Math Operators – Declaring an Operator with Automatic Argument Bracketing

delimitersmath-operators

I'd like to have an easy way to declare an operator that brackets its argument with a particular delimiter. As an example I'd like to be able to write

\Pr{X}

to mean the same as

\operatorname{Pr}\left[X\right]

FYI I know how to do

\newcommand{\Pr}[1]{\operatorname{Pr}\left[#1\right]}

but I thought I once saw a package that provided some command like

\DeclareBracketedOperator{\Pr}{Pr}{[}{]}

sort of a combination of \DeclarePairedDelimiter and \DeclareMathOperator and I think it also defined a starred version of the operator that didn't take any argument and didn't produce the delimiters.

Best Answer

I don't remember of any such package. Your \Pr command maybe obtained by

\usepackage{mathtools}
\DeclarePairedDelimiter{\Prfences}{[}{]}
\renewcommand{\Pr}{\operatorname{Pr}\Prfences}

With this definition, \Pr would behave exactly as if it was defined with \DeclarePairedDelimiter; that is, the *-form would use \left and \right.

If you want to always use \left and \right, you might follow Werner's suggestion, or go the hard way:

\makeatletter
\DeclareRobustCommand{\Pr}{\operatorname{Pr}\@ifstar\@firstofone\@Pr}
\newcommand{\@Pr}[1]{\left[#1\right]}
\makeatother

(requires amsmath, of course). Note: \renewcommand{\Pr}{...} is necessary, because \Pr is already defined in LaTeX.

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