krahets/hello-algo · error · IndexError

Queue is empty

Error message

Queue is empty

What it means

Raised by LinkedListQueue#peek (en/codes/ruby/chapter_stack_and_queue/linkedlist_queue.rb:55) when the queue is empty. peek dereferences @front.val; the guard rejects it because @front is nil when size == 0. pop calls peek first, so dequeue on an empty queue surfaces the same message.

Source

Thrown at en/codes/ruby/chapter_stack_and_queue/linkedlist_queue.rb:55

      @rear.next = node
      @rear = node
    end

    @size += 1
  end

  ### Dequeue ###
  def pop
    num = peek
    # Delete head node
    @front = @front.next
    @size -= 1
    num
  end

  ### Access front element ###
  def peek
    raise IndexError, 'Queue is empty' if is_empty?

    @front.val
  end

  ### Convert linked list to Array and return ###
  def to_array
    queue = []
    temp = @front
    while temp
      queue << temp.val
      temp = temp.next
    end
    queue
  end
end

### Driver Code ###
if __FILE__ == $0

View on GitHub (pinned to 69932aed18)

Solutions

1. Check queue.is_empty? before peek or pop.
2. Track enqueued/dequeued counts and never dequeue more than enqueued.
3. Rescue IndexError around dequeue and treat empty as a normal idle state.

Example fix

# before
queue = LinkedListQueue.new
queue.pop # raises IndexError, "Queue is empty"

# after
queue.pop unless queue.is_empty?

queue.pop unless queue.is_empty?

def queue_nonempty?(q); q.respond_to?(:is_empty?) && !q.is_empty?; end

begin
  queue.pop
rescue IndexError
  nil
end

When it happens

Trigger: Calling queue.peek or queue.pop when queue.size == 0 — before the first enqueue, or after the head has advanced past every node via successive pops.

Common situations: A BFS work-queue drained to empty then peeked again; a producer/consumer loop where the consumer outruns the producer; test code that pops more than it enqueues.

Related errors

• очередь пуста
• 队列为空
• Queue is empty
• Deque is empty
• Stack is empty

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/f0b94675b404b811. Report an issue: GitHub.