Repeat command n times?

Question

Is it possible to define a command, which repeats the following command n-times? Call it for example \Repeat, then

\Repeat[4] \command{...}

should be equivalent to

\command{...} \command{...} \command{...} \command{...}

Note that \repeat is already defined by LaTeX as end-macro for \loop.

Joseph Wright · Accepted Answer · 2011-04-20 12:22:28Z

This can be done in an expandable form using \csname. I would personally use the 'pre-packed' version in expl3:

\documentclass{article}
\usepackage{expl3}
\ExplSyntaxOn
\cs_new_eq:NN \Repeat \prg_replicate:nn
\ExplSyntaxOff
\begin{document}
\Repeat{4}{\command{...}}
\end{document}

For those who would code by hand, the basic approach (thanks to David Kastrup) is

\catcode `\@ = 11\relax
\def\replicate#1{%
  \csname empty\expandafter \replicate@first@aux
    \romannumeral - `0\number#1\endcsname
  \endcsname
}
\def\replicate@first@aux#1{%
  \csname replicate@first@#1\replicate@aux
}
\expandafter\long\expandafter\def\csname replicate@first@-\endcsname
  #1{\endcsname\NegativeReplication}
\expandafter\long\expandafter\def\csname replicate@first@0\endcsname
  #1{\endcsname}
\expandafter\long\expandafter\def\csname replicate@first@1\endcsname
  #1{\endcsname #1}
\expandafter\long\expandafter\def\csname replicate@first@2\endcsname
  #1{\endcsname #1#1}
\expandafter\long\expandafter\def\csname replicate@first@3\endcsname
  #1{\endcsname #1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@4\endcsname
  #1{\endcsname #1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@5\endcsname
  #1{\endcsname #1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@6\endcsname
  #1{\endcsname #1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@7\endcsname
  #1{\endcsname #1#1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@8\endcsname
  #1{\endcsname #1#1#1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@first@9\endcsname
  #1{\endcsname #1#1#1#1#1#1#1#1#1}
\def\replicate@aux#1{%
  \csname replicate@#1\replicate@aux
}
\expandafter\long\expandafter\def\csname replicate@\endcsname#1{}
\expandafter\long\expandafter\def\csname replicate@0\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}}
\expandafter\long\expandafter\def\csname replicate@1\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1}
\expandafter\long\expandafter\def\csname replicate@2\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1}
\expandafter\long\expandafter\def\csname replicate@3\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1}
\expandafter\long\expandafter\def\csname replicate@4\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@5\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@6\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@7\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@8\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1#1#1#1#1}
\expandafter\long\expandafter\def\csname replicate@9\endcsname
  #1{\endcsname{#1#1#1#1#1#1#1#1#1#1}#1#1#1#1#1#1#1#1#1}
\catcode `\@ = 12\relax
\edef\test{\replicate{20}{abc}}
\show\test
\bye

In the expl3 version, the \number#1 is (effectively) replaced by \number\numexpr#1\relax, which allows the 'number' used to be a calculation. If you try a negative number, the deliberately-undefined control sequence raises an error as part of the expansion, rather than having some odd error later.

A second approach which is expandable and does not require e-TeX is to use \romannumeral, for example

\catcode `\@ = 11\relax
\def\replicate#1{%
  \expandafter\replicate@aux\romannumeral\number #1000Q{}
}
\def\replicate@aux#1{\csname replicate@aux@#1\endcsname}
\long\def\replicate@aux@m#1Q#2#3{\replicate@aux#1Q{#2#3}{#3}}
\long\def\replicate@aux@Q#1#2{#1}
\edef\test{\replicate{5}{a}}
\show\test
\bye

This is clearer to code than the \csname approach, but is effectively a loop again and so gets slow for large numbers of repetitions.

Ah, I should have known that LaTeX3 is having something like this. I coded something by myself and just saw your post.
Note that while this seems like a overkill-approach, it should be (much?) more efficient than the simpler looping code proposed in other answers.
As @Will says, this approach scales well (I use it for testing purposes often with 100k repetitions).
The important point to notice about the code is that it replicates for each power of ten. So as the number gets very big, rather than lots of single repetitions each 'level' requires only one pass.
However, for huge number of repetitions, since all the repetitions are held in TeX's memory, we can overflow it. A solution in that case is \prg_replicate:nn {10000}{\prg_replicate:nn {10000}{...}}. Not needed often.

Tobi · Answer 2 · 2011-04-19 20:38:57Z

up vote 5 down vote

You can use the \foreach-command from PGF/TikZ.

\documentclass{minimal}
\usepackage{pgffor}
\newcommand{\cmd}{-x-}
% to provide your syntax
\newcommand{\Repeat}[2]{% \repeat already defined
    \foreach \n in {1,...,#1}{#2}
}
\begin{document}
\foreach \n in {1,...,4}{\cmd}

\Repeat{6}{\cmd}
\end{document}

For more information see the pgfmanual.pdf, section 56, pp. 504 an following.

