---
myst:
  html_meta:
    keywords: LaTeX,programmation"
---
# Comment mettre des caractères autres que des lettres dans les noms de commande ?

Les utilisateurs débutants avec LaTeX sont souvent étonnés de voir que des commandes contenant des caractères autres que des lettres ne fonctionnent pas. Par exemple :

```{noedit}
\newcommand{\a2main}{À demain !}
```

En effet, contrairement à d'autres langages de programmation, TeX n'autorise que [des lettres dans les noms de commandes](/2_programmation/macros/que_sont_les_macros). Il existe cependant des techniques pour contourner cette limitation mais elles ne sont pas sans défaut.

## Utilisation de « `\csname` » et « `\endcsname` »

Voici un exemple de la méthode utilisant les commandes `\csname` et `\endcsname`.

```{noedit}
\expandafter\newcommand\csname a2main\endcsname{À demain !}
Je vous dis « \csname a2main\endcsname ».
```

Cette technique a l'unique désavantage de demander d'être bien trop verbeuse.

:::{todo} *Traduction à poursuivre.*
:::

2. Define a "special-command generator", and use the resulting commands :

```{noedit}
\newcommand{\DefineRemark}[2]{%
  \expandafter\newcommand\csname rmk-#1\endcsname{#2}%
}
\newcommand{\Remark}[1]{\csname rmk-#1\endcsname}
 ...
\DefineRemark{cul8r}{Goodbye!}
 ...
\Remark{cul8r}
```

- **Pro:** Straightforward to use, not too untidy
- **Con:** It's hardly doing what we set out to do (experts will see that you are defining a macro, but others likely won't)

3. Convince TeX that `8` is a letter :

```{noedit}
\catcode`8 = 11
\newcommand{\cul8r}{Goodbye!}
I said, ``\cul8r''.
```

- **Pro:** `\cul8r` can be used directly
- **Con:** Likely to break other uses of `8` (such as numbers or dimensions; so `\setlength{\paperwidth}{8in}` tells us :

```text
! Missing number, treated as zero.
<to be read again>
                   8
```

As a general rule, changing category codes is something to use *in extremis*, after detailed examination of options. It is conceivable that such drastic action could be useful for you, but most ordinary users are well advised not even to try such a technique.

4. Define a macro `\cul` which must always be followed by `8r` :

```{noedit}
\def\cul8r{Goodbye!}
I said, ``\cul8r''.
```

- **Pro:** `\cul8r` can be used directly
- **Con #1:** Breaks if `\cul` is followed by anything other than `8r`, with a confusing diagnostic, as `\cul99` produces :

```text
! Use of \cul doesn't match its definition.
<*> \cul9
         9
```

(which would confuse someone who hadn't even realised there *was* a definition of `\cul` in the document).

- **Con #2:** Silently redefines existing `\cul`, if any; as a result, the technique cannot be used to define both a `\cul8r` and, say, a `\cul123` macro in the same document.

Technique 3 is in fact commonly used --- in a limited form --- within most LaTeX packages and within LaTeX itself. The convention is to use `@` within the names of internal macros to hide them from the user and thereby prevent naming conflicts. To this end, LaTeX automatically treats `@` as a letter while processing classes and packages and as a non-letter while processing the user's document. The key to this technique is the separation : internally a non-letter is used for macro names, and the user doesn't see anything of it, while the status remains "frozen" in all the definitions created within the class or package. See [`\@` and @ in macro names](faquk:FAQ-atsigns) for more information.

Note that analogous use of technique 3 in this example would give us

```{noedit}
\begingroup
  \catcode`8 = 11
  \gdef\cul8r{Goodbye!}
  \gdef\later{\cul8r}
\endgroup
I said, ``\later''.
```

which works, but rather defeats the object of the exercise. (`\later` has the "frozen" catcode for "8, even though the value has reverted to normal by the time it's used; note, also, the use of the primitive command `\gdef`, since `\newcommand` can't make a macro that's available outside the group.)

*Recommendation* : Either choose another mechanism (such as `\DefineRemark` above), or choose another name for your macro, one that contains only ordinary letters. A common approach is to use roman numerals in place of arabic ones :

```{noedit}
\newcommand{\culVIIIr}{Goodbye!}
```

which rather spoils the intent of the joke implicit in the example `\cul8r`!

:::{sources}
[Non-letters in macro names](faquk:FAQ-linmacnames)
:::