left aligning the conditional statement inside an array

Question

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?

Answer 1

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.

Answer 2

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:

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.