krahets/hello-algo · error · IndexError

Stack is empty

Error message

Stack is empty

What it means

Raised by LinkedListStack#peek (en/codes/ruby/chapter_stack_and_queue/linkedlist_stack.rb:41) when the stack is empty. peek returns @peek.val; @peek is nil at size == 0, so the guard prevents a nil dereference. pop calls peek first, so popping an empty linked-list stack also surfaces this message.

Source

Thrown at en/codes/ruby/chapter_stack_and_queue/linkedlist_stack.rb:41

  ### Push ###
  def push(val)
    node = ListNode.new(val)
    node.next = @peek
    @peek = node
    @size += 1
  end

  ### Pop ###
  def pop
    num = peek
    @peek = @peek.next
    @size -= 1
    num
  end

  ### Access top element ###
  def peek
    raise IndexError, 'Stack is empty' if is_empty?

    @peek.val
  end

  ### Convert linked list to Array and return ###
  def to_array
    arr = []
    node = @peek
    while node
      arr << node.val
      node = node.next
    end
    arr.reverse
  end
end

### Driver Code ###
if __FILE__ == $0

View on GitHub (pinned to 69932aed18)

Solutions

1. Guard with stack.is_empty? before peek/pop.
2. Keep push and pop counts balanced; assert before unwinding.
3. Rescue IndexError and return a sentinel for the empty-top case.

Example fix

# before
top = stack.peek # raises on empty

# after
top = stack.is_empty? ? nil : stack.peek

stack.peek unless stack.is_empty?

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

begin
  stack.peek
rescue IndexError
  nil
end

When it happens

Trigger: Calling stack.peek or stack.pop when stack.size == 0 — before any push, or after the list has been fully unwound by pops.

Common situations: A recursive/backtracking stack drained by unwind; a delimiter/matching stack where the input closes more groups than it opened; peeking the top before the first push.

Related errors

• 栈为空
• Stack is empty
• Deque is empty
• Queue is empty
• スタックは空です

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/543c554077f939b4. Report an issue: GitHub.