String interpolation

I’m confronted with some problems in strings.
If I create string like this one:

junk = “You junked %{item}”
Then I can put text into it like this:
junk % { :item => ‘some item’ }
But if I get this string:
junk = “You junked %{item} and you can choose %{reward($condition=Bark five times)}”

I can’t just access to :reward symbol until I doesn’t know the ($condition=Bark five times).
How can I handle with that?

Please forgive me for such a dumb question and weird english.

I'm not sure what the standard is, but I expect you'd handle the
conditional when entering the value into the hash.

hash = {}
hash[:reward] = condition ? "Bark five times" : "Bark four times"
"%{reward}" % hash

Thanks you for reply!
I already solved originaly problem with Interpolate features from I18n.