krahets/hello-algo · error · IndexError
索引越界
Error message
索引越界
What it means
Raised by the get() method of a custom dynamic-array list (Traditional Chinese) when the requested index falls outside the valid range [0, _size). The list distinguishes between _size (current element count) and _capacity (allocated slot count), so accessing any index at or beyond _size — even if the underlying array has room — is rejected. This is identical to how Python's built-in list guards index access, but implemented explicitly for pedagogical purposes.
Source
Thrown at zh-hant/codes/python/chapter_array_and_linkedlist/my_list.py:30
"""建構子"""
self._capacity: int = 10 # 串列容量
self._arr: list[int] = [0] * self._capacity # 陣列（儲存串列元素）
self._size: int = 0 # 串列長度（當前元素數量）
self._extend_ratio: int = 2 # 每次串列擴容的倍數
def size(self) -> int:
"""獲取串列長度（當前元素數量）"""
return self._size
def capacity(self) -> int:
"""獲取串列容量"""
return self._capacity
def get(self, index: int) -> int:
"""訪問元素"""
# 索引如果越界，則丟擲異常，下同
if index < 0 or index >= self._size:
raise IndexError("索引越界")
return self._arr[index]
def set(self, num: int, index: int):
"""更新元素"""
if index < 0 or index >= self._size:
raise IndexError("索引越界")
self._arr[index] = num
def add(self, num: int):
"""在尾部新增元素"""
# 元素數量超出容量時，觸發擴容機制
if self.size() == self.capacity():
self.extend_capacity()
self._arr[self._size] = num
self._size += 1
def insert(self, num: int, index: int):
"""在中間插入元素"""View on GitHub (pinned to 69932aed18)
Solutions
- Validate that 0 <= index < lst.size() before calling get()
- Use len() or size() as the loop upper bound, not capacity()
- Audit all index arithmetic for off-by-one errors
Example fix
# before
val = lst.get(i) # crashes if i >= size
# after
if 0 <= i < lst.size():
val = lst.get(i)
else:
val = -1 Defensive patterns
Strategy: validation
Validate before calling
if 0 <= index < lst.size():
val = lst.get(index)
else:
raise ValueError(f"index {index} out of range [0, {lst.size()})") Try / catch
try:
val = lst.get(index)
except IndexError:
val = -1 Prevention
- Always use size() as the loop upper bound, never capacity()
- Validate externally-computed indices before passing to get()
- Remember that capacity() >= size(); slots between them are inaccessible
When it happens
Trigger: Calling lst.get(5) when the list holds only 3 elements; passing a negative index; computing an index from an off-by-one length calculation and using it with get().
Common situations: Looping from 0 to capacity() instead of size(); reusing an index variable after the list was shrunk by remove(); porting code from a language with 1-based indexing.
Related errors
AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13).
Data as JSON: /api/errors/8dafa2ac7ae403c7.
Report an issue: GitHub.