Cfrgtkky

Let's say we have a command foo with 1 argument. Can we produce a command setfoo{aaa}{<some thing>} which redefines foo to:
if an argument of foois equal to 'aaa', do ; otherwise do what foo was supposed to do previously? I would also like to use setfoo many times, so

setfoo{a}{part1}

setfoo{b}{part2}

setfoo{c}{part3}

foo{a}  foo{b} foo{c}

would produce

part1 part2 part3

I've tried the following:

documentclass{article}

usepackage{ifthen}



newcommand{foo}[1]{}

%letoldfoofoo%

newcommand{setfoo}[2]{

letoldfoofoo %<- to save current foo

renewcommand{foo}[1]{

ifthenelse{equal{##1}{#1}}{#2}{oldfoo{##1}}%

}

}

begin{document}

setfoo{a}{part1}

setfoo{b}{part2}

%setfoo{c}{part3}



foo{a} foo{a} foo{c}



end{document}

But it works only if there are <=2 uses of setfoo. Otherwise, compilation doesn't stop for a few minutes so I guess something is not working. How can this be fixed? Thanks in advance.

This is a job for csname:

newcommand{setfoo}[2]{%

  expandafternewcommandcsname tomasz@#1endcsname{#2}%

}

newcommand{foo}[1]{csname tomasz@#1endcsname}

You might use a check in foo that the command is actually defined:

makeatletter

newcommand{foo}[1]{%

  ifcsname tomasz@#1endcsname

     expandafter@firstoftwo

  else

     expandafter@secondoftwo

  fi

  {csname tomasz@#1endcsname}%

  {BUMMER!}%

}

makeatother

In your special case, where foo is always to process a single undelimited non-optional argument, you can probably apply some expansion trickery for "flushing" the tokens that form the current definition of foo into the new definition of foo—one of the expansion-tricks is (ab)using romannumeral for triggering expansion until LaTeX has found the number 0. romannumeral's underlying conversion-routine does "swallow" non-positive numbers while silently not delivering any token at all:

documentclass{article}

usepackage{ifthen}



newcommandPassFirstToSecond[2]{#2{#1}}%



newcommand{foo}[1]{There is no setting for value #1.}



newcommand{setfoo}[2]{%

  expandafterPassFirstToSecond

  expandafter{%

    romannumeralexpandafterPassFirstToSecondexpandafter{foo{##1}}%

    {0 ifthenelse{equal{##1}{#1}}{#2}}%

  }{renewcommand{foo}[1]}%

}



parindent=0ex

parskip=bigskipamount



begin{document}



texttt{stringfoospace currently is defined as:\ meaningfoo}



setfoo{a}{part1}

texttt{stringfoospace currently is defined as:\ meaningfoo}



setfoo{b}{part2}

texttt{stringfoospace currently is defined as:\ meaningfoo}



setfoo{c}{part3}

texttt{stringfoospace currently is defined as:\ meaningfoo}



hrulefill



Some testing:



verb|foo{a}| yields: foo{a} 



verb|foo{b}| yields: foo{b} 



verb|foo{c}| yields: foo{c}



verb|foo{y}| yields: foo{y}



end{document}