找到数组的第二个元素

Question

I don't understand how this isn't working. The program is supposed to take instance method second in the class Array and return the 2nd object in the array

class Array
  def second(*arr)
  arr.length <= 1 ? nil : arr[1]
  end
end

#Test cases
Test.assert_equals(Array([1, 2, 3]), 2,) #Getting nil
Test.assert_equals(Array([]), nil) #passes
Test.assert_equals(Array([1]), nil) #passes

What am I doing wrong? if I remove class Array and test on second it works fine?

Answer 1

Why use *arr ? If you're monkey-patching Array , then use self :

class Array
  def second
    self.length <= 1 ? nil : self[1]
  end
end

p [1,2,3].second #=> 2
p [1].second #=> nil
p [].second #=> nil 

Answer 2

In answer to what you're doing wrong, your code as written doesn't need the splat ( * ) operator (it also doesn't need to be patched into the Array class). While patching into Array and using self allows you to call it like [1,2].second , you could also write it as follows without patching into Array :

def second(arr)
  arr.length <= 1 ? nil : arr[1]
end

Which would need to be called like second([1,2]) .

To find out more about the splat operator * , try something like this explanation (I confess - the first Google result, but seems ok), but what it's doing in your example is turning your passed-in array into an array of an array - eg [1,2,3] becomes [[1,2,3]] .