5

I often use xtring's \IfStrEqCase for conditional processing. This works great, but wondering if there is an easy way to enhance the current behaviour and allow to group mutliple cases into a single match.

In the MWE below, the output for case 1, case 3 and case 4 is identical, but I have to duplicate the text in several places. When this text is long it is rather error prone so would be nice to be able to lump them all togeter in a syntax similar to:

\IfStrEqCaseModified{#1}{% <--- Need to define this
    {{case 1}{case 3}{case 4}}{%
        some long text which is identical for case 1, case 3 and case 4.%
    }%
    {{case 2}{case 5}}{%
        other text for case 2 and case 5.%
    }%
}[{Error: Unknown parameter ``#1'' to ConditionalTextDesired}]%

References:

Notes:

  • One solution of course would be to define a macro

    \newcommand{\RepeatedText}{some long text which is identical for case 1, case 3 and case 4.}
    

    but would prefer to avoid that if possible.

Code:

\documentclass{article}
\usepackage{xstring}

\newcommand*{\ConditionalText}[1]{%
    \IfStrEqCase{#1}{%
        {case 1}{%
            some long text which is identical for case 1, case 3 and case 4.%
        }%
        {case 2}{%
            other text for case 2 and case 5.%
        }%
        {case 3}{%
            some long text which is identical for case 1, case 3 and case 4.%
        }%
        {case 4}{%
            some long text which is identical for case 1, case 3 and case 4.%
        }%
        {case 5}{%
            other text for case 2 and case 5.%
        }%
    }[{Error: Unknown parameter ``#1'' to ConditionalText}]%
}%

\newcommand*{\ConditionalTextDesired}[1]{%
    \IfStrEqCaseModified{#1}{% <--- Need to define this
        {{case 1}{case 3}{case 4}}{%
            some long text which is identical for case 1, case 3 and case 4.%
        }%
        {{case 2}{case 5}}{%
            other text for case 2 and case 5.%
        }%
    }[{Error: Unknown parameter ``#1'' to ConditionalTextDesired}]%
}%


\begin{document}
    \par\ConditionalText{case 1}
    \par\ConditionalText{case 2}
    \par\ConditionalText{case 3}
    \par\ConditionalText{case 4}
    \par\ConditionalText{case 5}

    %\bigskip\par
    %This should produce identical output
    %\par\ConditionalTextDesired{case 1}
    %\par\ConditionalTextDesired{case 2}
    %\par\ConditionalTextDesired{case 3}
    %\par\ConditionalTextDesired{case 4}
    %\par\ConditionalTextDesired{case 5}
\end{document}
3

Slow, but working. The code massages the input in order to make it acceptable to \str_case:nnTF

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\cs_new_protected:Nn \grill_str_multicase:nnTF
 {
  \seq_set_split:Nnn \l__grill_str_multicase_cases_seq { } { #2 }
  \tl_clear:N \l__grill_str_multicase_cases_tl
  \int_step_inline:nnnn { 1 } { 2 } { \seq_count:N \l__grill_str_multicase_cases_seq }
   {
    \seq_set_split:Nnx \l__grill_str_multicase_subcases_seq { }
     { \seq_item:Nn \l__grill_str_multicase_cases_seq { ##1 } }
    \seq_map_inline:Nn \l__grill_str_multicase_subcases_seq
     {
      \tl_put_right:Nx \l__grill_str_multicase_cases_tl
       {
        {\exp_not:n{####1}}{\seq_item:Nn \l__grill_str_multicase_cases_seq { ##1 + 1}}
       }
     }
   }
   \str_case:nVTF { #1 } \l__grill_str_multicase_cases_tl { #3 } { #4 }
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\cs_new_protected:Nn \grill_str_multicase:nn
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { } { }
 }
\cs_new_protected:Nn \grill_str_multicase:nnT
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { #3 } { }
 }
\cs_new_protected:Nn \grill_str_multicase:nnF
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { } { #3 }
 }

\NewDocumentCommand{\IfStringCaseX}{mmo}
 {
  \IfNoValueTF{#3}
   { \grill_str_multicase:nn { #1 } { #2 } }
   { \grill_str_multicase:nnF { #1 } { #2 } { #3 } }
 }
\ExplSyntaxOff

