Why does uniq! return nil if there are no duplicates

Question

I'm just starting with Ruby and I personally find the following to be a violation of the "principle of least surprise". And that is, quoting from the documentation, that uniq! "removes duplicate elements from self. Returns nil if no changes are made (that is, no duplicates are found)."

Can anybody explain this, which seems completely counter-intuitive to me? This means that rather than being able to write one line of code below by appending .uniq! to end the first line, I instead have to write the following two lines:

  hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
  hooks = hooks.uniq

Or am I missing something, a better way?

EDIT:

I understand that uniq! modifies its operand. Here's the problem illustrated better I hope:

  hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
  puts hooks.length #50
  puts hooks.uniq!.length #undefined method `length' for nil:NilClass

I contend that the way uniq! works makes it completely senseless and useless. Sure in my case as pointed out I could just append .uniq to the first line. However later in the same program I am pushing elements onto another array inside of a loop. Then, under the loop, I'd like to "de-dupe" the array, but I dare not write 'hooks_tested.uniq!' because it could return nil; instead I must write hooks_tested = hooks_tested.uniq

Indeed I contend this is a particularly egregious mis-feature in that it is a well known principle that, when devising a method that returns an array, one should always at least return an empty array, rather than nil

Answer 1

This is because uniq! modifies self and if uniq! would return a value you wouldn't be able to know whether a change actually occurred in the original object.

var = %w(green green yellow)
if var.uniq!
  # the array contained duplicate entries
else
  # nothing changed
end

In your code you can simply write

hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
hooks.uniq!
# here hooks is already changed

If you need to return the value of hook perhaps because it's the last method statement just do

def method
  hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
  hooks.uniq
end

or otherwise

def method
  hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
  hooks.uniq!
  hooks
end

Answer 2

The exclamation point on uniq! indicates that it modifies the array instead of returning a new one. You should do this:

hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/).uniq

or this

hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)
hooks.uniq!
puts hooks.length

Answer 3

You can append uniq (no exclamation mark at the end) to the end of the first line.

Or, if you insist on using uniq!, use

(hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/)).uniq!

Answer 4

This is not an answer to why, but rather, a workaround.

Since uniq doesn't return nil, I use uniq and assign the the result to a new variable instead of using the bang version

original = [1,2,3,4]
new = original.uniq

#=> new is [1,2,3,4]
#=> ... rather than nil

Having a new variable is a small price to pay. It sure as hell beats doing if checks, with repeated complex calls to uniq! and uniq and checking for nil

Answer 5

Since Ruby 1.9, Object#tap is available:

hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/).tap do |hooks|
  hooks.uniq!
end
puts hooks.length

And perhaps more succinctly (h/t @Aetherus):

hooks = IO.read(wt_hooks_impl_file).scan(/wt_rt_00\w{2}/).tap(&:uniq!)
puts hooks.length