How do I make ditto marks?

Question

In France, students are taught not to repeat the same text twice when there is little difference between a line and the one below. It's a quick way to take notes.

I reproduced this effect by tinkering:

The code is as follows:

\documentclass[aspectratio=1610]{beamer} 
\usepackage[T1]{fontenc}
\usepackage[french]{babel} 
\usepackage{amsmath, amsfonts, amssymb}
\usefonttheme[onlymath]{serif}

\begin{document}
\begin{frame}
Lorsque le \og \textit{carreau carré} \fg{}   mesure:
\begin{itemize}
\item $\rm 1\,cm$ de côté, l'unité d'aire est appelée le \og \textrm{centimètre carré} \fg{} et est noté $\rm cm^2$;
\item $\rm 1\,dm$ \hspace{1mm}'' \hspace{3mm}'' \hspace{8mm}'' \hspace{8mm}'' \hspace{5mm}''\hspace{8mm}'' \hspace{5.5mm}''\hspace{1mm} \og \textrm{décimètre carré} \fg{} \hspace{2mm}'' \hspace{2mm}'' \hspace{4mm}'' \hspace{1.5mm} $\rm dm^2$;
\item $\rm 1\,m$ \hspace{3mm}'' \hspace{3mm}'' \hspace{8mm}'' \hspace{8mm}'' \hspace{5mm}'' \hspace{7mm}'' \hspace{5.5mm}''\hspace{1mm} \og \textrm{mètre carré} \fg{}  \hspace{9mm}'' \hspace{2mm}'' \hspace{4mm}'' \hspace{1mm} $\rm m^2$;
\item etc.
\end{itemize}
\end{frame}
\end{document}

How can I get the same effect more simply than by tinkering like I just did?

Answer 1

Since @samcarter was faster than me, I had to build some special functionality in my answer, borrowing some code from @egreg (link):

\documentclass[aspectratio=1610]{beamer}
\usepackage[french]{babel}
\usepackage[T1]{fontenc}
\usepackage{amsmath, amsfonts, amssymb}
\usefonttheme[onlymath]{serif}

% from egreg's answer at
%https://tex.stackexchange.com/a/280981/97512
\usepackage{xparse}

\ExplSyntaxOn
\NewDocumentCommand{\xsameword}{m}
{
  \leavevmode\maieul_xsameword:n { #1 }
}

\tl_new:N \l__maieul_xsameword_list_tl

\cs_new_protected:Nn \maieul_xsameword:n
{
  \tl_set:Nn \l__maieul_xsameword_list_tl
  {
    \__maieul_xsameword_start:n { #1 }
  }
  \regex_replace_all:nnN
  { (\c{footnote}\cB..*?\cE.) }
  { \cE\} \1 \c{__maieul_xsameword_start:n} \cB\{ }
  \l__maieul_xsameword_list_tl
  \tl_use:N \l__maieul_xsameword_list_tl
}
\cs_new_protected:Nn \__maieul_xsameword_start:n
{
  \tl_set:Nn \l__maieul_xsameword_list_tl { #1 }
  \regex_replace_all:nnN
  { ([^\s,.!?]+) }% Delete what should not be printed again here
  { \c{maieul_xsameword_format:n} \cB\{ \1 \cE\} }
  \l__maieul_xsameword_list_tl
  \tl_use:N \l__maieul_xsameword_list_tl
}

\NewDocumentCommand{\setxsamewordformat}{m}
{
  \cs_set_protected:Nn \maieul_xsameword_format:n { #1 }
}
\ExplSyntaxOff
%%%%%%%%%%%%%%%%%%%%%

\newbox\mytmpbox

\newcommand{\repword}[1]{%
  \sbox{\mytmpbox}{#1}%
  \hbox to \wd\mytmpbox{\hss\hspace{.2em}''\hss}%
}

\setxsamewordformat{\repword{#1}}

\newcommand{\keepword}[2]{%
  \sbox{\mytmpbox}{#2}%
  \hbox to \wd\mytmpbox{\hss#1\hss}%
}

\begin{document}
\begin{frame}

Lorsque le \og \textit{carreau carré} \fg{}   mesure:
\begin{itemize}
  \item {1}\,$cm$ de côté, l'unité d'aire est appelée le \og \textit{\textrm{centimètre carré}} \fg{} et est noté $cm^2$;

  \item 1\,\keepword{$dm$}{$cm$} \xsameword{de côté, l'unité d'aire est appelée le} \keepword{\og \textit{\textrm{décimètre carré}} \fg{}}{\og \textit{\textrm{centimètre carré}} \fg{}} \xsameword{et est noté} $cm^2$;

  \item 1\,\keepword{$m$}{$cm$} \xsameword{de côté, l'unité d'aire est appelée le}  \keepword{\og \textit{\textrm{mètre carré}} \fg{}}{\og \textit{\textrm{centimètre carré}} \fg{}} \xsameword{et est noté} \keepword{$m^2$}{$cm^2$};
\end{itemize}

\end{frame}
\end{document}

This allows you to put a whole sentence in the argument of \xsamewords and they will be replaced. Also, I built a macro that allows you to insert words taking the same amount of space as another word: \keepwords{this line}{line above}.

edit: Now I put the first $dm$ in \keepwords and it is aligned properly, thanks @marmot for pointing me there! :-)

edit2: Example with three lines added.

edit3: Added \usepackage[T1]{fontenc} and inserted a comment showing which line should be edited to remove, e.g., commas.

