Create simple repetitive macro with optional argument

Question

I would like the simplest (in terms of length of characters) way to create a macro that does something or, optionally, does something n times.

In fact, all I want to do is to create something like --------X where the -`s go from 1 to n. I do not want to have to supply 1 to the macro as that is default and the most used case.

So

\dash{X} gives -X
\dash[3]{X} gives ---X
or
\dash{X}[3] gives ---X
or 
\dash{X}{3} gives ---X
or
\dash{3}{X} gives ---X

I don't care about the syntax. Just that it is short and works as intended and should be efficient. I probably don't want to have nested groups used because there would be no point and it would be inefficient, but either way doesn't matter.

I'd like to use as little extra packages as possible unless some package does this well to make it pretty short.

Something like

\newcommand{\dash}[2][1]{\directlua{for i=1,#1 do tex.print('-') end}#2}%

works with LuaLaTeX but tex.print seems to add a space after/before each dash resulting in - - - -X.

Answer 1

Using xparse (with options):

\documentclass{article}

\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\dash}{O{1} O{-} m}{%
  \prg_replicate:nn {#1} {#2}#3
}
\ExplSyntaxOff

\begin{document}

\dash{X}

\dash[2]{X}

\dash[15][{{-}}]{X}

\dash[7][a]{X}

\end{document}

Reference: Repeat characters n times

Answer 2

Just use one of David Kastrup's good old \replicate-macros that are described here:

http://www.gust.org.pl/projects/pearls/2005p/david-kastrup/bachotex2005-david-kastrup-pearl3.pdf

Example:

\documentclass{article}

\makeatletter
\newcommand\xii[2]{\if#2m#1\expandafter\xii\else\expandafter\@gobble\fi{#1}}
\newcommand\xiii{}\long\def\xiii#1\relax#2{\xii{#2}#1\relax}
\newcommand\replicate[1]{\expandafter\xiii\romannumeral\number\number#1 000\relax}
%
\newcommand\dash[1][1]{\replicate{#1}{-}\@firstofone}
%
% You can avoid consecutive dashes yielding en-dash-ligatures and em-dash-ligatures
% by wrapping the single dash into braces---\hyphendash yields hyphens:
%
\newcommand\hyphendash[1][1]{\replicate{#1}{{-}}\@firstofone}
\makeatother

\begin{document}

\verb|\dash[0]{X}| should yield X and indeed yields \dash[0]{X}

\verb|\dash{X}| should yield -X and indeed yields \dash{X}

\verb|\dash[1]{X}| should yield -X and indeed yields \dash[1]{X}

\verb|\dash[2]{X}| should yield --X and indeed yields \dash[2]{X}

\verb|\dash[3]{X}| should yield ---X and indeed yields \dash[3]{X}

\leavevmode\leaders\hbox{-}\hfill\null

\verb|\hyphendash[0]{X}| should yield X and indeed yields \hyphendash[0]{X}

\verb|\hyphendash{X}| should yield {-}X and indeed yields \hyphendash{X}

\verb|\hyphendash[1]{X}| should yield {-}X and indeed yields \hyphendash[1]{X}

\verb|\hyphendash[2]{X}| should yield {-}{-}X and indeed yields \hyphendash[2]{X}

\verb|\hyphendash[3]{X}| should yield {-}{-}{-}X and indeed yields \hyphendash[3]{X}

\end{document}  

If it is only about dashes/hyphens and if you don't want dashes/hyphens to be broken across lines, you can probably also fill a horizontal box of predetermined width with horizontal \leaders:

\documentclass{article}

\makeatletter
\newbox\mytempbox
\newcommand\nobreakhyphendash[1][1]{%
  \begingroup
  \setbox\mytempbox\hbox{-}%
  \vbox{\hbox to#1\wd\mytempbox{\leaders\box\mytempbox\hfill}}%
  \endgroup 
  \@firstofone
}
\makeatother

\begin{document}

\verb|\nobreakhyphendash[0]{X}| should yield X and indeed yields \nobreakhyphendash[0]{X}

\verb|\nobreakhyphendash{X}| should yield {-}X and indeed yields \nobreakhyphendash{X}

\verb|\nobreakhyphendash[1]{X}| should yield {-}X and indeed yields \nobreakhyphendash[1]{X}

\verb|\nobreakhyphendash[2]{X}| should yield {-}{-}X and indeed yields \nobreakhyphendash[2]{X}

\verb|\nobreakhyphendash[3]{X}| should yield {-}{-}{-}X and indeed yields \nobreakhyphendash[3]{X}

\verb|\nobreakhyphendash[15]{X}| should yield {-}{-}{-}{-}{-}{-}{-}{-}{-}{-}{-}{-}{-}{-}{-}X and indeed yields \nobreakhyphendash[15]{X}

\end{document}

Answer 3

You might enjoy this generalization:

\documentclass{article}
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\pattern}{>{\SplitList{,}}m}
 {
  \ProcessList{#1}{\MakePattern}
 }
\NewDocumentCommand{\MakePattern}{m}
 {
  \MakePatternAux #1 \MakePatternAux
 }
\NewDocumentCommand{\MakePatternAux}{O{1}u{\MakePatternAux}}
 {
  \prg_replicate:nn { #1 } { {#2} }
 }
\ExplSyntaxOff

\begin{document}

\pattern{[3]-,X,-,Y,[5]abc}

$\pattern{[3]-,X,-,Y,[5]+}$

\end{document}

You describe a pattern by a comma separated list of items: X means “print one copy of X”, while [5]Y means “print five copies of Y). The additional braces make TeX into not adding automatic spacing between atoms, because all are treated as ordinary atoms. Items can be more than one token.