UK TeX FAQ -- question label moren9

### How to break the 9-argument limit

If you think about it, you will realise that Knuth’s command definition syntax:

>     \def\blah#1#2 ... #9{<macro body>}

is intrinsically limited to just 9 arguments. There’s no direct way round this: how would you express a 10th argument? — and ensure that the syntax didn’t gobble some other valid usage?

If you really must have more than 9 arguments, the way to go is:

>     \def\blah#1#2 ... #9{%
>       \def\ArgI{{#1}}%
>       \def\ArgII{{#2}}%
>       ...
>       \def\ArgIX{{#9}}%
>       \BlahRelay
>     }
>     \def\BlahRelay#1#2#3{%
>       % arguments 1-9 are now in
>       %   \ArgI-\ArgIX
>       % arguments 10-12 are in
>       %   #1-#3
>       <macro body>%
>     }

This technique is easily extendible by concert pianists of the TeX keyboard, but is really hard to recommend.

LaTeX users have the small convenience of merely giving a number of arguments in the `\``newcommand` that defines each part of the relaying mechanism: Knuth’s restriction applies to `\``newcommand` just as it does to `\``def`. However, LaTeX users also have the way out of such barbarous command syntax: the keyval package. With keyval, and a bit of programming, one can write really quite sophisticated commands, whose invocation might look like:

>     \flowerinstance{species=Primula veris,
>       family=Primulaceae,
>       location=Coldham's Common,
>       locationtype=Common grazing land,
>       date=1995/04/24,
>       numplants=50,
>       soiltype=alkaline
>     }

The merit of such verbosity is that it is self-explanatory: the typist doesn’t have to remember that argument twelve is `soiltype`, and so on: the commands may be copied from field notes quickly and accurately.

`keyval.sty`
  Distributed as part of macros/latex/required/graphics (or browse the directory); catalogue entry

* faq-mac-prog.tex (q-keyval): tweak words about getoptk

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