\newcommand{\ConditionalText}[1]{%
  \IfStringCaseX{#1}
   {
    {{case 1}{case 3}{case 4}}
    {%
     some long text which is identical for case 1, case 3 and case 4.%
    }
    {{case 2}{case 5}}
    {%
     other text for case 2 and case 5.%
    }%
   }[{Error: Unknown parameter ``#1'' to ConditionalText}]%
}

\begin{document}

\ConditionalText{case 1}

\ConditionalText{case 2}

\ConditionalText{case 3}

\ConditionalText{case 4}

\ConditionalText{case 5}

\ConditionalText{case 6}

\end{document}

enter image description here

Using variables can be accommodated:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\cs_new_protected:Nn \grill_str_multicase:nnTF
 {
  \seq_set_split:Nnn \l__grill_str_multicase_cases_seq { } { #2 }
  \tl_clear:N \l__grill_str_multicase_cases_tl
  \int_step_inline:nnnn { 1 } { 2 } { \seq_count:N \l__grill_str_multicase_cases_seq }
   {
    \seq_set_split:Nnx \l__grill_str_multicase_subcases_seq { }
     { \seq_item:Nn \l__grill_str_multicase_cases_seq { ##1 } }
    \seq_map_inline:Nn \l__grill_str_multicase_subcases_seq
     {
      \tl_put_right:Nx \l__grill_str_multicase_cases_tl
       {
        {\exp_not:n{####1}}{\seq_item:Nn \l__grill_str_multicase_cases_seq { ##1 + 1}}
       }
     }
   }
   \str_case:nVTF { #1 } \l__grill_str_multicase_cases_tl { #3 } { #4 }
 }
\cs_generate_variant:Nn \seq_set_split:Nnn { Nnx }
\cs_new_protected:Nn \grill_str_multicase:nn
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { } { }
 }
\cs_new_protected:Nn \grill_str_multicase:nnT
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { #3 } { }
 }
\cs_new_protected:Nn \grill_str_multicase:nnF
 {
  \grill_str_multicase:nnTF { #1 } { #2 } { } { #3 }
 }
\cs_generate_variant:Nn \grill_str_multicase:nn { o }
\cs_generate_variant:Nn \grill_str_multicase:nnF { o }

\NewDocumentCommand{\IfStringCaseX}{mmo}
 {
  \IfNoValueTF{#3}
   { \grill_str_multicase:nn { #1 } { #2 } }
   { \grill_str_multicase:nnF { #1 } { #2 } { #3 } }
 }
\NewDocumentCommand{\IfStringCaseXO}{mmo}
 {
  \IfNoValueTF{#3}
   { \grill_str_multicase:on { #1 } { #2 } }
   { \grill_str_multicase:onF { #1 } { #2 } { #3 } }
 }
\ExplSyntaxOff

\NewDocumentCommand{\ConditionalText}{sm}{%
  \IfBooleanTF{#1}{\IfStringCaseXO{#2}}{\IfStringCaseX{#2}}%
   {
    {{case 1}{case 3}{case 4}}
    {%
     some long text which is identical for case 1, case 3 and case 4.%
    }
    {{case 2}{case 5}}
    {%
     other text for case 2 and case 5.%
    }%
   }[{Error: Unknown parameter ``#2'' to ConditionalText}]%
}
\ExplSyntaxOff

\begin{document}

\ConditionalText{case 1}

\ConditionalText{case 2}

\ConditionalText{case 3}

\ConditionalText{case 4}

\ConditionalText{case 5}

\ConditionalText{case 6}

\def\casesix{case 6}

\ConditionalText*{\casesix}

\end{document}
  • Sorry for not catching this until now. But, I can't quite get this to work when used through a variable: \def\CaseNumber{case 1} \ConditionalText{\CaseNumber}. If it is difficult to support both direct and macro use, I'd prefer macro use as shown here. – Peter Grill Apr 23 '17 at 10:57
  • @PeterGrill Easy. ;-) – egreg Apr 23 '17 at 11:22
  • Looks like the * version can handle both cases so \IfStringCaseX is not needed. At least for this test case. – Peter Grill Apr 23 '17 at 11:45
1

Here is a listofitems approach. The key is in how the separator is defined. Using \setsepchar{case 1||case 3||case 4/case 2||case 5}, the top level parsing searches for cases 1, 3 and 4. The 2nd tier parsing looks for cases 2 and 5. The parsing is performed with \readlist\tmp{#1}. Then, \listlen\tmp[] reveals how many items are created by splitting along the parse level 1 phrase. Likewise, \listlen\tmp[1] reveals how many items are created by splitting along the parse level 2 phrase. If the phrase is found, the answer is 2, indicating the space (even if null) prior to and following the parsed phrase, and if the answer is 1, it means no phrase was parsed.

