# RFC - Recursive Array Method call acting on element batches

Discussion in 'Ruby' started by rubydeckard@gmail.com, Nov 24, 2007.

1. ### Guest

I work for the Department of Redundancy Department and I'm thinking
there must be a better, more elegant way to doing the following:

With an array of 200 elements
Call a method to act on 10 elements at a time
Repeat until all elements are processed.
Return the processed array.

Here's my solution to the above:

__BEGIN__

class Array
def process(method_obj, batch_size = 10, fin = [])
if (this_batch = self.first(batch_size)).size > 0
fin.concat method_obj.call(this_batch)
(self - this_batch).process(method_obj, batch_size, fin)
else
return fin
end
end
end

def munge(ary)
ary.collect { |e| e.+ 10 }
end

my_meth = methodmunge)

ARY = [1,2,3,4,5,6]
p ARY.process(my_meth, 2) # run my_meth against 2 elements at a time

__END__

The above produces: [11, 12, 13, 14, 15, 16]

This is just a trivial example, but, for what I'm actually doing
there's a limit of 100 items that can be processed by this mysterious
(dynamic) method, hence the need to "batch" process the array.

Does my solution scream newb? Anyone have a suggestion that would
"own me with easy"?

Ben

, Nov 24, 2007

2. ### 7stud --Guest

unknown wrote:
>
> With an array of 200 elements
> Call a method to act on 10 elements at a time
> Repeat until all elements are processed.
> Return the processed array.
>

Pre ruby 1.9:

require 'enumerator'

def sum(arr)
total = 0

arr.each {|elmt| total += elmt}
#faster than the ponderously slow inject method

total
end

results = []
arr = Array.new(15) {|i| i}
p arr

arr.each_slice(10) do |portion|
results << sum(portion)
end

p results

--output:--
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14]
[45, 60]
--
Posted via http://www.ruby-forum.com/.

7stud --, Nov 24, 2007

3. ### Guest

Thanks for your input guys. That put me on the right track.

def proc_array(method_obj, batch_size = 2)
result = []
self.each_slice(batch_size) do |batch|
result.concat method_obj.call(batch)
end
result
end

That turns out to be twice as fast as the recursive method. The only
trouble I experienced was getting each_slice into my class. Turns out
that there's an error with the ruby core library documentation
produced by RDoc. #each_slice is available via the *standard* library
'enumerator.rb', not the core module library Enumerable as is
documented at www.ruby-doc.org. Once I figured that out, all is
peachy keen.

Thanks,

Ben

, Nov 25, 2007
4. ### Eivind EklundGuest

On Nov 25, 2007 8:30 PM, <> wrote:
> Thanks for your input guys. That put me on the right track.
>
> def proc_array(method_obj, batch_size = 2)
> result = []
> self.each_slice(batch_size) do |batch|
> result.concat method_obj.call(batch)
> end
> result
> end

Better:
def proc_array(batch_size = 2, &method_obj)
result = []
self.each_slice(batch_size) do |batch|
result.concat method_obj.call(batch)
end
result
end

Can be called as

...proc_array do |batch|
end

or

...proc_array(2, &method_obj)

Eivind.

Eivind Eklund, Nov 27, 2007