Real nuby question.

This works fine...
  irb> puts root.elements['foo/bar[3]'].text
three

I can't work out how to do this...
  irb> i = 3
  irb> puts root.elements['foo/bar[i]'].text

I keep getting error message
NoMethodError: undefined method `text' for nil:NilClass

I've tried defining "i" every which way -- [i], [@i] and others.
  
  irb> $i = 3
  irb> [${i}] 
doesn't error, but it outputs as if i = 1.

All of the REXML examples for text nodes that I have been able to track
down have the index hard coded, or use .each

Can someone point me in the right direction?