What causes "キューが空です" in krahets/hello-algo?

Guard with `if not queue.is_empty(): queue.peek()`.

krahets/hello-algo · error · IndexError

キューが空です

Error message

キューが空です

What it means

This IndexError (Japanese: 'キューが空です' = 'queue is empty') is raised by ArrayQueue.peek() (array_queue.py:51) when _size == 0. peek() returns _nums[_front]; pop() calls peek() first, so the same exception propagates through pop() on an empty queue. The circular-array layout means _front alone does not indicate a valid element — only _size > 0 does.

Source

Thrown at ja/codes/python/chapter_stack_and_queue/array_queue.py:51

        # 末尾ポインタを計算し、末尾インデックス + 1 を指す
        # 剰余演算により、rear が配列末尾を越えた後に先頭へ戻るようにする
        rear: int = (self._front + self._size) % self.capacity()
        # num をキュー末尾に追加
        self._nums[rear] = num
        self._size += 1

    def pop(self) -> int:
        """デキュー"""
        num: int = self.peek()
        # 先頭ポインタを1つ後ろへ進め、末尾を越えたら配列先頭に戻す
        self._front = (self._front + 1) % self.capacity()
        self._size -= 1
        return num

    def peek(self) -> int:
        """キュー先頭の要素にアクセス"""
        if self.is_empty():
            raise IndexError("キューが空です")
        return self._nums[self._front]

    def to_list(self) -> list[int]:
        """表示用のリストを返す"""
        res = [0] * self.size()
        j: int = self._front
        for i in range(self.size()):
            res[i] = self._nums[(j % self.capacity())]
            j += 1
        return res


"""Driver Code"""
if __name__ == "__main__":
    # キューを初期化
    queue = ArrayQueue(10)

    # 要素をエンキュー

View on GitHub (pinned to 69932aed18)

Solutions

Guard with `if not queue.is_empty(): queue.peek()`.
Drain with `while not queue.is_empty(): queue.pop()`.
Base pop-count on queue.size(), not capacity or a stale length.
In producer/consumer code, check is_empty() before attempting to consume.

Example fix

// before
val = queue.pop()  # IndexError on empty

// after
val = queue.pop() if not queue.is_empty() else None

Defensive patterns

Strategy: validation

Validate before calling

if not queue.is_empty():
    val = queue.peek()
# pop() calls peek() first — guard pop the same way

Type guard

def queue_has_front(q: ArrayQueue) -> bool:
    return not q.is_empty()

Try / catch

try:
    val = queue.pop()
except IndexError:
    val = None

Prevention

Check is_empty() before peek() or pop() (pop delegates to peek).
Base consume counts on queue.size(), not capacity.
Drain with `while not queue.is_empty():`.
In producer/consumer flows, gate consumption on is_empty().

When it happens

Trigger: Calling peek() or pop() on a freshly constructed ArrayQueue. Calling pop() more times than push(). The constructor sets _size = 0 and _front = 0, so any immediate peek/pop raises.

Common situations: Consuming faster than producing. Looping a fixed number of pops based on capacity rather than size(). Forgetting that pop() delegates to peek() and will raise the same IndexError.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/ac57dd02488fb09c. Report an issue: GitHub.