The approach also will permit more than 2 levels to be set, merely be using \setsepchar to set 3 or more tiers of parsing.

\documentclass{article}
\usepackage{xstring,listofitems}
\newcommand*{\ConditionalText}[1]{%
  \setsepchar{case 1||case 3||case 4/case 2||case 5/case 6}
  \readlist\tmp{#1}%
  \IfStrEqCase{\listlen\tmp[]\listlen\tmp[1]\listlen\tmp[1,1]}{%
      {211}{some long text which is identical for case 1, case 3 and case 4.}%
      {121}{other text for case 2 and case 5.}%
      {112}{for case 6 only.}%
  }[{Error: Unknown parameter ``#1'' to ConditionalText}]%
}
\begin{document}
    \par\ConditionalText{case 1}
    \par\ConditionalText{case 2}
    \par\ConditionalText{case 3}
    \par\ConditionalText{case 4}
    \par\ConditionalText{case 5}
    \par\ConditionalText{case 6}
    \par\ConditionalText{case 7}
\end{document}

enter image description here

Of course, xstring can be eliminated from the process altogether, by way of nested \ifnums:

\documentclass{article}
\usepackage{listofitems}
\setsepchar{\\}
\readlist*\thecases{
some long text which is identical for case 1, case 3 and case 4.\\
other text for case 2 and case 5.\\
for case 6 only.
}
\newcommand*{\ConditionalText}[1]{%
  \setsepchar{case 1||case 3||case 4/case 2||case 5/case 6}
  \readlist\tmp{#1}%
  \edef\tmpnum{\listlen\tmp[]\listlen\tmp[1]\listlen\tmp[1,1]}
  \ifnum\tmpnum=211\relax\thecases[1]%
  \else\ifnum\tmpnum=121\relax\thecases[2]%
  \else\ifnum\tmpnum=112\relax\thecases[3]%
  \else Error: Unknown parameter ``#1'' to ConditionalText%
  \fi\fi\fi
}
\begin{document}
    \par\ConditionalText{case 1}
    \par\ConditionalText{case 2}
    \par\ConditionalText{case 3}
    \par\ConditionalText{case 4}
    \par\ConditionalText{case 5}
    \par\ConditionalText{case 6}
    \par\ConditionalText{case 7}
\end{document}

ORIGINAL ANSWER

Why not just define \msgA and \msgB and indirectly refer to them by case? This avoids the need to retype the identical message multiple times.

\documentclass{article}
\usepackage{xstring}
\def\msgA{some long text which is identical for case 1, case 3 and case 4.}
\def\msgB{other text for case 2 and case 5.}
\newcommand*{\ConditionalText}[1]{%
    \IfStrEqCase{#1}{%
        {case 1}{\msgA}%
        {case 2}{\msgB}%
        {case 3}{\msgA}%
        {case 4}{\msgA}%
        {case 5}{\msgB}%
    }[{Error: Unknown parameter ``#1'' to ConditionalText}]%
}
\begin{document}
    \par\ConditionalText{case 1}
    \par\ConditionalText{case 2}
    \par\ConditionalText{case 3}
    \par\ConditionalText{case 4}
    \par\ConditionalText{case 5}
\end{document}
  • Yes that is one way to get this to work. However, as mentioned in the Notes I would prefer to avoid this as it is one more step to do. Not an issue normally, but this happens often and some times need to pass in paramters, etc so would prefer a solution that does not need a definition of new macro. – Peter Grill Jan 28 '17 at 0:35
  • @PeterGrill Please see my revision. – Steven B. Segletes Jan 28 '17 at 1:01
1

Some years ago I wrote a routine \UD@KeepKthOfLArguments which lets you choose an arbitrary undelimited argument from an arbitrary amount of undelimited arguments which—due to recursion—is not bound to the 9-argument-limit usually imposed on TeX' macro-programming.

