How to split an array in sub arrays of the same length

Ruby
1

Hi,

Is there a way to split an array in sub arrays of the same length, that
is :

[ 1, 2, 3, 4, 5, 6, 7, 8 ].unknown_function(3)

that returns:

[ [1, 2, 3], [4, 5, 6], [7, 8] ]

thanks in advance.

2

Hi,

Paolo B. [email protected] writes:

Is there a way to split an array in sub arrays of the same length, that
is :

[ 1, 2, 3, 4, 5, 6, 7, 8 ].unknown_function(3)

that returns:

[ [1, 2, 3], [4, 5, 6], [7, 8] ]

% irb --prompt simple

require ‘enumerator’
=> true
[ 1, 2, 3, 4, 5, 6, 7, 8 ].enum_slice(3).to_a
=> [[1, 2, 3], [4, 5, 6], [7, 8]]

3

fr paulo:

Is there a way to split an array in sub arrays of the same

length, that

is :

[ 1, 2, 3, 4, 5, 6, 7, 8 ].unknown_function(3)

that returns:

[ [1, 2, 3], [4, 5, 6], [7, 8] ]

irb(main):021:0> x
=> [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
irb(main):022:0> y=[]
=> []
irb(main):023:0> x.each_slice(3){|s| y << s}
=> nil
irb(main):024:0> y
=> [[0, 1, 2], [3, 4, 5], [6, 7, 8], [9]]

kind regards -botp

4

Although you could use Enumerator::each_slice as others have
suggested, it might be simpler to augment Array with a partition
function.

class Array

def partition(n, r=[])
raise ArgumentError if n <= 0
if n <= size then
r << first(n)
last(size - n).partition(n, r)
else
r << self unless empty?
return r
end
end

end

p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(1) => [[1], [2], [3], [4],
[5], [6], [7], [8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(3) => [[1, 2, 3], [4, 5, 6],
[7, 8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(4) => [[1, 2, 3, 4], [5, 6, 7,
8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(6) => [[1, 2, 3, 4, 5, 6], [7,
8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(8) => [[1, 2, 3, 4, 5, 6, 7, 8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(10) => [[1, 2, 3, 4, 5, 6, 7, 8]]
p [ 1, 2, 3, 4, 5, 6, 7, 8 ].partition(-2) =>
untitled text:6:in `partition’: ArgumentError (ArgumentError)
from untitled text:24

Hope this is helps.
Regards, Morton

5

Paolo B. wrote:

thanks in advance.

Always remember that a program without inject is a
pointless program.

[ 1, 2, 3, 4, 5, 6, 7, 8 ].inject([[]]){|a,x|
a.last.size<3 ? a.last << x : a << [x]; a }

6

Il giorno gio, 20/07/2006 alle 06.53 +0000, Paolo B. ha scritto:

thanks in advance.

thanks to all for responding.

7

Paolo B. wrote:

Is there a way to split an array in sub arrays of the same length, that

Stolen from the facets[1] extensions for Arrays[2]:

class Array
def each_slice(n=nil, &yld)
n = yld.arity.abs unless n
i=0
while i < self.length
yld.call(*self.slice(i,n))
i+=n
end
end
end

[1] http://facets.rubyforge.org/
[2] http://facets.rubyforge.org/api/core/classes/Array.html#M000163

8

Logan C. wrote:

   yld.call(*self.slice(i,n))
   arrays.each do |array|
 end

                user     system      total        real

Facets: 1.250000 0.010000 1.260000 ( 1.315879)
Enumerator: 1.390000 0.010000 1.400000 ( 1.447592)

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

9

On Jul 20, 2006, at 4:40 PM, Phrogz wrote:

while i < self.length
  yld.call(*self.slice(i,n))
  i+=n
end

end
end

[1] http://facets.rubyforge.org/
[2] http://facets.rubyforge.org/api/core/classes/Array.html#M000163

This is in enumerator, enumerator comes with ruby. Do we really need
an “Optimized for Array” version? (That is its purpose according to
the docs.)

Well surprising to me, there really is a noticeable speed difference.
I gues sit makes sense, each_slice in enumerator.c is written to use
each, and not just call to_a first either. (Which makes sense, Files
are Enumerables after all, maybe you want it in chunks of N lines at
a time. You wouldn’t want to have to slurp the whole file into memory
just to do that).

I wonder if we can get this each_slice stuck in the standard lib for
Array.

% cat enumerator_vs_facets.rb
#!/usr/bin/env ruby
require ‘enumerator’
require ‘benchmark’

class Array
def facets_each_slice(n=nil, &yld)
n = yld.arity.abs unless n
i=0
while i < self.length
yld.call(*self.slice(i,n))
i+=n
end
end
end

arrays = [ (1…97).to_a, (0…99).to_a, [“hello”, “world”] ]
N = 1000
Benchmark.bmbm do |bm|
bm.report("Facets: ") do
N.times do
arrays.each do |array|
array.facets_each_slice(3) { |*x| “#{x}” }
end
end
end

bm.report("Enumerator: ") do
N.times do
arrays.each do |array|
array.each_slice(3) { |*x| “#{x}” }
end
end
end
end

% ruby enumerator_vs_facets.rb
Rehearsal ------------------------------------------------
Facets: 1.250000 0.010000 1.260000 ( 1.364671)
Enumerator: 1.380000 0.010000 1.390000 ( 1.442132)
--------------------------------------- total: 2.650000sec

                user     system      total        real

Facets: 1.250000 0.010000 1.260000 ( 1.315879)
Enumerator: 1.390000 0.010000 1.400000 ( 1.447592)

10

bbiker wrote:

  #########################################################################
  	length = self.size
  	0.upto(n-1) do |idx|

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

11

bbiker wrote:

  # evenly split array 'a' into 'n' sub arrays
  	while buckets > 0
  		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

12

William J. wrote:

def split(n, b=[], d=[])
  	buckets = n
  	start = 0  # start

Sorry, but I pressed the Post message button too soon.

    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

13

Paolo B. wrote:

Is there a way to split an array in sub arrays of the same length, that
is :

[ 1, 2, 3, 4, 5, 6, 7, 8 ].unknown_function(3)

that returns:

[ [1, 2, 3], [4, 5, 6], [7, 8] ]

Amuse yourself here:

http://redhanded.hobix.com/bits/matchingIntoMultipleAssignment.html

14

William J. wrote:

    a.last << e  :  a << [e] ;  a }

end
end

a = (1…23).to_a
p a.split(5)
p a

Yes! it is a much better way !!!
Since obviously I am not familiar with the inject method, I took to the
pick axe and irb to work it out until I understood exactly what you
were doing.

The only question left is how to best handle bad inputs?

n <= 0, n not specified, and an empty array, n > [1,2,3,4,5].size

for [1,2,3,4,5].split(0) => Exception: divided by 0

for [1,2,3,4,5].split(-n) => [[], [1], [2], [3], [4], [5]]

for [].split(+/-n) => [[]] where n != 0
for [].split(0) => Exception: divided by 0

for [1,2,3,4,5].split() =>
Exception: wrong number of arguments (0 for 1)

for [1,2,3,4,5].split(8) => [[1], [2], [3], [4], [5]]

I would guess this might be the correct way. This could cause a problem
if the user were to check the size of the returned array For the above
example it would be5 instead of the expected 8.

You could say that for n == 0 or n not specifiend are already handled
by existing Exceptions. Do need to handle negative values of n, n >
array.size and array.size == 0