5

I have equations of the form

a = b + d     for i <= 4, j < 3
  = c         for i = 5
  = d - e + u for i > 5 

I am writing them using the following code in latex:

\begin{equation} 
  a = \left\{
 \begin{array}{lr}
 b + d  & \mbox{for } i \le 4, j < 3 \\
 c & \mbox{for } i = 5 \\
 d - e + u & \mbox{for } i > 5
\end{array}
\right.
\label{eqn:simple_one} 
\end{equation}

But the problem is that the conditional statements starting with "for" are always right aligned and the output looks like this (with a left brace after the "=" sign, which i cannot draw :) ).

a = b + d     for i <= 4, j < 3
    c                 for i = 5
    d - e + u         for i > 5 

I wanted to make the condtional part left aligned such that the words "for" come exactly one below the other. How can this be done?

10

They are right aligned because you have {array}{lr} change that to {array}{ll} to left align both columns. But generally it is best not to use array for laying out whole equations, it is designed for matrices/arrays. Here I think the amsmath package and its cases environment would be better.

  • @David Carlisle: Maybe its also good to note the other cases environment if needed such as: rcases (used for a right-aligned brace), dcases (used for display-math inside of the cases environment, and others that maybe available. – night owl Jun 27 '12 at 22:51
10

using amsmath and \cases allows this to be accomplished with simpler coding:

\begin{equation} 
  a = \begin{cases}
 b + d  & \text{for $i \le 4$, $j < 3$} \\
 c & \text{for $i = 5$} \\
 d - e + u & \text{for $i > 5$}
\end{cases}
\label{eqn:simple_one} 
\end{equation}

with this result:

enter image description here

also note the use of \text instead of \mbox. \text will produce the correct size whether it's used at the base level or in sub- or superscripts or limits.

  • 4
    I'd type \text{for $i \le 4$, $j < 3$}, but it's a matter of taste. – egreg Jun 27 '12 at 21:33
  • @egreg -- you're absolutely right. updated in example. – barbara beeton Jun 27 '12 at 21:42

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