donnemartin/interactive-coding-challenges · error · Exception
Stack is empty
Error message
Stack is empty
What it means
n_stacks' pop raises Exception('Stack is empty') when the stack pointer for that stack_index is -1, meaning no elements have been pushed (or all were popped) on that virtual stack. It prevents reading uninitialized slots of the backing array.
Solutions
- Track or check emptiness before popping: peek at stack_pointers[stack_index] == -1
- Ensure every pop has a corresponding successful push for the same stack_index
- Wrap pop in try/except Exception and treat underflow as a no-op or signal if unavoidable
Example fix
# before
data = stacks.pop(0) # may raise 'Stack is empty'
# after
if stacks.stack_pointers[0] != -1:
data = stacks.pop(0)
else:
data = None Defensive patterns
Strategy: validation
Validate before calling
if stacks.stack_pointers[stack_index] != -1:
data = stacks.pop(stack_index)
else:
data = None Try / catch
try:
data = stacks.pop(i)
except Exception as e:
if str(e) == 'Stack is empty':
data = None # treat underflow as empty
else:
raise Prevention
- Check the stack pointer before popping
- Keep push/pop calls paired per stack_index
- Loop with an emptiness condition rather than exception-driven control flow
When it happens
Trigger: Calling pop(stack_index) on a freshly created stack, or popping more times than you pushed on that same stack_index. Each of the N stacks tracks underflow independently.
Common situations: Mismatched push/pop pairing across code paths; popping in a loop without checking remaining elements; mixing up stack indices so you pop a stack you never pushed to.
Related errors
AI-assisted analysis of donnemartin/interactive-coding-challenges@358f2cc604 (2026-08-28).
Data as JSON: /api/errors/cbef0fc8c21985b7.
Report an issue: GitHub.
Appendix: source
Thrown at stacks_queues/n_stacks/n_stacks_solution.ipynb:136
" def __init__(self, num_stacks, stack_size):\n",
" self.num_stacks = num_stacks\n",
" self.stack_size = stack_size\n",
" self.stack_pointers = [-1] * self.num_stacks\n",
" self.stack_array = [None] * self.num_stacks * self.stack_size\n",
"\n",
" def abs_index(self, stack_index):\n",
" return stack_index * self.stack_size + self.stack_pointers[stack_index]\n",
"\n",
" def push(self, stack_index, data):\n",
" if self.stack_pointers[stack_index] == self.stack_size - 1:\n",
" raise Exception('Stack is full')\n",
" self.stack_pointers[stack_index] += 1\n",
" array_index = self.abs_index(stack_index)\n",
" self.stack_array[array_index] = data\n",
"\n",
" def pop(self, stack_index):\n",
" if self.stack_pointers[stack_index] == -1:\n",
" raise Exception('Stack is empty')\n",
" array_index = self.abs_index(stack_index)\n",
" data = self.stack_array[array_index]\n",
" self.stack_array[array_index] = None\n",
" self.stack_pointers[stack_index] -= 1\n",
" return data"
]
},
{
"cell_type": "markdown",
"metadata": {},
"source": [
"## Unit Test\n",
"\n"
]
},
{
"cell_type": "code",
"execution_count": 2,View on GitHub (pinned to 358f2cc604)