Distance between array rows

Question

I am using the array environment to create a multiple row bracket expression, as folows:

\documentclass{article}

\begin{document}

\begin{equation}
\left\{ \begin{array}{lll}
C(\tau_{k,i})=0.5Q_e\ddot{q}_t^T\ddot{q}_t + 0.5R_e \theta^T \theta & when & t\neq t_v\\
\\
C(\tau_{k,i})=0.5Q_e\ddot{q}_t^T\ddot{q}_t + 0.5R_e \theta^T \theta +    P_{viapoint} & when & t=t_v\\
\\
\quad\quad P_{viapoint}=10^{10}(q_{t}-q_{viapoint})^T(q_{t}-q_{viapoint})

\end{array}\right.  
\label{cost2}
\end{equation}

\end{document}

which results in something like this:

However, the vertical spacing between the equations is much greater that the line spacing in the text, and is eating too much space on the paper. Is there a way to reduce the vertical spacing?

Answer 1

The array (and tabular) environment sets struts in rows. The size of the struts can be reduced by using smaller values than 1 or \arraystretch:

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation}
\left\{
\renewcommand*{\arraystretch}{.5}
\begin{array}{lll}
C(\tau_{k,i})=0.5Q_e\ddot{q}_t^T\ddot{q}_t + 0.5R_e \theta^T \theta
& \text{when} &
t\neq t_v\\
\\
C(\tau_{k,i})=0.5Q_e\ddot{q}_t^T\ddot{q}_t + 0.5R_e \theta^T \theta +
P_{\text{viapoint}} & \text{when} & t=t_v\\
\\
\quad\quad P_{\text{viapoint}}
=10^{10}(q_{t}-q_{\text{viapoint}})^T(q_{t}-q_{\text{viapoint}})
\end{array}\right.
\end{equation}
\end{document}

You can also take a look at AmSLaTeX (package amsmath) that provides many environments for aligned/non-aligned equations and cases structures.