Unfortunately \IfStrEqCase itself is not fully expandable and therefore cannot be used within \UD@KeepKthOfLArguments's first argument for evaluating the number K.
Thus you have to do it the other way, i.e., call \UD@KeepKthOfLArguments from within \IfStrEqCase for selecting one of three arguments:

\documentclass{article}
\usepackage{xstring}

\makeatletter
%%=========================================================================
%% Paraphernalia:
%%    \UD@firstoftwo, \UD@secondoftwo, \UD@PassFirstToSecond
%%.........................................................................
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@PassFirstToSecond[2]{#2{#1}}%
%%-------------------------------------------------------------------------
%% Check whether argument is empty:
%%.........................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is empty>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
%%
%% A concern in his posting is that the argument is hit with \string
%% after some expansions which in edge cases might result in unbalancing
%% surrounding \if..\fi-constructs if the macro is used inside of such
%% \if..\fi-constructs.
%%
%% That challenging concern sickened me. ;-)
%%
%% Therefore I decided to implerment a variant where this cannot happen
%% as expansion is forced by \romannumeral:
%%
%% After the first expansion-step, \string is not applied yet.
%% After the second expansion-step, any possibly disturbing remainders
%% are already removed due to \romannumeral-expansion.
%%
%% No eTeX- or whatsoever extensions. No \if.. .Only \romannumeral,
%% digit 0, space token for terminating \romannumeral-expansion,
%% \string, \expandafter, \UD@firstoftwo, \UD@secondoftwo, {, }.
%%
%% May 20, 2016
%%
%% Ulrich Diez (e-mail: ud.usenetcorrespondence@web.de)
%%
\newcommand\UD@CheckWhetherNull[1]{%
  \romannumeral0\expandafter\UD@secondoftwo\string{\expandafter
  \UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
  \UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
  \UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
  \UD@secondoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
%%-------------------------------------------------------------------------
%% Keep only the K-th of L consecutive undelimited arguments.
%%   ( IF K < 1 OR K > L just remove L consecutive undelimited arguments. )
%%.........................................................................
%% \UD@KeepKthOfLArguments{<integer number K>}%
%%                        {<integer number L>}%
%%                        {<Tokens to insert before K-th argument>}%
%%                        <L consecutive undelimited arguments>
%%
%% If K >= 1 and K <= L  yields:
%%       <Tokens to insert before K-th argument><K-th undelimited argument>
%% If K < 1 or K > L
%%       (-> there is no K-th argument in the set
