krahets/hello-algo · error · IndexError

スタックは空です

Error message

スタックは空です

What it means

Raised by ArrayStack#pop (ja/codes/ruby/chapter_stack_and_queue/array_stack.rb:31) when the stack is empty. It calls @stack.pop; the guard rejects the operation with an explicit message rather than returning nil as stdlib Array#pop would. Message: "スタックは空です" (Stack is empty). Japanese mirror of error 541.

Source

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

  ### スタックの長さを取得 ###
  def size
    @stack.length
  end

  ### スタックが空か判定 ###
  def is_empty?
    @stack.empty?
  end

  ### プッシュ ###
  def push(item)
    @stack << item
  end

  ### ポップ ###
  def pop
    raise IndexError, 'スタックは空です' if is_empty?

    @stack.pop
  end

  ### スタックトップ要素を参照 ###
  def peek
    raise IndexError, 'スタックは空です' if is_empty?

    @stack.last
  end

  ### 表示用のリストを返す ###
  def to_array
    @stack
  end
end

### Driver Code ###

View on GitHub (pinned to 69932aed18)

Solutions

1. Check stack.is_empty? before pop.
2. Balance push and pop counts; assert before unwinding.
3. Rescue IndexError and return a default when empty is a valid state.

Example fix

# before
while true
  stack.pop # raises once drained
end

# after
stack.pop until stack.is_empty?

stack.pop unless stack.is_empty?

def stack_nonempty?(s); s.respond_to?(:is_empty?) && !s.is_empty?; end

begin
  stack.pop
rescue IndexError
  nil
end

When it happens

Trigger: Calling stack.pop when stack.is_empty? is true — after popping as many items as pushed, or before the first push.

Common situations: Unbalanced push/pop in an expression-evaluation or DFS routine; popping in a loop with the emptiness check against the wrong variable; reusing a stack across recursive calls that drain it.

Related errors

• Stack is empty
• Stack is empty
• インデックスが範囲外です
• ヒープが空です
• 両端キューは空です

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/a660e2373a840f2d. Report an issue: GitHub.