newcommand and math environment

Question

So I've created a latex command using newcommand. This is its definition

\newcommand{\twopartdef}[3]
{
\left\{
    \begin{array}{ll}
        #1 & \mbox{if } #2 \\
        #3 & \mbox{otherwise}
    \end{array}
\right
}

But when I try to use the command with this example

f$(x)$ = \twopartdef{$\lbrace \alpha \rbrace}{$x >$ 10}{\lbrace \beta \rbrace}

Latex tells me that it added the symbol $ because it was unmatched..

What am doing wrong? What corrections should I make?

Answer 1

I assume you are writing this entire array construction in a display math environment (since it's at least two lines in height. As such, you don't need any math-switching delimiters inside your usage:

\documentclass{article}
\newcommand{\twopartdef}[3]{%
  \left\{
    \begin{array}{ll}
      #1 & \mbox{if } #2 \\
      #3 & \mbox{otherwise}
    \end{array}
  \right.
}
\begin{document}
\[
  f(x) = \twopartdef{\{ \alpha \}}{x > 10}{\{ \beta \}}
\]
\end{document}
​

You have a \arraycolsep gap between the extensible left brace and the start of your "two part definition". If you want to remove that, use the array column specification {@{}ll}. Also, I've used \right. (not just \right) to end off the extensible brace pair.

amsmath provides a cases environment that does something similar. Here's your definition using that:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\newcommand{\twopartdef}[3]{%
  \begin{cases}
    #1 & \text{if #2} \\
    #3 & \text{otherwise}
  \end{cases}
}
\begin{document}
\[
  f(x) = \twopartdef{\{ \alpha \}}{$x > 10$}{\{ \beta \}}
\]
\end{document}

For completeness, the entire condition is set in \text, which requires you to wrap x>10 in math mode again.