%%           of <L consecutive undelimited arguments> )
%% yields:
%%       <Tokens to insert before K-th argument>
%%
%% Examples:
%%
%% X\UD@KeepKthOfLArguments{3}{7}{<Tokens to insert>}{A}{B}{C}{D}{E}{F}{G}X
%% yields: X<Tokens to insert>CX
%%
%% X\UD@KeepKthOfLArguments{5}{2}{<Tokens to insert>}{A}{B}X
%% yields X<Tokens to insert>X
%%
%% X\UD@KeepKthOfLArguments{0}{2}{<Tokens to insert>}{A}{B}X
%% yields X<Tokens to insert>X
%%
%% X\romannumeral0%
%%  \UD@KeepKthOfLArguments{3}{7}{ <Tokens to insert>}{A}{B}{C}{D}{E}{F}{G}X
%% yields: X\romannumeral0 <Tokens to insert>CX
%% yields: X<Tokens to insert>CX
%% In case of embedding the whole thing in other expansion-contexts,
%% you need to have "hit" \romannumeral by only one \expandafter-chain for
%% obtaining the result...
%%
%% January 17, 2005
%%
%% Ulrich Diez (e-mail: ud.usenetcorrespondence@web.de)
%%
\newcommand\UD@KeepKthOfLArguments[2]{%
  \expandafter\UD@KeepKthOfLArgumentsA
  \expandafter{\romannumeral\number\number#1 000\expandafter}%
  \expandafter{\romannumeral\number\number#2 000}%
}%
\newcommand\UD@KeepKthOfLArgumentsA[3]{%
  \UD@CheckWhetherNull{#1}{%
    \UD@RemoveNArguments{#2}{#3}{}%
  }{%
    \expandafter\UD@PassFirstToSecond
    \expandafter{%
    \UD@firstoftwo{}#1}{\UD@KeepKthOfLArgumentsB{#1}{#2}}{#2}{#3}%
  }%
}%
\newcommand\UD@KeepKthOfLArgumentsB[5]{%
  \UD@CheckWhetherNull{#1}{%
     \UD@RemoveNArguments{#3}{\UD@RemoveNArguments{#2}{#5}}{}%
  }{%
    \UD@CheckWhetherNull{#2}{%
      \UD@RemoveNArguments{#4}{#5}{}%
    }{%
      \expandafter\UD@PassFirstToSecond
      \expandafter{%
      \UD@firstoftwo{}#2}{\expandafter\UD@KeepKthOfLArgumentsB
                       \expandafter{%
                       \UD@firstoftwo{}#1}}{#3}{#4}{#5}%
    }%
  }%
}%
\newcommand\UD@RemoveNArguments[3]{%
  \UD@CheckWhetherNull{#1}{#2#3}{%
    \UD@firstoftwo{%
      \expandafter\UD@RemoveNArguments
      \expandafter{\UD@firstoftwo{}#1}{#2}{#3}%
    }%
  }%
}%
%----------------------------------------------------------------------------
%----------------------------------------------------------------------------
\newcommand*{\ConditionalTextDesired}[1]{%
  % \IfStrEqCase will deliver the call to \UD@KeepKthOfLArguments plus
  % \UD@KeepKthOfLArguments's first argument:
  \IfStrEqCase{#1}{%
       {case 1}{\UD@KeepKthOfLArguments{1}}%
       {case 2}{\UD@KeepKthOfLArguments{2}}%
       {case 3}{\UD@KeepKthOfLArguments{1}}%
       {case 4}{\UD@KeepKthOfLArguments{1}}%
       {case 5}{\UD@KeepKthOfLArguments{2}}%
  }[{\UD@KeepKthOfLArguments{3}}]%
  % Here come the remaining arguments of \UD@KeepKthOfLArguments:
  {3}{}%
  {some long text which is identical for case 1, case 3 and case 4.}%
  {other text for case 2 and case 5.}%
  {Error: Unknown parameter ``#1'' to ConditionalText.}%
}%
\makeatother

