General ⇒ Question about expandafter chains...

[kreyszig]

Hello,

could someone explain to me why in the following code the macro \msg is not defined as "Hello World"

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\def\hello#1{Hello #1}%
\def\world{World}%
\expandafter\def\expandafter\msg\expandafter{\expandafter\hello\world}%
\show\def%

The output is

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> \msg=macro:
->\hello World.

but I was expecting it to be

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> \msg=macro:
->Hello World.

So \hello is not expanded as I was expecting.

I need a single level expansion of both \world and \hello and the the expansion of \world to be used as argument of \hello.

Thanks!

[josephwright]

After TeX has followed your \expandafter run, you end up with

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\def\msg{\hello world}%

which then gives the definition you see. This is is because \def does no expansion, and your \expandafter run only affects \world.

If we assume you have e-TeX available, then you can do

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\edef\msg{%
  \expandafter\unexpanded\expandafter{\hello}%
  \expandafter\unexpanded\expandafter{\world}%
}

Here the \expandafter instructions do one level expansion, while the \unexpand's prevent any further expansion. Without e-TeX, you could use token registers:

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\newtoks\toksa
\newtoks\toksb
\toksa\expandafter{\hello}
\toksb\expandafter{\world}
\edef\msg{\the\toksa\the\toksb}

but this is obviously more awkward (especially as the number of items grows).

[phi]

After TeX has followed your \expandafter run, you end up with

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\def\msg{\hello world}%

which then gives the definition you see. This is is because \def does no expansion, and your \expandafter run only affects \world.

If we assume you have e-TeX available, then you can do

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\edef\msg{%
  \expandafter\unexpanded\expandafter{\hello}%
  \expandafter\unexpanded\expandafter{\world}%
}

Here the \expandafter instructions do one level expansion, while the \unexpand's prevent any further expansion. Without e-TeX, you could use token registers:

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\newtoks\toksa
\newtoks\toksb
\toksa\expandafter{\hello}
\toksb\expandafter{\world}
\edef\msg{\the\toksa\the\toksb}

but this is obviously more awkward (especially as the number of items grows).

Your suggestions don't work here because \hello is itself a macro with a parameter. Here we either need two \expandafter chains or exhaustive expansion:

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1{Hello #1}%
\def\world{World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\expandafter{\world}%
}
\meaning\msg

\edef\msg{\hello\world}
\meaning\msg

% \edef\msg{%
%   \expandafter\unexpanded\expandafter{\hello}%
%   \expandafter\unexpanded\expandafter{\world}%
% }
% \meaning\msg

% \newtoks\toksa
% \newtoks\toksb
% \toksa\expandafter{\hello}
% \toksb\expandafter{\world}
% \edef\msg{\the\toksa\the\toksb}
% \meaning\msg

\end{document}

[kreyszig]

Thanks! I think I get the idea now. I was somehow misleaded by expandafter definition that says that

\expandafter\a\b

"expands \b, then \a". A more correct statement I think would be that it expands \b than put \a back in front of it, not preventing it from being expanded. What I mean is that expandafter does not itself expands \a, right?

Thanks!

[kreyszig]

After TeX has

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1{Hello #1}%
\def\world{World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\expandafter{\world}%
}
\meaning\msg

\end{document}

I am afraid this does not work either for the more general case where \hello can take more arguments, for example if it is defined as

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\def\hello#1 #2{Hello #1 #2}

and that \world is defined as

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\def\world{The World}

[josephwright]

I dodn't follow. If you avoid wrapping the expansion of \world in braces, then what is wrong with

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1 #2{Hello #1:#2:}%
\def\world{World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\world
}
\meaning\msg

\end{document}

[kreyszig]

I dodn't follow. If you avoid wrapping the expansion of \world in braces, then what is wrong with

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1 #2{Hello #1:#2:}%
\def\world{World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\world
}
\meaning\msg

\end{document}

yes I agree that not using the braces does not work either. What I meant is that the code with the braces only works if the \hello command only takes a single argument...

[phi]

I dodn't follow. If you avoid wrapping the expansion of \world in braces, then what is wrong with

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1 #2{Hello #1:#2:}%
\def\world{World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\world
}
\meaning\msg

\end{document}

This doesn't even compile because the replacement text of \world should contain a space. If you say

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\def\world{The World}

the result is "Hello The:W:orld", while the OP probably wants "Hello The:World:". Here you need another delimiter, e.g.

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\documentclass{minimal}

\begin{document}

\ttfamily

\def\hello#1 #2\nil{Hello #1:#2:}%
\def\world{The World}%

\expandafter\expandafter\expandafter\def
\expandafter\expandafter\expandafter\msg
\expandafter\expandafter\expandafter{%
  \expandafter\hello\world\nil
}
\meaning\msg

\end{document}

[kreyszig]

ok,

I have found a solution that works:

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\def\hello#1 #2{Hello #1 #2}%
\def\world{{The} {World}}%

\def\exec#1#2{\expandafter\def\expandafter\result\expandafter{#1#2}}%
\expandafter\exec\expandafter\hello\expandafter{\world}%

\show\result

> \result=macro:
->Hello The World.

[josephwright]

This doesn't even compile because the replacement text of \world should contain a space.

Mu apologies: I of course meant

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\def\hello#1#2{Hello #1:#2:}%

which then does work to give the expected "Hello W:o:rld" (i.e. each token collected separately).