Whitespace characters

Question

I am defining a command that uses an subscript somewhere. I want the subscript to dissappear when I leave its corresponding command-input open, but then LaTeX will trow an error since no character was put in the subscript. Now I want to type a dummy character after the input to ensure that at least one character is printed at all times. However, I don't want this dummy character to show up. It just needs to be there to decieve LaTeX.

If I type \infty, I get an infinity symbol, if I print the character I am asking for, LaTeX prints a character that has no black.

In this code, the command \spacechar takes the role of the character I want

\newcommand{\tup}[3]{(-\; #1 -\! \underset{\! #2 \,\,\,}{)_{#3\spacechar}}}

instead of

\newcommand{\tup}[3]{(-\; #1 -\! \underset{\! #2 \,\,\,}{)_{#3}}}

which breaks when #3 is empty.

Some example of the code in action

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\newcommand{\tup}[3]{(-\; #1 -\! \underset{\! #2 \,\,\,}{)_{#3}}}
\begin{document}
\noindent $\tup{x_i}{i}{n}$\\
$(-\; x_i -\! \underset{\!  \,\,\,}{)}$
\end{document}

Something like this is also useful in many other places where LaTeX, such as when LaTeX wants to remove whitespace at the beginning of a sentence.

This is a slightly related but unsatisfying answer. Whitespace as wide as a character It gives whitespace commands, which are not actual characters. They are whitespace, and whitespace can't decieve LaTeX.

This is also not what I'm looking for. Explicit space character? Instead of invisible characters it gives me visible whitespace.

In short, I want a character that prints absolutely nothing.

Welcome. It would help if you posted some code, so that we could see the context. In general $a_{}$ works just fine. — Steven B. Segletes, Oct 18 '19 at 15:02
Your question isn't clear (to me, at least). If I understand correctly, you want braces after the subscript, so that either a_{} or a_{b} will work. — Phelype Oleinik, Oct 18 '19 at 15:03
You probably want to have a look at xparse and all of its goodies of parsing arguments, graping subscripts etc. — daleif, Oct 18 '19 at 15:40
I see, when arg #3 is absent, it compiles, but moves #2 relative to the closing paren. What is it you actually want when #3 is missing? — Steven B. Segletes, Oct 18 '19 at 15:45
what do you mean by "breaks when empty" ? _{} is not an error? — David Carlisle, Oct 18 '19 at 15:59
@DavidCarlisle True. It does work now, for some reason. I think I made a typo in my file and corrected it here, and fixed it while playing around with it. — Niels Korpel, Oct 18 '19 at 16:03

score 3 · Answer 1 · answered Oct 18 '19 at 15:49

Your problem in using \underset is that it tries to center the underset material. You want the underset to be left aligned with the paren, if I understand your intentions. To do that, I use a left-aligned stack.

Here, I show the 4 cases where #2 and #3 are either present or absent.

\documentclass{article}
\usepackage{stackengine}
\usepackage{mathtools}
\newcommand{\tup}[3]{(-\; #1 -\! 
  \ensurestackMath{\stackengine{1pt}{)_{#3}}{\scriptstyle#2}{U}{l}{F}{F}{S}}}
\begin{document}
\noindent $\tup{x_i}{i}{n}$\\[4pt]
$\tup{x_i}{i}{}$\\[4pt]
$\tup{x_i}{}{n}$\\[4pt]
$\tup{x_i}{}{}$
\end{document}

Thanks, it does make it better. – Niels Korpel Oct 18 '19 at 16:11 — Niels Korpel, Oct 18 '19 at 16:11
@NielsKorpel You can upvote the answer if you like it. – Dr. Manuel Kuehner Oct 18 '19 at 16:15 — Dr. Manuel Kuehner, Oct 18 '19 at 16:15

egreg · Answer 2 · 2019-10-18T16:38:58.357

There is no need for a “\spacechar”.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\newcommand{\tup}[3]{%
  ({-}\; #1 \;{-}%
  \mathclose{\underset{\mathclap{#2\,}}{)}}%
  \if\relax\detokenize{#3}\relax\else
    \mathclose{}_{#3}%
  \fi
}

\begin{document}

$\tup{x_i}{i}{n}$

$\tup{x_i}{i}{}$

\end{document}

With \underset{\mathclap{#1\,}{)} we set the argument below the parentheses, making the whole construction a Close atom; if the third argument is not empty, a phantom parentheses receives the subscript.

