If you do not need the value of $i, you could use the XOR operator:

$i = 0
for( $j = 0; $j < 10 ; $j++ )
{
   if( $i == 0 )
   {
     $class = "even" ;
   }
   else
   {
     $class = "even" ;
   }
   $i ^= 1 ;
}
[download]

Or you could toggle $i between two values:

$i = 1 ;
for( $j = 0; $j < 10 ; $j++ )
{
   $i *= -1 ;
   if( $i == -1 )
   {
     $class = "even" ;
   }
   else
   {
     $class = "even" ;
   }
}
[download]

Or you could just flip the value of $class:

if( $class =~ /even/ )
{
   $class = "odd" ;
}
else
{
   $class = "even" ;
}
[download]

Or

$class = ( $class ~= /even/ ) ? "odd" : "even" ;
[download]

Enjoy
Keith