krahets/hello-algo · error · IndexError

キューは空です

Error message

キューは空です

What it means

Raised by LinkedListQueue#peek (ja) when `is_empty?` is true. Because `pop` is implemented as `num = peek; @front = @front.next; ...`, the same IndexError surfaces for BOTH `peek` and `pop` on an empty queue — the failure originates at line 55 inside peek. The guard protects against dereferencing the nil @front.

Source

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

      @rear.next = node
      @rear = node
    end

    @size += 1
  end

  ### デキュー ###
  def pop
    num = peek
    # 先頭ノードを削除
    @front = @front.next
    @size -= 1
    num
  end

  ### 先頭要素にアクセス ###
  def peek
    raise IndexError, 'キューは空です' if is_empty?

    @front.val
  end

  ### 連結リストを Array に変換して返す ###
  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?` (or `queue.size.zero?`) before calling peek or pop.
  2. Cap your consumer loop on `queue.size > 0` rather than a fixed count.
  3. If you need non-raising semantics, wrap pop in a helper that returns nil when empty.

Example fix

// before
queue.pop  # raises 'キューは空です' when drained

// after
item = queue.is_empty? ? nil : queue.pop
Defensive patterns

Strategy: validation

Validate before calling

item = queue.pop unless queue.is_empty?

Type guard

def queue_nonempty?(q) = !q.is_empty?

Try / catch

begin
  item = queue.pop  # also covers peek
rescue IndexError
  item = nil
end

Prevention

When it happens

Trigger: Calling `queue.peek` or `queue.pop` on an empty LinkedListQueue. Any dequeue-after-drain, or a consumer that pops in a loop without a size check, will hit this on the first empty operation.

Common situations: A producer/consumer loop where the consumer outpaces the producer; popping the last item then popping once more; a driver block that peeks before enqueueing anything.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/8d09371475c06ca2. Report an issue: GitHub.