\begin{document}
    \par\ConditionalTextDesired{case 1}
    \par\ConditionalTextDesired{case 2}
    \par\ConditionalTextDesired{case 3}
    \par\ConditionalTextDesired{case 4}
    \par\ConditionalTextDesired{case 5}
\end{document}

By the way: In case you wish to do such choosing within pure-expansion-contexts, here is an example of a strategy based on using delimited arguments:

\documentclass{article}
\makeatletter
%%=========================================================================
%% Paraphernalia:
%%    \UD@firstoftwo, \UD@secondoftwo, \UD@PassFirstToSecond
%%.........................................................................
\newcommand\UD@firstoftwo[2]{#1}%
\newcommand\UD@secondoftwo[2]{#2}%
\newcommand\UD@PassFirstToSecond[2]{#2{#1}}%
%%-------------------------------------------------------------------------
%% Check whether argument is empty:
%%.........................................................................
%% \UD@CheckWhetherNull{<Argument which is to be checked>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is empty>}%
%%                     {<Tokens to be delivered in case that argument
%%                       which is to be checked is not empty>}%
%%
%% The gist of this macro comes from Robert R. Schneck's \ifempty-macro:
%% <https://groups.google.com/forum/#!original/comp.text.tex/kuOEIQIrElc/lUg37FmhA74J>
%%
%% A concern in his posting is that the argument is hit with \string
%% after some expansions which in edge cases might result in unbalancing
%% surrounding \if..\fi-constructs if the macro is used inside of such
%% \if..\fi-constructs.
%%
%% That challenging concern sickened me. ;-)
%%
%% Therefore I decided to implerment a variant where this cannot happen
%% as expansion is forced by \romannumeral:
%%
%% After the first expansion-step, \string is not applied yet.
%% After the second expansion-step, any possibly disturbing remainders
%% are already removed due to \romannumeral-expansion.
%%
%% No eTeX- or whatsoever extensions. No \if.. .Only \romannumeral,
%% digit 0, space token for terminating \romannumeral-expansion,
%% \string, \expandafter, \UD@firstoftwo, \UD@secondoftwo, {, }.
%%
%% May 20, 2016
%%
%% Ulrich Diez (e-mail: ud.usenetcorrespondence@web.de)
%%
\newcommand\UD@CheckWhetherNull[1]{%
  \romannumeral0\expandafter\UD@secondoftwo\string{\expandafter
  \UD@secondoftwo\expandafter{\expandafter{\string#1}\expandafter
  \UD@secondoftwo\string}\expandafter\UD@firstoftwo\expandafter{\expandafter
  \UD@secondoftwo\string}\expandafter\expandafter\UD@firstoftwo{ }{}%
  \UD@secondoftwo}{\expandafter\expandafter\UD@firstoftwo{ }{}\UD@firstoftwo}%
}%
%%-------------------------------------------------------------------------
%% Keep only the K-th of L consecutive undelimited arguments.
%%   ( IF K < 1 OR K > L just remove L consecutive undelimited arguments. )
%%.........................................................................
%% \UD@KeepKthOfLArguments{<integer number K>}%
%%                        {<integer number L>}%
%%                        {<Tokens to insert before K-th argument>}%
%%                        <L consecutive undelimited arguments>
%%
%% If K >= 1 and K <= L  yields:
%%       <Tokens to insert before K-th argument><K-th undelimited argument>
%% If K < 1 or K > L
%%       (-> there is no K-th argument in the set
%%           of <L consecutive undelimited arguments> )
%% yields:
%%       <Tokens to insert before K-th argument>
%%
%% Examples:
%%
%% X\UD@KeepKthOfLArguments{3}{7}{<Tokens to insert>}{A}{B}{C}{D}{E}{F}{G}X
%% yields: X<Tokens to insert>CX
%%
%% X\UD@KeepKthOfLArguments{5}{2}{<Tokens to insert>}{A}{B}X
%% yields X<Tokens to insert>X
%%
%% X\UD@KeepKthOfLArguments{0}{2}{<Tokens to insert>}{A}{B}X
%% yields X<Tokens to insert>X
%%
%% X\romannumeral0%
%%  \UD@KeepKthOfLArguments{3}{7}{ <Tokens to insert, be aware of the space behind the opening brace!>}{A}{B}{C}{D}{E}{F}{G}X
%% yields: X\romannumeral0 <Tokens to insert>CX
%% yields: X<Tokens to insert>CX
%% In case of embedding the whole thing in other expansion-contexts,
%% you need to have "hit" \romannumeral by only one \expandafter-chain for
%% obtaining the result...
%%
%% January 17, 2005
%%
%% Ulrich Diez (e-mail: ud.usenetcorrespondence@web.de)
%%
\newcommand\UD@KeepKthOfLArguments[2]{%
  \expandafter\UD@KeepKthOfLArgumentsA
  \expandafter{\romannumeral\number\number#1 000\expandafter}%
  \expandafter{\romannumeral\number\number#2 000}%
}%
\newcommand\UD@KeepKthOfLArgumentsA[3]{%
  \UD@CheckWhetherNull{#1}{%
    \UD@RemoveNArguments{#2}{#3}{}%
  }{%
    \expandafter\UD@PassFirstToSecond
    \expandafter{%
    \UD@firstoftwo{}#1}{\UD@KeepKthOfLArgumentsB{#1}{#2}}{#2}{#3}%
  }%
}%
\newcommand\UD@KeepKthOfLArgumentsB[5]{%
  \UD@CheckWhetherNull{#1}{%
