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Is there a convenient way to typeset the floor or ceiling of a number, without needing to separately code the left and right parts? For example, is there some way to do $\ceil{x}$ instead of $\lceil x \rceil$?

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    In case anyone else got here looking for mathjax notation, in mathjax, \lceil and \rceil are the appropriate markups. – Hugh Perkins Aug 21 '17 at 8:15
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    For me, $\lceil x \rceil$ was already the solution I was looking for ;) – Christopher K. Aug 29 '19 at 16:24
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\usepackage{mathtools}
\DeclarePairedDelimiter{\ceil}{\lceil}{\rceil}

The command \ceil will do; if called as \ceil*{x} it will add \left and \right; you can also call it as

\ceil[\big]{x} \ceil[\Big]{x} \ceil[\bigg]{x} \ceil[\Bigg]{x}

to state explicitly the size of the delimiters.

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    Even after more than 10 years of LaTeXing one still learns some new tricks! – yo' Jan 25 '12 at 17:32
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    Don't tell me! I learn new tricks even after 25 years! – egreg Jan 25 '12 at 17:34
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    @jamaicanworm There is, I believe also on this site. But I won't tell you: use \left and \right only when they are really needed. – egreg Apr 21 '12 at 17:50
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    @jamaicanworm Swap definition of starred and non-starred command discusses switching the starred with non-starred version. – Qrrbrbirlbel Nov 11 '12 at 1:36
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    To typeset the floor function, just replace "ceil" with "floor". This may be obvious, but it might save you the trouble of consulting documentation. – void-pointer Sep 30 '13 at 7:19
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Here is a simple xparse implementation of \ceil, similar to that provided by mathtools' \DeclarePairedDelimiter:

xparse implementation of \ceil

\documentclass{article}
\usepackage{xparse}% http://ctan.org/pkg/xparse
\NewDocumentCommand{\ceil}{s O{} m}{%
  \IfBooleanTF{#1} % starred
    {\left\lceil#3\right\rceil} % \ceil*[..]{..}
    {#2\lceil#3#2\rceil} % \ceil[..]{..}
}
\begin{document}
\[\ceil[\big]{x} \quad \ceil[\Big]{x} \quad \ceil[\bigg]{x} \quad \ceil[\Bigg]{x} \quad \ceil*[\big]{\frac{1}{2}}\]
\end{document}

The optional argument is ignored in the starred version of \ceil*[..]{..}.

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    You could eliminate the \IfNoValueTF {\lceil#3\rceil} if you used {s O{} m}. Then the second arg #2 will be defined so only need #2\lceil#3#2\rceil. – Peter Grill Jan 25 '12 at 18:29

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