From: Niklas Frykholm <r2d2@...>
Date: 2002-06-07T07:33:28+09:00
Subject: Re: Unflatten

[Shashank Date]:
> I am trying to convert  a flat list (array) of  tokens  into a nested list
> (hence the name UNflatten) using a simple rule:
> Every token which looks like  '(' begins a nested list and the corresponding
> ')' token ends it ... in a recursive fashion.
> e.g:
> ["a", "(","b", ")"]                 ==> ["a",[ "b" ]]
> [ "(", "a", "(","b", ")", ")"]    ==> [["a",["b"]]]

My suggestions:

def unflatten(list, left='(', right=')')
    current = []
    stack = []

    list.each do |x|
        if x == left
            stack << (current << [])
            current = current[-1]
        elsif x == right
            raise "unbalanced" if stack.size == 0
            current = stack.pop
        else
            current << x
        end
    end
    raise "unbalanced" unless stack.size == 0
    return current
end

def reflatten(list, left='(', right=')')
    acc = []
    list.each do |x|
        if Array === x
            acc << left 
            acc += reflatten(x, left, right)
            acc << right
        else
            acc << x
        end
    end
    return acc
end

// Niklas