Ruby: Use the return of the conditional for variable assignment and comparison

I have a method and in order to check whether it is being passed a block, I do the following:

if block_given?
    res = yield(array[i], array[i+1])
  else
    res = array[i] - array[i+1]
  end

However RuboCop is giving me a warning which I don't really understand in the if block_given? line:

Use the return of the conditional for variable assignment and comparison

Is there any other more rubyist way of doing this?

Thanks

What the warning is telling you to do is:

res = if block_given?
        yield(array[i], array[i+1])
      else
        array[i] - array[i+1]
      end

That is, having a single assignment instead of two (or even more).