}

                }
            }(\tcp[h]{HIGHLIGHT ELSE IN ANOTHER COLOR})
            {
                \If{$P[j..j+x-1] = X[start(w)..end(w)]$}{
                    $p \gets p-x$;

                    $j \gets j+x$;

                    $v \gets w$;
                    $failed \gets \False$;
                    \Break \myfor ;
                }
            }
        }
    }
}
\Return{"KEIN MATCH"}
\end{algorithm}}
\end{frame}
\end{document}

Answer 1

In the tikzmark documentation, we can read:

I wrote the original \tikzmark macro in 2009 for use in lecture slides prepared with the beamer package. Its original definition was:

\newcommand{\tikzmark}[1]{\tikz[overlay,remember picture] \node (#1) {};}

And a little higher:

Using the \tikzmark is extremely simple. You need to load the tikz package and then load tikzmark as a tikzlibrary. Thus in your preamble you should have something like: \usepackage{tikz} \usetikzlibrary{tikzmark} In your document, you can now type \tikzmark{} at a point that you want to remember. This will save a mark with name for use later (or earlier). To use it in a \tikz or tikzpicture, simply use the pic coordinate system:

\tikz[remember picture] \draw[overlay] (0,0) -- (pic cs:<name>);

EDIT2: To have the colored frame behind the text, you must put the tikzpicture environment before the algorithm environment. It's sharper, the opacity is no longer useful. With the code:

    \documentclass{beamer}
\usepackage{tikz}
\usetikzlibrary{calc,tikzmark}
\usepackage[linesnumbered,noend]{algorithm2e}
\newcommand\mycommfont[1]{\tiny\ttfamily\textcolor{blue}{#1}}
\SetCommentSty{mycommfont}
\SetKw{Or}{or}
\SetKw{And}{and}
\SetKw{Break}{break}
\SetKw{True}{true}
\SetKw{False}{false}
\SetKw{myfor}{for loop}
\begin{document}
\begin{frame}
\scalebox{0.5}{
  %%%%%%%%%%%%%%%%%%%%%%%%%    before the algo 
  \begin{tikzpicture}[remember picture,overlay]
    \draw [fill=yellow] ($(pic cs:a) + (-0.2,0.3)$) rectangle ($(pic cs:b)+(7,-0.1)$);
    \draw [fill=blue!50] ($(pic cs:c) + (-0.2,0.3)$) rectangle ($(pic cs:d)+(5.,-0.1)$);
  \end{tikzpicture}
  %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
    \begin{algorithm}[H]
\SetInd{0.3em}{0.8em}
\DontPrintSemicolon
\KwIn{Compact Suffix Trie T für Text X, Pattern P}
\KwOut{Startindex eines Matchs in X oder "KEIN MATCH"}
$p \gets P.length()$;

$j \gets 0$         ;

$v \gets T.root()$;
\Repeat{failed \Or T.isExternal(v)}{
    $failed \gets$ \True ;

    \ForEach{Kindknoten w von v}{
        $i \gets start(w)$;
        \If{P[j] = X[i]}
        {
            $x \gets end(w)-i+1$;
            \eIf(\tcp[h]{HIGHLIGHT FROM HERE IN ONE COLOR}){$p \leq x$}{
              \tikzmark{a}\eIf{$P[j..j+p-1] = X[i..i+p-1]$}{
                    \Return{i-j}

                }{
                    \Return{"KEIN MATCH"}

                }\tikzmark{b}
            }(\tcp[h]{HIGHLIGHT ELSE IN ANOTHER COLOR})
            {
              \tikzmark{c}\If{$P[j..j+x-1] = X[start(w)..end(w)]$}{
                    $p \gets p-x$;

                    $j \gets j+x$;

                    $v \gets w$;
                    $failed \gets \False$;
                    \Break \myfor \tikzmark{d};
                }
            }
        }
    }
}
\Return{"KEIN MATCH"}
%%%%%%%%%%%%%%%%%%%%%%%%  MODIFIED 
% \begin{tikzpicture}[remember picture,overlay]
%   \draw [fill=yellow,opacity=0.5] ($(pic cs:a) + (-0.2,0.3)$) rectangle ($(pic cs:b)+(7,-0.1)$);
%   \draw [fill=blue,opacity=0.5] ($(pic cs:c) + (-0.2,0.3)$) rectangle ($(pic cs:d)+(5.,-0.1)$);
% \end{tikzpicture}
\end{algorithm}
}
\end{frame}
\end{document}