Alternately taking elements from an array

Given the task of having to return an array that alternately takes elements from two arrays, what would be most Ruby way of completing the task? As of now, I have this code, but it doesn't feel very elegant.

def alternateTake (a, b)
  raise ArgumentError, "Unequal length" if a.length != b.length
  build = []
  (0..a.length-1).each do |x|
    build << (x.even? ? a[x] : b[x])
  end
  build
end

Example input & output:

method([1, 4, 3, 7], [1, 4, 2, 33]) = [1, 4, 3, 33]
method([98, 12, 41], [35, 22, 14]) = [98, 22, 41]
method([12, 33], [66, 45, 3]) = ERROR

This one-liner should be what you're looking for:

def alternate_take(xs, ys) 
  raise ArgumentError, "Unequal length" if xs.length != ys.length
  xs.zip(ys).map.with_index{|(x,y),i| i.even? ? x : y} 
end

Follow Up

Tokland's answer to OP's followup about a solution without indexes is really clever. Here's another variation, which I don't like as much as the index version, but it answers the question:

def alternate_take(xs, ys) 
  raise ArgumentError, "Unequal length" if xs.length != ys.length
  xs.zip(ys).reduce([]){|m,(x,y)| m + (m.size.even? ? [x] : [y])} 
end

StackExchange Code Review Q#128535, answer score: 5