Weird behavior of arrays in recursive function

Question

I don't understand this, as far as I can tell these two snippets of code should do the same thing. But they aren't. Can someone shine some light as to why this is happening?

I am simply trying to find the total files inside a folder. This count should include all nested folders files also.

Here is my test case:

A - File 1 - File 2 - File 3 -- B - File 4 -- D - File 5 -- C - File 6

When I run this snippet,

def get_all_files(myfolder, filearray = Array.new)
    filearray += myfolder.myfiles.pluck(:id)
    myfolder.children.each { |c| get_all_files(c, filearray) }
    return filearray
end

It returns only 3 files. (Only A's file.)

When I run this snippet,

def get_all_files(myfolder, filearray = Array.new)
    myfolder.myfiles.pluck(:id).each do |id|
      filearray.push(id)
    end
    myfolder.children.each { |c| get_all_files(c, filearray) }
    return filearray
end

It runs the proper number of files, which is 6. I thought both .push and + are just normal Ruby array methods. So why does this happen?

No idea if this solves your problem/question, but the typical way to push onto an array is using the shovel << operator. filearray << myfolder.myfiles.pluck(:id) — Jeff Price
– Jeff Price, Commented Jun 5, 2015 at 17:05
Try file_array += myfolder.children.each { |c| get_all_files(c, filearray) }. You can also delete return file_array. — Cary Swoveland
– Cary Swoveland, Commented Jun 5, 2015 at 17:07
Yeah that's a mistake when I copied it to Stackoverflow. I named it better here (had it named recur when testing). — Swaathi Kakarla
– Swaathi Kakarla, Commented Jun 5, 2015 at 17:09

msergeant · Accepted Answer · 2015-06-05 20:20:32Z

2

+= returns a new instance of the array. Your recursion is not doing anything with the return value so that new instance dies at the end of the method.

When you do push or <<, you are operating on the original instance of the array. That is why you see different behavior.

answered Jun 5, 2015 at 20:20

msergeant

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Comments

Cary Swoveland · Accepted Answer · 2015-06-06 02:32:40Z

1

Try this:

def get_all_files(myfolder)
    folder_files   = myfolder.myfiles.pluck(:id))
    children_files = myfolder.children.each_with_object([]) do |c,a|
      a += get_all_files(c, filearray))
    end if my_folder.children.any?
    folder_files + children_files
end

The problem is with your line:

myfolder.children.each { |c| get_all_files(c, filearray) }

which obtains children's files, but does nothing with them.

To see what's happening, consider the following:

def a(h, x=[])
  puts h[:name]
  puts "x=#{x}, x.object_id 1 = #{x.object_id}"
  x += [1]
  puts "x=#{x}, x.object_id 2 = #{x.object_id}"
  h[:children].each do |g| 
    a(g,x)
    puts h[:name]
    puts  "x=#{x}, x.object_id 3 = #{x.object_id}"
  end
  x
end

h = { name: "Bob",
      children: [
        { name: "Marsha", children: [] },
        { name: "Bippy" , children: [] }
      ]
    }

a(h)
  # Bob
  # x=[],         x.object_id 1 = 70286033312340
  # x=[1],        x.object_id 2 = 70286033311720
  #   Marsha
  #     x=[1],    x.object_id 1 = 70286033311720
  #     x=[1, 1], x.object_id 2 = 70286033311280
  # Bob
  # x=[1],        x.object_id 3 = 70286033311720
  #   Bippy
  #     x=[1],    x.object_id 1 = 70286033311720
  #     x=[1, 1], x.object_id 2 = 70286033310660
  # Bob
  # x=[1],        x.object_id 3 = 70286033311720
  # => [1]

edited Jun 6, 2015 at 2:32

answered Jun 5, 2015 at 17:40

Cary Swoveland

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2 Comments

Dave Newton Over a year ago

I liked your other answer better.

Cary Swoveland Over a year ago

@Dave, for me, a tough choice. Still disturbed?

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