Macro for \left( and \right)

Question

I know there has been discussion about this topic, but I haven't seen a working solution. Is it possible to define a macro within LaTeX, such that inside math mode, ( would produce \left( and ) would produce \right)?

I know it is possible to define a different macro, say \lparen, but it has the disadvantage of not being as visually clear, and also not as simple, as (. I would like to override the ( command only in math mode. This issue is pretty annoying when writing a lot of math.

Answer 1

without using amsmath:

\documentclass{article}
\begingroup
  \catcode`(\active \xdef({\left\string(}  % ( is defined as \left(
  \catcode`)\active \xdef){\right\string)} % ) is defined as \right)
\endgroup
\mathcode`(="8000 \mathcode`)="8000        % active in mathmode

\begin{document}
\[ 
   (\frac{(3-\frac{1}{x})^2}{(\frac4x-1)x}) 
\]    
\end{document}

and if you are using package amsmath

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\def\resetMathstrut@{%
  \setbox\z@\hbox{%
    \mathchardef\@tempa\mathcode`\[\relax
    \def\@tempb##1"##2##3{\the\textfont"##3\char"}%
    \expandafter\@tempb\meaning\@tempa \relax
  }%
  \ht\Mathstrutbox@\ht\z@ \dp\Mathstrutbox@\dp\z@}
\makeatother
\begingroup
  \catcode`(\active \xdef({\left\string(}
  \catcode`)\active \xdef){\right\string)}
\endgroup
\mathcode`(="8000 \mathcode`)="8000
\begin{document}

\begin{align}
 f(x) &= (\frac{(3-\frac{1}{x})^2}{(\frac4x-1)x}-1)x^2 \\
\end{align}

\end{document}

Answer 2

Have a look at the nath package. It does automatic scaling of all delimiters. For example

\documentclass{article}
\usepackage{nath}

\begin{document}
\[ ( \sum_{i=1}^n a_i b_i ) \]
\end{document}

gives

Answer 3

You can redefine the category codes for ( and ) to be \active. And then, condition on whether or not you're in math mode via \ifmmode...\else...\fi:

\documentclass{article}

\def\lparen{(}% left parenthesis (
\catcode`(=\active
\newcommand{(}{\ifmmode\left\lparen\else\lparen\fi}
\def\rparen{)}% right parenthesis )
\catcode`)=\active
\newcommand{)}{\ifmmode\right\rparen\else\rparen\fi}

\begin{document}
This is some regular (bracketed) stuff in text mode. Here is some stuff in $(xyz)$ text style math mode. Here is some display style math stuff:
\[
  (\sum_{i=1}^n i)
\]
\end{document}

Answer 4

I strongly advise against automatically replacing ( and ) with \left( and right). This has a serious drawback when formulas have to be squeezed in order to fit them into a line. The problem is that the stuff between \left( and right) is set with natural spacing, whereas the top level math list is subject to stretching and shrinking (see E-TeX: Guidelines for Future TeX Extensions by Frank Mittelbach, Section 11). See yourself:

In the first line, the space around the first + is shrunk to 0, whereas the space around the second + is set to the natural width. In the second line, both +'s are treated in the same way.

\documentclass{article}
\textwidth=4.715cm
\begin{document}
$a+\left(b+c\right)$ doesn't look good \par
$a+(b+c)$ doesn't\llap{\rule[0.6ex]{1.1em}{0.7pt}\hskip0.1em} look good
\end{document}

Let me point out that the above does not apply to AMS environments like align or gather since in those the spaces are always set to their natural width.

Answer 5

I learned that making your file too "special" has its minus - in particular when you want to collaborate. Therefore, your source should be as simple as possible. In order to make the typing quicker, I got to know YASNIPPET for emacs; you can find similar tools, namely snippets, for other editors as well. For example in order to type

\( \left( x^2 \right) \)

I type:

m[TAB] pare[TAB]x^2[TAB][TAB]

yasnippet takes care of what I need according to the snippets I defined. This way I get a standard .tex file, easily entered. If you want further details, let me know.