Bug #14107: Enumerable#each_with_object partly mutate object

• Status: Rejected
• Assignee: -
• Target version: -
• ruby -v: 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux] , ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]
• Backport: 2.3: UNKNOWN, 2.4: UNKNOWN

[fanantoxa (Anton Sivakov)]

Hi. I was working with each_with_object and found a bug.
I was needed to iterate through the array and have an memo object at the same time so I've tried to use next code:

Here is an array:

d = [1, 2, 3, 4, 5, 6, 7]

And what I've tried to do:

d.each_with_object([1, {}]) do |item, (index, chunks)|
  index +=1 if item % 3 == 0

  chunks[index] ||= []
  chunks[index].push(item)

  memo = [index, chunks]
end

Second variation is:

d.each_with_object([1, {}]) do |item, memo|
  index, chunks = memo
  index +=1 if item % 3 == 0

  chunks[index] ||= []
  chunks[index].push(item)

  [index, chunks]
end

Or even if I mutate memo directly at the end like:

d.each_with_object([1, {}]) do |item, memo|
  index, chunks = memo
  index +=1 if item % 3 == 0

  chunks[index] ||= []
  chunks[index].push(item)

  memo = [index, chunks]
end

It always returns:

[1, { 1 => [1, 2, 4, 5, 7], 2 => [3, 6] }]

For some reason It's mutate an Hash inside the array but not mutate an Integer.

I was expecting that index will increase each time and I'll get:

[3, { 1 => [1, 2], 2 => [3, 4, 5], 3 => [6, 7] }]

Ruby -v:

ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]
ruby 2.3.1p112 (2016-04-26 revision 54768) [x86_64-linux]

[nobu (Nobuyoshi Nakada)]

• Description updated (diff)
• Status changed from Open to Rejected

fanantoxa (Anton Sivakov) wrote:

Or even if I mutate memo directly at the end like:

It doesn't mutate the argument object.

d.each_with_object([1, {}]) do |item, memo|
  index, chunks = memo
  index +=1 if item % 3 == 0

  chunks[index] ||= []
  chunks[index].push(item)

  memo[0] = index
end

or

d.inject([1, {}]) do |(index, chunks), item|
  index +=1 if item % 3 == 0

  chunks[index] ||= []
  chunks[index].push(item)

  [index, chunks]
end