krahets/hello-algo · error · IndexError
队列为空
Error message
队列为空
What it means
IndexError '队列为空' (queue is empty) raised by ArrayQueue.peek, and propagated by pop which calls peek first. Reading the front slot of an empty circular queue would return stale data, so the guard short-circuits.
Source
Thrown at codes/python/chapter_stack_and_queue/array_queue.py:51
# 计算队尾指针，指向队尾索引 + 1
# 通过取余操作实现 rear 越过数组尾部后回到头部
rear: int = (self._front + self._size) % self.capacity()
# 将 num 添加至队尾
self._nums[rear] = num
self._size += 1
def pop(self) -> int:
"""出队"""
num: int = self.peek()
# 队首指针向后移动一位，若越过尾部，则返回到数组头部
self._front = (self._front + 1) % self.capacity()
self._size -= 1
return num
def peek(self) -> int:
"""访问队首元素"""
if self.is_empty():
raise IndexError("队列为空")
return self._nums[self._front]
def to_list(self) -> list[int]:
"""返回列表用于打印"""
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__":
# 初始化队列
queue = ArrayQueue(10)
# 元素入队View on GitHub (pinned to 69932aed18)
Solutions
- Guard with is_empty() before peek or pop.
- Drive consumption with while not q.is_empty(): item = q.pop().
- For optional access, expose a get-or-default wrapper.
Example fix
// before front = q.peek() # raises on empty // after front = q.peek() if not q.is_empty() else None
Defensive patterns
Strategy: validation
Validate before calling
def safe_peek(q):
return q.peek() if not q.is_empty() else None Type guard
def queue_non_empty(q) -> bool:
return not q.is_empty() Try / catch
try:
front = q.peek()
except IndexError:
front = None Prevention
- Drive consumers with while not q.is_empty().
- Return a sentinel for optional front reads.
- Avoid popping more items than were pushed.
When it happens
Trigger: Calling queue.peek() or queue.pop() when _size == 0.
Common situations: BFS that pops after exhausting the queue; producer/consumer where the consumer starts before any push; off-by-one drain loop popping n+1 items.
Related errors
AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13).
Data as JSON: /api/errors/3e8e28e0b6a660de.
Report an issue: GitHub.