Split an array of elements into a set of smaller arrays of equal size. Extra elements are preferentially assigned to earlier arrays. If there are no elements in a given returned array it will be [] (empty array)

# use as standalone function
def chunk_array(array, pieces=2)
  len = array.length;
  mid = (len/pieces)
  chunks = []
  start = 0
  1.upto(pieces) do |i|
    last = start+mid
    last = last-1 unless len%pieces >= i
    chunks << array[start..last] || []
    start = last+1
  end
  chunks
end

# use as array.chunk
class Array
  def chunk(pieces=2)
    len = self.length;
    mid = (len/pieces)
    chunks = []
    start = 0
    1.upto(pieces) do |i|
      last = start+mid
      last = last-1 unless len%pieces >= i
      chunks << self[start..last] || []
      start = last+1
    end
    chunks
  end
end

Examples of use:

>> chunk_array [1,2,3,4,5,6], 2
=> [[1, 2, 3], [4, 5, 6]]

>> chunk_array [1,2,3,4,5,6], 3
=> [[1, 2], [3, 4], [5, 6]]

>> chunk_array [1,2,3,4,5,6], 4
=> [[1, 2], [3, 4], [5], [6]]

>> chunk_array [1,2,3,4,5,6,7,8,9,10], 4
=> [[1, 2, 3], [4, 5, 6], [7, 8], [9, 10]]

>> chunk_array [1,2,3], 4
=> [[1], [2], [3], []]

>> chunk_array [], 2
=> [[], []]

if you prefer the second form (more ruby-ish, but not always appropriate)

>> [1,2,3,4,5,6,7,8,9,10].chunk
=> [[1, 2, 3, 4, 5], [6, 7, 8, 9, 10]]

>> [1,2,3,4,5,6,7,8,9,10].chunk 3
=> [[1, 2, 3, 4], [5, 6, 7], [8, 9, 10]]

This is handy when used with a splat because you can do things like:

left, right = *chunk_array(all,2)