The line { ([^\s,.!?]+) } represents a regular expression, or short regex. These can be used for advanced pattern matching, if a simple find or find & replace is not sufficient. Here, this regex represents the words that should be replaced by '' your ditto marks:

• [..] brackets match one character from the inside. [abc] operated on foobar would match the b or the a (but only one of them, a single character).
• [^..] is the negation of the characters inside, so this regex here is matching anything not present. [^abc] operated on foobar would match f, o, o, or r.
• \s is any white space character (spaces, tabs, sometimes also new lines)
• ,.!? are self-explanatory I assume ;)
• + after a character (remember, brackets match a single character) makes the character match as many multiple ("identical") characters, as possible (usual behavior) but at least one. So, e.g., o+ in regex operated on the string fooobar would match ooo, not just one or two os. [abc]+ operated on foobar would match ba.

You can see now, this regex matches everything that is not a white space or punctuation mark, i.e., everything else, that is words with all other special characters. You can think of the symbols inside the negated brackets to be the word delimiters that you still want to print and not replace in this context.

edit4: If you add \leavevmode before calling (the old) \xsameword, it should work fine at the beginning of a line as well. Now, I added it in the definition of \xsameword, so there's no need to take care of it manually.

Answer 2

Instead of guessing the required spaces, you could measure the width of the words you want to replace:

\documentclass[aspectratio=1610]{beamer} 
\usepackage[french]{babel} 
\usepackage{amsmath, amsfonts, amssymb}
\usefonttheme[onlymath]{serif}
\usepackage{siunitx}

\newlength{\mywidth}

\newcommand{\test}[1]{%
\settowidth{\mywidth}{#1}%
\makebox[\mywidth][c]{''}%
}

\newlength{\centwidth}
\settowidth{\centwidth}{<< centimètre carré >>}

\newlength{\foowidth}
\settowidth{\foowidth}{\SI{1}{\centi\meter}}

\begin{document}
\begin{frame}
Lorsque le \og \textit{carreau carré} \fg{}   mesure:
\begin{itemize}
\item \makebox[\foowidth][l]{\SI{1}{\centi\meter}} de côté, l'unité d'aire est appelée le \makebox[\centwidth][c]{\og \textit{\textrm{centimètre carré}} \fg} et est noté \si{\centi\meter\square};
\item \makebox[\foowidth][l]{\SI{1}{\deci\meter}} \test{de} \test{côté,} \test{l'unité} \test{d'aire} \test{est} \test{appelée} \test{le} \makebox[\centwidth][c]{\og \textit{\textrm{décimètre carré}} \fg} \test{et} \test{est} \test{noté} \si{\deci\meter\square};
\item \makebox[\foowidth][l]{\SI{1}{\meter}} \test{de} \test{côté,} \test{l'unité} \test{d'aire} \test{est} \test{appelée} \test{le} \makebox[\centwidth][c]{\og \textit{\textrm{mètre carré}} \fg} \test{et} \test{est} \test{noté} \si{\meter\square};
\end{itemize}
\end{frame}
\end{document}

Another approach could be to use a tabular

\documentclass[aspectratio=1610]{beamer} 
\usepackage[french]{babel} 
\usepackage{amsmath, amsfonts, amssymb}
\usefonttheme[onlymath]{serif}
\usepackage{siunitx}

\begin{document}
\begin{frame}
Lorsque le \og \textit{carreau carré} \fg{}   mesure:

\begin{tabular}{ll*{11}{@{~}c}@{~}l}
\usebeamertemplate{itemize item} & \SI{1}{\centi\meter} & de & côté,& l'unité &d'aire &est& appelée& le &\og \textit{\textrm{centimètre carré}} \fg{}& et &est& noté& \si{\centi\meter\square};\\
\usebeamertemplate{itemize item} & \SI{1}{\deci\meter} & " & " & " & " & " & " & " &\og \textit{\textrm{décimètre carré}} \fg{}& " & " & " & \si{\deci\meter\square};\\
\usebeamertemplate{itemize item} & \SI{1}{\meter} & " & " & " & " & " & " & " &\og \textit{\textrm{mètre carré}} \fg{}& " & " & " & \si{\meter\square};\\
\end{tabular}

\end{frame}
\end{document}