There’s also a TeX-Way see e.g. this page (in german …)

edited Apr 19 '11 at 20:38

answered Apr 19 '11 at 19:19

Tobi
15.7k13792

1

pgfs \foreach has the drawback that the command is executed in a group, which might be a big problem or none at all depending on the application. BTW: there is no reason the write {} after \cmd inside the loop, except if you have it as a placeholder for a possible argument (then you should write it as {...} as the OP did). – Martin Scharrer♦ Apr 19 '11 at 19:33

@MartinScharrer There’s no special reason why I used the {}. You’re right an I edited my post. Could you please outline a situation where the \foreach-group causes problems? – Tobi Apr 19 '11 at 20:38

All assignments done in the loop will be executed in their own local group, which could have serious side effects depending on the code. Because you never known what code the user will use it is best to avoid these situations. – Martin Scharrer♦ Apr 19 '11 at 21:06

@MartinScharrer Thank’s. Maybe I remember that in case of getting problem with \foreachif not I’ll be back to ask here ;-) – Tobi Apr 19 '11 at 21:57

morbusg · Answer 3 · 2011-04-20 07:45:24Z

Plain and simple (pun intended):

\def\foo{keke}
\def\bar#1#2{\count0=#1 \loop \ifnum\count0>0 \advance\count0 by -1 #2\repeat}
\bar3\foo % results in kekekekekeke
\bye

Token list registers expand more quickly, so if it suits you, you could also do:

\newtoks\foo \foo={keke}
\def\bar#1#2{\count0=#1 \loop \ifnum\count0>0 \advance\count0 by -1 \the#2\repeat}
\bar3\foo % results in kekekekekeke
\bye

Repeating in an expendable way using e-tex additions (from http://www.tug.org/TUGboat/tb29-2/tb92jackowski.pdf):

\def\foo{keke}
\def\gobbleone#1{}
\long\def\replicate#1#2{% 
  \ifnum\numexpr#1>0 
    #2\expandafter\replicate\expandafter 
    {\number\numexpr#1-1\expandafter}% 
  \else 
    \expandafter\gobbleone 
  \fi{#2}}
\replicate3\foo % results in kekekekekeke
\bye

Yes, but not expandable. Ok, it isn't really necessary in the majority of the cases.
Christian forgot to notify @Martin I guess, since I don't know the answer to that question.
@Christian: Something is expandable if you can use it inside an \edef or \write an TeX converts it fully to what you want. Assignments are not expandable, and so a loop using a count will not turn into a series of repeated commands inside an \edef.
@Christian: I thought that the general question had been asked, but cannot find it. You might wish to ask about expandability in general, and those of us who understand it can try to explain!

Martin Scharrer · Answer 4 · 2011-04-19 19:55:09Z

up vote 2 down vote

Here some implementation I came up with which doesn't need any extra package. It uses \numexpr to avoid counters and is fully expandable.

\documentclass{article}

\makeatletter
\newcommand{\Repeat}[1]{%
    \expandafter\@Repeat\expandafter{\the\numexpr #1\relax}%
}

\def\@Repeat#1{%
    \ifnum#1>0
        \expandafter\@@Repeat\expandafter{\the\numexpr #1-1\expandafter\relax\expandafter}%
    \else
        \expandafter\@gobble
    \fi
}
\def\@@Repeat#1#2{%
    \@Repeat{#1}{#2}#2%
}
\makeatother

\begin{document}

\Repeat{0}{test }

\Repeat{1}{test }

\Repeat{2}{test }

\Repeat{3}{test }

\Repeat{4}{test }

\Repeat{5}{test }

\edef\TEST{\Repeat{5}{test }}
\texttt{\meaning\TEST}

\end{document}

answered Apr 19 '11 at 19:55

Martin Scharrer♦
102k8290470

	If you look at the LaTeX3 implementation, it's expandable even without needing e-TeX. This is some clever code that has been around for many years. – Joseph Wright♦ Apr 19 '11 at 20:33
	@Joseph: I have a really hard time reading the `expl3` code, but it looks like it needs e-TeX to me. It uses `\int_eval:w` a.k.a. `\numexpr`. – TH. Apr 20 '11 at 7:42
	@TH. The `expl3` implementation does indeed need `\numexpr`, as that makes the nature of the 'number' you give be more flexible. However, the original version of this approach just uses `\number`, and thus no e-TeX. Later on today I'll post the code 'translated' to plain TeX as a separate answer. – Joseph Wright♦ Apr 20 '11 at 8:09
	@TH. I've added the plain TeX code to my answer, avoiding e-TeX but explaining why it is used for the `expl3` version. – Joseph Wright♦ Apr 20 '11 at 8:34
	@Joseph: I see now, that's very clever! Thanks. (One of these days, I really just need to learn `expl3`.) – TH. Apr 20 '11 at 8:49

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Repeat command n times?

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