Check if key is set in xkeyval

Question

I am working with the xkeyval package.

Sometimes I want to check by hand if a key is contained in a list of keys.

That is, I want a if-command like the following:

\contains@key{<family>}{<key value pairs>}{<key>}
{<execute if <key> is contained in <key value pairs>>}
{<execute if key is not contained>}

Are there already commands in xkeyval to achieve this?

Answer 1

The following defines a fully expandable macro that checks whether the key is in the list. It uses expkv to loop over the list of key=value pairs and pdftexcmds to make \pdf@strcmp available in all engines. The key names are not expanded in any way in this process. The macro needs exactly two steps of expansion to do its job.

\documentclass{article}

\usepackage{expkv}
\usepackage{pdftexcmds}

\makeatletter
\newcommand\ifkeyinlistTF[1]
  {\romannumeral\expandafter\ifkeyinlistTF@\detokenize{#1}\relax}
\long\def\ifkeyinlistTF@#1\relax#2%
  {%
    \ekvparse{\ifkeyinlistTF@k{#1}}{\ifkeyinlistTF@v{#1}}{#2}%
    \ifkeyinlistTF@false
  }
\newcommand\ifkeyinlistTF@k[2]
  {\if0\pdf@strcmp{#1}{\unexpanded{#2}}\ifkeyinlistTF@true\fi}
\newcommand\ifkeyinlistTF@v[3]
  {\if0\pdf@strcmp{#1}{\unexpanded{#2}}\ifkeyinlistTF@true\fi}
\newcommand\ifkeyinlistTF@false[2]{\z@#2}
\long\def\ifkeyinlistTF@true\fi#1\ifkeyinlistTF@false#2#3{\fi\z@#2}
\makeatother

\begin{document}
\edef\foo
  {%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i}{.}{a}%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i,key}{i}{.}%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i,keya}{.}{q}%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i,key=a}{u}{.}%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i,key=}{i}{.}%
    \ifkeyinlistTF{key}{a,b=c,d,e=f,g,h=i, key=}{t}{.}%
    \ifkeyinlistTF{\key}{a,b=c,d,e=f,g,h=i, \key =}{a}{.}%
  }
\texttt{\meaning\foo}
\end{document}

Output:

Answer 2

I figured out myself how to achieve a command that takes a key, a list of key-value pairs, and executes some of two code blocks depending on whether the key occurs in the list.

The solution is build upon low level commands of the xkeyval package, thus well-suited if you already use the package. At least I use the command along with setting the keys in xkeyval.

For comparision I use the ifthen package which may be replaced by any other sufficient mechanism.

The solution is as follows:

% !TEX TS-program = pdflatex
\documentclass{article}

\usepackage{xkeyval}
\usepackage{ifthen}

\begin{document}
    
    
    \makeatletter
        
        \newcommand{\if@contains@key}[4]{%
            \XKV@checksanitizea{#2}\XKV@resb%
            \newif\ifkey@found\key@foundfalse% set to true right after the key was found, otherwise remains false
            \XKV@for@o\XKV@resb\XKV@tempa{% loop over the key-value pairs
                % the following two lines extract the key from the pair and remove trailing whitespace (same solution as in \setkeys of xkeyval)
                \expandafter\XKV@g@tkeyname\XKV@tempa=\@nil\XKV@tempa%
                \expandafter\KV@@sp@def\expandafter\XKV@tempa\expandafter{\XKV@tempa}%
                \ifthenelse{\equal{#1}{\XKV@tempa}}%
                    {\key@foundtrue}{}%
            }%
            \ifkey@found%
                #3%
            \else%
                #4%
            \fi%
        }
        
    
        \if@contains@key{welt}{hallo={}, welt={}}
            {Hallo}
            {Welt}
        
    \makeatother

\end{document}

Note that the solution works on the list of key-value pairs directly such that it does not check whether the key belongs to a certain family of keys.