What causes "佇列為空" in krahets/hello-algo?

Check queue.is_empty() before calling peek() or pop()

krahets/hello-algo · error · IndexError

佇列為空

Error message

佇列為空

What it means

Raised by the peek() method of a circular-array-based queue when _size equals 0. Since pop() delegates to peek() to read the front element before advancing _front, both operations fail on an empty queue. The _front pointer is meaningless when the queue is empty, so the guard prevents returning stale data.

Source

Thrown at zh-hant/codes/python/chapter_stack_and_queue/array_queue.py:51

        # 計算佇列尾指標，指向佇列尾索引 + 1
        # 透過取餘操作實現 rear 越過陣列尾部後回到頭部
        rear: int = (self._front + self._size) % self.capacity()
        # 將 num 新增至佇列尾
        self._nums[rear] = num
        self._size += 1

    def pop(self) -> int:
        """出列"""
        num: int = self.peek()
        # 佇列首指標向後移動一位，若越過尾部，則返回到陣列頭部
        self._front = (self._front + 1) % self.capacity()
        self._size -= 1
        return num

    def peek(self) -> int:
        """訪問佇列首元素"""
        if self.is_empty():
            raise IndexError("佇列為空")
        return self._nums[self._front]

    def to_list(self) -> list[int]:
        """返回串列用於列印"""
        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__":
    # 初始化佇列
    queue = ArrayQueue(10)

    # 元素入列

View on GitHub (pinned to 69932aed18)

Solutions

Check queue.is_empty() before calling peek() or pop()
Use while not queue.is_empty() as the loop condition
Wrap in try/except IndexError for defensive consumer code

Example fix

# before
val = queue.peek()

# after
val = queue.peek() if not queue.is_empty() else None

Defensive patterns

Strategy: validation

Validate before calling

if not queue.is_empty():
    val = queue.peek()
else:
    val = None

Try / catch

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

Prevention

Check is_empty() before peek() or pop() in consumer code
Use while not queue.is_empty() as the draining loop condition
In producer-consumer setups, guard the consumer side with is_empty()

When it happens

Trigger: Calling peek() or pop() on a freshly constructed ArrayQueue with no push() calls; popping all elements and then calling pop() once more; a consumer reading faster than the producer enqueues.

Common situations: Producer-consumer pipelines where the queue is temporarily empty; BFS traversals that pop the last node and then call peek; testing with empty input sequences.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/b6b6e10b55763690. Report an issue: GitHub.