CODEWITHSUNDEEP
rubypass-by-reference
user612308
user612308

Reputation: 1833

pass array to Array#each, internally, it uses reference or value

array1 = [0, 1]

# CASE 1:  
[array1].each do |arr| 
  arr = [3, 4] 
end

=> [0, 1]

# CASE 2:
[array1].each do |arr| 
  arr.delete_if { |ele| ele == 0 }
end

=> [[1]]

I thought ruby always passes array by reference. Why didn't it change the value of array in CASE 1 but did change it in CASE 2? thanks.

Upvotes: 0

Views: 918

Answers (3)

newacct
newacct

Reputation: 122489

.each simply calls the block with each element of the collection it's called on (in this case [array1]) in turn. Passing an argument just assigns it to a parameter. Your example can be simplified to the following:

# CASE 1:  
array1 = [0, 1]
arr = array1
arr = [3, 4]
array1

=> [0, 1]

# CASE 2:
array1 = [0, 1]
arr = array1
arr.delete_if { |ele| ele == 0 }
array1

=> [1]

Upvotes: 0

dnch
dnch

Reputation: 9605

In your first situation, all you've done is change which object arr points at - you haven't actually modified the original. This can be proven with the following script:

# Given our test value...
test_array = [1, 2]

# we can verify the values and the object_ids
puts "Value of `test_array`: #{test_array.inspect}"
puts "Object_id of `test_array`: #{test_array.object_id}"

# now, let's put it in a container and run it through a block
@array_container = [test_array]

@array_container.each do |arr|

  # Just to prove that `arr` points at test_array
  puts "Object_id of `arr`: #{arr.object_id}"

  # and that it's the same as the first element in our container
  puts "@container.first.object_id: #{@array_container.first.object_id}"

  # but, when we re-assign our block variable
  arr = [3, 4]

  # we get different values
  puts "Object_id of `arr`: #{arr.object_id}"
  puts "@container.first.object_id: #{@array_container.first.object_id}"
end

Which outputs...

Value of `test_array`: [1, 2]
Object_id of `test_array` : 2150710260

Object_id of `arr`        : 2150710260
@container.first.object_id: 2150710260

Object_id of `arr`        : 2150708040
@container.first.object_id: 2150710260

So how is that different in Case 2? In case 2, you're actually calling a self-destructive method, which will make changes to the original object that is being referenced by arr

Upvotes: 2

mattexx
mattexx

Reputation: 6606

I think your issue has to do with scope. The do-end construct is really a new block, so when you declare arr = [3, 4] you are instantiating a new arr = [3, 4] within that block. Whereas in Case 2 you are modifying the reference directly. To do the same thing in Case 1 would be something like:

[array1].each do |arr| 
  arr.each_index do |i|
    arr[i]+=3
  end 
end

=> [[3,4]]

Upvotes: 0

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