krahets/hello-algo · error · IndexError

堆疊為空

Error message

堆疊為空

What it means

Raised by the pop method of the ArrayStack teaching class (backed by Ruby's Array) when is_empty? is true. Although Ruby's own Array.pop returns nil on empty, this teaching implementation explicitly raises IndexError to demonstrate proper stack-underflow handling. The guard fires before delegating to @stack.pop.

Source

Thrown at zh-hant/codes/ruby/chapter_stack_and_queue/array_stack.rb:31

  ### 獲取堆疊的長度 ###
  def size
    @stack.length
  end

  ### 判斷堆疊是否為空 ###
  def is_empty?
    @stack.empty?
  end

  ### 入堆疊 ###
  def push(item)
    @stack << item
  end

  ### 出堆疊 ###
  def pop
    raise IndexError, '堆疊為空' if is_empty?

    @stack.pop
  end

  ### 訪問堆疊頂元素 ###
  def peek
    raise IndexError, '堆疊為空' if is_empty?

    @stack.last
  end

  ### 返回串列用於列印 ###
  def to_array
    @stack
  end
end

### Driver Code ###

View on GitHub (pinned to 69932aed18)

Solutions

1. Check stack.is_empty? before calling pop.
2. Use while !stack.is_empty? to drain the stack safely.
3. Return nil when empty: stack.is_empty? ? nil : stack.pop.
4. Be aware this is stricter than Ruby's native Array.pop — do not assume nil-return semantics.

Example fix

# before
3.times { val = stack.pop }  # raises if stack has fewer than 3 elements

# after
until stack.is_empty?
  val = stack.pop
end

return nil if stack.is_empty?
stack.pop

# Ruby: safe pop
def safe_pop(stack)
  stack.is_empty? ? nil : stack.pop
end

begin
  val = stack.pop
rescue IndexError
  val = nil  # stack underflow
end

When it happens

Trigger: Calling stack.pop on a newly constructed stack, calling pop more times than elements were pushed, or popping in a drain loop without an emptiness check.

Common situations: Assuming the stack has elements when it does not; mismatched push/pop counts; treating this stack like a raw Ruby Array (which silently returns nil on empty pop).

Related errors

• 堆疊為空
• スタックは空です
• стек пуст
• стек пуст
• 堆積為空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/9ddc6e6fd44d1d33. Report an issue: GitHub.