I also propose a friendlier syntax, where the subscript is specified with the standard _:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{xparse}

\NewDocumentCommand{\tup}{mme{_}}{%
  ({-}\; #1 \;{-}
  \mathclose{\underset{\mathclap{#2\,}}{)}}%
  \IfValueT{#3}{\mathclose{\vphantom)}_{#3}}%
}


\begin{document}

$\tup{x_i}{i}_{n}$

$\tup{x_i}{i}$

\end{document}

Similarly, but with also the second argument optional, in case you sometimes need no variable below the parenthesis.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{xparse}

\NewDocumentCommand{\tup}{mme{_}}{%
  ({-}\; #1 \;{-}
  \mathclose{\underset{\mathclap{#2\,}}{)}}%
  \IfValueT{#3}{\mathclose{\vphantom)}_{#3}}%
}


\begin{document}

$\tup{x_i}{i}_{n}$

$\tup{x_i}{i}$

\end{document}

score 0 · Answer 3 · answered Oct 18 '19 at 16:10

0

The definition in the question is correct and does not break. I presume I made forgot the braces in the actual file. The above answer from @Steven B. Segletes does make the command better.

However, as for my question on invisible characters, I have found an imperfect solution.

\usepackage{xcolor}
\newcommand{\spacechar}{\text{\textcolor{white}{\tiny .}}}

This will print a tiny white dot, which works so long as the background isn't white.

answered Oct 18 '19 at 16:10

Niels Korpel

123

2

why would you ever want a coloured . ? not say a thin space like \, or nothing? I don't think this is really an answer to the question as asked. If you turned on tracing to see how much work tex does to evaluate \text{\textcolor{white}{\tiny .} you would see several hundred lines, \text evaluates its argument four times, and \tiny loads an entire font setup, \textcolor inserts additional colour whatsits.... – David Carlisle Oct 18 '19 at 16:12
@DavidCarlisle why \text evaluates its argument four times? I don't understand. Thank you! – manooooh Oct 19 '19 at 05:52
@manooooh https://tex.stackexchange.com/a/110637/1090 – David Carlisle Oct 19 '19 at 07:07
@DavidCarlisle I still do not understand. What does \mathchoice command have to do with \text? – manooooh Oct 27 '19 at 01:05
1

@manooooh the difference between mbox and text is just that text uses mathchoice so it is smaller in subscripts – David Carlisle Oct 27 '19 at 08:17

score 0 · Answer 4 · answered Oct 18 '19 at 16:22

With xparse optional arguments:

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}
\usepackage{xparse}
\ExplSyntaxOn
\NewDocumentCommand \otup { m O{} O{} }{
    (-\; #1 -\! \underset{\tl_if_empty:nTF{#3}{}{\!} #2 \,\,\,}{\tl_if_empty:nTF{#3}{\!\!}{})\sb{#3}}
}
\ExplSyntaxOff
\begin{document}
$\otup{x_i}[i][n]$

$\otup{x_i}[i]$

$\otup{x_i}$

\end{document}

This does not use the stackengine solution of the other answer, instead when the 3rd argument is empty (not given) then the right parenthesis is shifted back a bit, if the argument is not empty then the underset argumentitself (#2) is shifted left.

Note that with \ExplSyntaxOn the _ character is no longer a math subscript, so \sb is used instead.

Whitespace characters

4 Answers4