krahets/hello-algo · error · IndexError

双向队列为空

Error message

双向队列为空

What it means

Raised by `LinkedListDeque#pop` (linkedlist_deque.rb:70) when `is_empty?` is true. This is the internal pop shared by `pop_first` and `pop_last`; on an empty deque there is no node to detach, and dereferencing `@front.val` or `@rear.val` would fail. The guard prevents nil-dereference on the linked-list head/tail pointers.

Source

Thrown at codes/ruby/chapter_stack_and_queue/linkedlist_deque.rb:70

      node.prev = @rear
      @rear = node # 更新尾节点
    end
    @size += 1 # 更新队列长度
  end

  ### 队首入队 ###
  def push_first(num)
    push(num, true)
  end

  ### 队尾入队 ###
  def push_last(num)
    push(num, false)
  end

  ### 出队操作 ###
  def pop(is_front)
    raise IndexError, '双向队列为空' if is_empty?

    # 队首出队操作
    if is_front
      val = @front.val # 暂存头节点值
      # 删除头节点
      fnext = @front.next
      unless fnext.nil?
        fnext.prev = nil
        @front.next = nil
      end
      @front = fnext # 更新头节点
    # 队尾出队操作
    else
      val = @rear.val # 暂存尾节点值
      # 删除尾节点
      rprev = @rear.prev
      unless rprev.nil?
        rprev.next = nil

View on GitHub (pinned to 69932aed18)

Solutions

  1. Check `deque.is_empty?` before `pop_first` or `pop_last`.
  2. Loop with `until deque.is_empty?` when draining.
  3. Rescue IndexError if popping past empty is recoverable.

Example fix

# before
val = deque.pop_first

# after
val = deque.is_empty? ? nil : deque.pop_first
Defensive patterns

Strategy: validation

Validate before calling

return nil if deque.is_empty?
deque.pop_first

Type guard

def ll_deque_popable?(deque)
  deque.respond_to?(:is_empty?) && deque.respond_to?(:pop) && !deque.is_empty?
end

Try / catch

begin
  deque.pop_first
rescue IndexError
  nil
end

Prevention

When it happens

Trigger: Calling `deque.pop_first` or `deque.pop_last` (both delegate to `pop`) on a deque with zero nodes. Occurs after removing all elements from both ends.

Common situations: Deque-based undo/redo fully consumed; deque used as a work-queue drained to empty; test calling pop on a new deque.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/1f7b79a9bdba78ca. Report an issue: GitHub.