     \UD@RemoveNArguments{#3}{\UD@RemoveNArguments{#2}{#5}}{}%
  }{%
    \UD@CheckWhetherNull{#2}{%
      \UD@RemoveNArguments{#4}{#5}{}%
    }{%
      \expandafter\UD@PassFirstToSecond
      \expandafter{%
      \UD@firstoftwo{}#2}{\expandafter\UD@KeepKthOfLArgumentsB
                       \expandafter{%
                       \UD@firstoftwo{}#1}}{#3}{#4}{#5}%
    }%
  }%
}%
\newcommand\UD@RemoveNArguments[3]{%
  \UD@CheckWhetherNull{#1}{#2#3}{%
    \UD@firstoftwo{%
      \expandafter\UD@RemoveNArguments
      \expandafter{\UD@firstoftwo{}#1}{#2}{#3}%
    }%
  }%
}%
%%-------------------------------------------------------------------------
%% Check whether argument contains no exclamation-mark on top-brace-level:
%%.........................................................................
%% \UD@CheckWhetherNoExclamationMark{<Argument which is to be checked>}%
%%                  {<Tokens to be delivered in case that
%%                    argument which is to be checked does not contain !>}%
%%                  {<Tokens to be delivered in case that
%%                    argument which is to be checked does contain !>}%
\long\def\UD@RemoveToExclamationMark#1!{}%
\long\def\UD@CheckWhetherNoExclamationMark#1{%
  \expandafter\UD@CheckWhetherNull\expandafter{\UD@RemoveToExclamationMark#1!}%
}%
%%-------------------------------------------------------------------------
%% Fork depending on some tokens:
%%.........................................................................
%% \ConditionalTextDesired{<Argument which is to be checked>}%
%%           {<Tokens to be delivered in case <Argument which is to be checked> is "case 1" or "case 3" or "case 4">}%
%%           {<Tokens to be delivered in case <Argument which is to be checked> is "case 2" or "case 5" >}%
%%
%% In case <Argument which is to be checked> is neither "case 1" nor
%% "case 2" the phrase "Error: Unknown parameter ``<Argument which is
%% to be checked>'' to \ConditionalTextDesired." will be delivered.
%%
\newcommand\@ConditionalTextDesired{}%
\long\def\@ConditionalTextDesired%
   #1!!case 1!case 2!case 3!case 4!case 5!#2#3!!!!{#2}%
\newcommand\ConditionalTextDesired[1]{%
  \romannumeral0%
  \UD@KeepKthOfLArguments{%
    \UD@CheckWhetherNoExclamationMark{#1}{%
      \@ConditionalTextDesired
      !#1!case 1!case 2!case 3!case 4!case 5!{1}%<- #1 is EMPTY
      !!#1!case 2!case 3!case 4!case 5!{2}% <- #1 = case 1
      !!case 1!#1!case 3!case 4!case 5!{3}% <- #1 = case 2
      !!case 1!case 2!#1!case 4!case 5!{2}% <- #1 = case 3
      !!case 1!case 2!case 3!#1!case 5!{2}% <- #1 = case 4
      !!case 1!case 2!case 3!case 4!#1!{3}% <- #1 = case 5
      !!case 1!case 2!case 3!case 4!case 5!{1}% <- #1 = something else without exclamation mark
      !!!!%
    }{1}% <- #1 = something else with exclamation mark
  }{3}{ }% <- The remaining arguments of \UD@KeepKthOfLArguments.
  % You could put the three text-arguments into the macro at this place.
  % You can also provide only the one here, where the macro-parameter #1
  % is needed and provide the other ones at run-time of the macro.
  % The latter will be done for demonstration-purposes.
  {%
    Error: Unknown parameter ``#1'' to \texttt{\string\ConditionalTextDesired}.%
    %In case eTeX-extensions are available you might wish to use \detokenize:
    %Error: Unknown parameter ``\detokenize{#1}'' to \texttt{\string\ConditionalTextDesired}.%
  }%
}%
\makeatother

\begin{document}
% This should yield "Error: Unknown parameter ``'' to \ConditionalTextDesired."
\ConditionalTextDesired{}
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "Error: Unknown parameter ``!case 1'' to \ConditionalTextDesired."
\ConditionalTextDesired{!case 1}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "Error: Unknown parameter ``case 6'' to \ConditionalTextDesired."
\ConditionalTextDesired{case 6}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "some long text which is identical for case 1, case 3 and case 4."
\ConditionalTextDesired{case 1}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "some long text which is identical for case 1, case 3 and case 4."
\ConditionalTextDesired{case 3}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "some long text which is identical for case 1, case 3 and case 4."
\ConditionalTextDesired{case 4}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "other text for case 2 and case 5."
\ConditionalTextDesired{case 2}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

% This should yield "other text for case 2 and case 5."
\ConditionalTextDesired{case 5}%
                       {some long text which is identical for case 1, case 3 and case 4.}%
                       {other text for case 2 and case 5.}%

\end{document}

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