Dtyjlui

Please consider the following, non-working, example:

documentclass{standalone}



makeatletter

  parindentz@

  @namedef{blafasel}{foo}

  deffoo{foo}

  defbar{bar}

makeatother



begin{document}

1:

expandafterifx@nameuse{blafasel}foo

  message{true}

else

  message{false}

fi    



2: 

expandafterifx@nameuse{blafasel}bar

  message{true}

else

  message{false}

fi



end{document}

How must I alter this code so that the output is "correct" (1: truen2: false)? So far, I tried up to 9 expandafter before ifx, but none gave me the desired output. Why is that?

EDIT

Okay, my mistake was that @ wasn't a letter anymore... Using three expandafters did the trick. Nonetheless i'd like to know what happens here. How does LaTeX expand @namedef and @nameuse resp.? Extending the example by the following code also gives weired results:

3: 

expandafterifxcsname blafaselendcsnamebar

  true

else

  false

fi



4: 

expandafterifxcsname blafaselendcsnamebar

  true

else

  false

fi

You're almost there. Two problems only.

First, @nameuse contains an @ in the name, so you need to makeatletter before using it.

Second, if you show@nameuse (after a makeatletter, of course) you see:

> @nameuse=macro:

#1->csname #1endcsname 

so one expansion of @nameuse{foo} yields csname blafaselendcsname. The csname...endcsname requires another expansion to make blafasel, which is what you want. So now you know you need two expansions of @nameuse.

With

expandafterifx@nameuse{blafasel}foo

you expand it once. To expand twice you need one more expandafter before each token that precedes @nameuse. That is, one expandafter for the "old" expandafter, and another for the ifx:

expandafterexpandafterexpandafterifx@nameuse{blafasel}foo

Now the code:

documentclass{standalone}



makeatletter

  parindentz@

  @namedef{blafasel}{foo}

  deffoo{foo}

  defbar{bar}

makeatother



begin{document}

1:

makeatletter

expandafterexpandafterexpandafterifx@nameuse{blafasel}foo

  message{true}

else

  message{false}

fi    



2:

expandafterexpandafterexpandafterifx@nameuse{blafasel}bar

  message{true}

else

  message{false}

fi



end{document}

prints true false somewhere in the terminal.

You need to expand two levels, so expandafterexpandafterexpandafter, besides making @ a letter in order to avoid TeX compare the first two tokens in the expansion of @ (precisely, spacefactor and @m).

A single expandafter is sufficient with expandafterifxcsname blafaselendcsnamefoo.

There are simpler ways to compare macros expanding to strings of characters, the simplest one being

ifnumpdfstrcmp{@nameuse{blafadel}}{foo}=0

  true

else

  false

fi

that doesn't need to define foo. Since pdfstrcmp is available with different names in other engines, it's best to do

usepackage{pdftexcmds}

and use pdf@strcmp.

Much more powerful string comparison functions are available with expl3.