Comment répéter une commande N fois?

TeX n'a pas été conçu comme un langage de programmation, mais il arrive de vouloir répéter une partie d'un document, tout comme certaines parties de programmes doivent être exécutées plusieurs fois. La conception de schémas est un exemple évident. Dans l'exemple ci-dessous, vous n'avez évidemment pas envie de répéter manuellement le dessin de chaque graduation sur l'axe, alors que l'ordinateur saurait très bien le faire pour vous:

Donc tous naturellement, l'environnement picture de LaTeX et PGF (notamment) fournissent des fonctions pour répéter des actions. Même en dehors du contexte de dessin, il est souvent plus simple d'utiliser ces fonctionnalités, plutôt que de les réimplémenter à l'aide d'obscures macros TeXniques.

Cette réponse traite de la répétition d'une opération un nombre donné de fois; si vous souhaitez exécuter une action pour chaque élément d'une liste d'objets lisez plutôt la réponse à la question «Comment répéter une action pour chaque élément d'un ensemble?».

Plain TeX itself provides a \loop … \repeat contruct, which enables you to repeat a command (or set of commands). The syntax is simple enough, but it use of TeX conditionals is different enough that many people find it confusing.

\newcount\foo
\foo=10
\loop
  \message{\the\foo}
  \advance \foo -1
\ifnum \foo>0
\repeat

In this slightly tricky code, \loop starts the construct ended by \repeat, but \repeat also “serves as” the \fi to the \ifnum. The loop above prints the numbers from 10 down to 1 via TeX \message (i.e., on the console output).

The multido package is also 'generic' (usable both in Plain TeX and LaTeX); it defines a command \multido with three arguments:

  \multido{<variables>}{<repetitions>}{<stuff to repeat>}

When the macro is executing, the <stuff to repeat> gets executed <repetitions> times; the <variables> gives a list of variables that can be used in <stuff>. Each variable is a composite of a command sequence and how it varies; so a variable \iz=2+4 sets \iz to 2 first time around, then 6 and 10 in the next two iterations, and so on. (The variable \iz is an integer; variables with other initial letters represent different data types.)

Both current LaTeX and (experimental) LaTeX3 have iteration commands for internal use and for package writers; their use is probably not recommendable.

The LaTeX distribution package ifthen offers the macro \whiledo:

\newcounter{ct}
...
\setcounter{ct}{1}
\whiledo {\value{ct} < 5}%
{%
  \thect\
  \stepcounter {ct}%
}

The forloop package provides nothing but \forloop:

\newcounter{ct}
...
\forloop{ct}{1}{\value{ct} < 5}%
{%
  \thect\
}

as you can see, the arguments are counter, starting value and termination condition; an optional argument supplies a step value (default step is 1).

The LaTeX picture environment has a simple command for repeated drawing:

\multiput(x,y)(xstep,ystep){n}{obj}

which places <obj> (intended to be a bit of picture) <n> times at positions (<x>, <y>), (<x>+<xstep>, <y>+<ystep>), (<x>+2<xstep>, <y>+2<ystep>) and so on, adding the displacement again each time. The command was designed for use in picture, but it makes no check, and may even be used to provide eccentric typesetting in a “regular” sentence, such as:

Here \multiput(0,0)(1,1){3}{we} are again.

with predictable (if not actually desirable) effect. It may be used with nothing but an iterative calculation in the braced argument, in which case its graphical capabilities have no effect.

The pgffor package, which is part of the PGF distribution, also provides iterations to support the needs of graphics. Its syntax is in the style of common programming languages:

\documentclass{article}
  \usepackage{pgffor}
 
\newcommand{\cmd}{-x-}
 
\begin{document}
  \foreach \n in {1,...,4}{\cmd{}}
\end{document}

The \foreach command has the potential drawback that its repeated unit is executed in a group, so that any calculations done within the loop are lost (unless their result is made \global); however, it does not 'build in' its graphical origins (as \multiput does) so its potential outside its own graphics environment “home” is more clear.

Source: Repeating a command ‘n’ times