From: William James <w_a_x_man@...>
Date: 2006-07-30T03:05:10+09:00
Subject: Re: how to split an array in sub arrays of the same length

William James wrote:
> bbiker wrote:
> > bbiker wrote:
> > > I believe the OP asked how to split an array in "x" sub arrays of equal
> > > length.
> > > To me, it appears that the response show how to partition an array into
> > > sub arrays of "y" elements.
> > >
> > > The following method does what the OP asked for, as far as I read it.
> > >
> > > class Array
> > > 	def split(n, b=[], d=[])
> > > 		#########################################################################
> > > 		# evenly split array 'a' into 'n' sub arrays
> > > 		#########################################################################
> > > 		if self.size == 0 or n <= 0
> > >     	b = nil
> > > 		elsif n == 1
> > >     	b = *self
> > > 		else
> > > 			# determine how many elements of the array should go in each sub
> > > arrays
> > > 			buckets = n
> > > 			length = self.size
> > > 			while buckets > 0
> > > 				elements = length / buckets + (length % buckets > 0 ? 1 : 0)
> > > 				d << elements
> > > 				length -= elements
> > > 				buckets -= 1
> > > 			end
> > > 			# p d
> > >
> > > 			# evenly distribute array elements into an array with 'n' sub arrays
> > > 			start = 0  # start
> > > 			0.upto(n-1) do |idx|
> > > 				len = d[idx]
> > > 				b[idx] = *self.slice(start,len)
> > > 				start += len
> > > 			end
> > > 		end
> > > 		b
> > > 	end
> > > end
> >
> > Sorry, but I pressed the Post message button too soon.
> >
> > c =
> > [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23].split(5)
> > =>
> >
> > [[1, 2, 3, 4, 5], [6, 7, 8, 9, 10], [11, 12, 13, 14, 15], [16, 17, 18,
> > 19], [20, 21, 22, 23]]
> >
> > Note that the first 3 sub arrays contain 5 elements and the last 2 sub
> > arrays contain 4 elements.
> >
> > I am a Ruby newbie and would appreciate a critique on the method I
> > posted.
> >
> > Thank You
>
> class Array
>   def split n
>     count , fat_ones = self.size / n , self.size % n
>     self.inject( [[]] ){ |a,e|
>       a.last.size < count + ( a.size <= fat_ones ? 1 : 0 ) ?
>         a.last << e  :  a << [e] ;  a }
>   end
> end
>
> a = (1..23).to_a
> p a.split(5)
> p a

Perhaps faster:

class Array
  def split n
    count , fat_ones = self.size / n , self.size % n
    cp, out = self.dup, []
    out << cp.
      slice!(0, count + (out.size < fat_ones ? 1 : 0 )) while cp!=[]
    out
  end
end