CODEWITHSUNDEEP
ruby
jibzoiderz
jibzoiderz

Reputation: 91

undefined method `<' for nil:NilClass

While creating a bubble sort program i ran into this error:

test.rb:8:in `block in bubble_sort': undefined method `<' for nil:NilClass (NoMethodError)
    from test.rb:6:in `downto'
    from test.rb:6:in `bubble_sort'
    from test.rb:16:in `<main>

does anyone know what does that mean? Here is the code:

def bubble_sort(arr)
    length = arr.length
    sorted = false


    length.downto(0) do |cntr|

        if arr[cntr] < arr[cntr + 1]

        end


    end
end

bubble_sort([2,6,8,1,0,2])

Upvotes: 0

Views: 2502

Answers (3)

spickermann
spickermann

Reputation: 107132

The error message undefined method '<' for nil:NilClass means that you are trying to call < on something that is nil.

In your example that must be the if arr[cntr] < arr[cntr + 1] comparison. In a next step we need to find out why arr[cntr] is nil. One reason could be that there is no element in the arr array at the cntr index, another reason might be that the index cntr is out of bounds of the array. In your example it is the second reason that is causing the problem.

Why is the index out of bounds? Let's have a closer look how the loop is build and use an example array [a, b, c] to do so:

length = arr.length              # length = 3             # [a, b, c].length
length.downto(0) do |cntr|       # 3.downto(0) do |cntr|
  if arr[cntr] < arr[cntr + 1]   #   if arr[3] < arr[4]   # in the first iteration

Ops, there aren't not indexes 3 and 4 in the arr array, because indexes start counting with 0 and there are only 3 elements in my example (that makes the last element's index 2).

The fix:

def bubble_sort(array)
  (array.length - 2).downto(0).each do |index|
    if array[index] < array[index + 1]
      # ...
    end
  end
end

Upvotes: 3

Junan Chakma
Junan Chakma

Reputation: 651

In your code length variable value will be 6(length = arr.length). When you iterate it down to 0 ... in the first iteration cntr variable value will be 6. so arr[cntr] is getting nil value because you are accessing elements out of bounds of the array. That's why you are getting undefined method < for nil:NilClass (NoMethodError) error.

Upvotes: 0

Sergio Tulentsev
Sergio Tulentsev

Reputation: 230531

does anyone know what does that mean?

It means that arr[cntr] is nil in this expression

if arr[cntr] < arr[cntr + 1]

Oh, and if this one is nil, then arr[cntr + 1] is definitely nil.

Hint: you're accessing elements out of bounds of the array.

Upvotes: 2

Related Questions