krahets/hello-algo · error · IndexError

栈为空

Error message

栈为空

What it means

Raised by `ArrayStack#pop` (array_stack.rb:31) when `is_empty?` is true. The stack delegates storage to Ruby's `Array#pop`; the guard exists to throw a descriptive IndexError rather than letting `Array#pop` silently return nil. It enforces the stack ADT contract that pop on an empty container is an error.

Source

Thrown at codes/ruby/chapter_stack_and_queue/array_stack.rb:31

  ### 获取栈的长度 ###
  def size
    @stack.length
  end

  ### 判断栈是否为空 ###
  def is_empty?
    @stack.empty?
  end

  ### 入栈 ###
  def push(item)
    @stack << item
  end

  ### 出栈 ###
  def pop
    raise IndexError, '栈为空' if is_empty?

    @stack.pop
  end

  ### 访问栈顶元素 ###
  def peek
    raise IndexError, '栈为空' if is_empty?

    @stack.last
  end

  ### 返回列表用于打印 ###
  def to_array
    @stack
  end
end

### Driver Code ###

View on GitHub (pinned to 69932aed18)

Solutions

1. Check `stack.is_empty?` before calling `pop`.
2. Track push/pop balance with a counter in the caller.
3. Rescue IndexError if an empty pop is a recoverable condition.

Example fix

# before
val = stack.pop

# after
val = stack.is_empty? ? nil : stack.pop

return nil if stack.is_empty?
stack.pop

def stack_popable?(stack)
  stack.respond_to?(:is_empty?) && stack.respond_to?(:pop) && !stack.is_empty?
end

begin
  stack.pop
rescue IndexError
  nil
end

When it happens

Trigger: Calling `stack.pop` when the backing `@stack` array is empty. Occurs after popping all pushed items, or on a freshly constructed stack.

Common situations: Unbalanced push/pop pairs (more pops than pushes); expression-evaluation or backtracking algorithms that over-pop; test calling pop before any push.

Related errors

• 栈为空
• стек пуст
• стек пуст
• 堆疊為空
• 堆疊為空

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/1e6d7892255b0f89. Report an issue: GitHub.