General ⇒ Calling macros with arguments - using \csname

[gkamel]

Is it possible to call a macro that takes in an argument, using the \csname primitive?

For example, if I have the following macro definition:

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\newcommand\MyMacro{
Hello world!
}

it can be called using:

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\csname MyMacro\endcsname

But if I have:

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\newcommand\MyMacro[1]{
Hello #1!
}

then

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\csname MyMacro{world}\endcsname

doesn't have quite the intended effect..

George

[josephwright]

Two things here. First, \newcommand is a LaTeX command, whereas \csname is a TeX primitive command. So the syntax is different. Second, \csname does not define anything: you need to use the TeX primitive \def. This also requires the \expandafter primitive.

To create a macro with arguments using \def, you give the arguments as #1, #2, and so on:

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\def\MyMacro#1{Some code #1}

and so with \csname:

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\expandafter\def\csname MyMacro\endcsname#1{Some code #1}

What happens here is that TeX sees the \expandafter, and moves on to \csname (constructing the name) before using \def to create the new macro. If you try this without the \expandafter things go wrong.

LaTeX provides and internal macro which covers up some of the messy detail:

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\makeatletter
\@namedef{MyMacro}#1{Some code #1}
\makeatother

has exactly the same effect as the primitive version above.

[nlct]

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\csname MyMacro{world}\endcsname

doesn't have quite the intended effect..

\csname MyMacro\endcsname expands to \MyMacro, so to get \MyMacro{world} you need to do

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\csname MyMacro\endcsname{world}

If you put {world} inside the \csname...\endcsname you make it a part of the command name rather than a separate object that happens to follow the command name.

Regards
Nicola Talbot

[gkamel]

Many thanks for the replies.

Joseph, I was actually looking for a way to call the macro using \csname rather than defining it. But what you have suggested may nevertheless come in handy for me later.

Nicola, the code you have given works fine if the macro is defined, but I need it to not display anything if the macro is undefined, which I didn't make clear in my initial post. So I thought of building on your suggestion by defining a wrapper macro as follows:

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\newcommand\DisplayOutput[2]{
\ifthenelse{\isundefined{#1}}{}{\csname#1\endcsname{#2}}
}

and calling:

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\DisplayOutput{\MyMacro}{world}

which works fine if the macro is undefined, but if it's defined, then the following error appears:

! Missing \endcsname inserted.

Any ideas?

With kind regards,

George

[josephwright]

Ah, sorry, mis-read things. I'd probably use the e-TeX primitive \ifcsname for this. I'd also use a few expansion tricks to make sure everything works out OK.

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\newcommand*{\DisplayOutput}[1]{%
  \def\tempa{\csname #1\endcsname}%
  \def\gobbleone##1{}%
  \ifcsname #1\endcsname
    \expandafter\tempa
  \else
    \expandafter\gobbleone
  \fi
}

The idea here is that the \ifcsname ... \endcsname construct looks for the definition of #1. If it is found, \tempa is executed (and turns into \csname #1\endcsname), but the \expandafter means this happens after the \fi. On the other hand, if #1 is not defined \gobbleone will aborb the next token, again after the \fi because of the \expandafter.

You could do this without e-TeX, but the tools are there and make the code easier to follow.

Hope this helps.

[phi]

The etoolbox package makes this a bit easier:

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\documentclass{article}

\usepackage{etoolbox}

\newrobustcmd*\TEST[2]{%
	\ifcsdef{#1}{\csuse{#1}{#2}}{}%
}


\begin{document}

No output:
\TEST{MyMacro}{world}

Now we define the macro
\newcommand*\MyMacro[1]{Hello #1!}

Output:
\TEST{MyMacro}{world}

\end{document}

[gkamel]

Many thanks for both of your suggestions... it works perfectly now.

Kind regards,

George

[gkamel]

Dear Joseph,

It turns out I need to make use of your suggestion on how to define a macro with csname sooner than I thought.

I have the following:

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\def\temp{MyMacro}

and I'm using your suggestion:

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\expandafter\def\csname temp\endcsname#1{Some code #1}

to define a new macro called MyMacro. But when I call this macro, LaTeX complains that MyMacro is undefined.

Any ideas what the problem could be?

Kind regards,

George

[josephwright]

Not without some context. Could you post an example?

[gkamel]

Well, simply I have the following macro defined:

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\def\temp{MyMacro}

and I'd like to define a new macro whose name is the expansion of temp, in this case, MyMacro.

Perhaps your code was achieving something else, but I'd be grateful for your guidance in this, to save my remaining hair.

Thanks,

George