Alternately taking elements from an array

Question

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

Answer 1

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

• tokland: Looks good. Some details: maybe use unless instead of if, give plural names to a and b (maybe: xs, ys) and unpack the pair |(x, y), idx| ...
• Michael: Is there any way to use iterators or something like that and avoid using indices?
• Jonah: @tokland, Both good suggestions, edited.
• tokland: @Michael. A lot of ways, for example: xs.zip(ys, [true, false].cycle).map { |x, y, take_x| take_x ? x : y }. Not that there is anything wrong with using with_index.
• tokland: Jonah, the second version is so horrible.. I don't think it's worth it. Children watching! :) There is nothing wrong with using with_index, it's not the same than accessing the array by indexes (which is indeed ugly).