What causes "стек пуст" in krahets/hello-algo?

Guard with `stack.is_empty?` before peek or pop.

krahets/hello-algo · error · IndexError

стек пуст

Error message

стек пуст

What it means

Raised by LinkedListStack#peek (ru) on an empty stack. `pop` is implemented as `num = peek; @peek = @peek.next; @size -= 1`, so this same raise fires for both `peek` and `pop` when empty — the failure originates at the peek call inside pop. It guards against nil @peek.

Source

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

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

  ### Извлечение из стека ###
  def pop
    num = peek
    @peek = @peek.next
    @size -= 1
    num
  end

  ### Доступ к верхнему элементу стека ###
  def peek
    raise IndexError, 'стек пуст' if is_empty?

    @peek.val
  end

  ### Преобразовать связный список в Array и вернуть ###
  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

Guard with `stack.is_empty?` before peek or pop.
Drive pop loops with `while stack.size > 0`.
Wrap pop in a helper returning nil on empty for optional semantics.

Example fix

// before
stack.pop  # raises 'стек пуст' when empty

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

Defensive patterns

Strategy: validation

Validate before calling

val = stack.pop unless stack.is_empty?

Type guard

def stack_nonempty?(s) = !s.is_empty?

Try / catch

begin
  val = stack.pop  # also covers peek
rescue IndexError
  val = nil
end

Prevention

pop routes through peek — guard covers both.
Drive pop loops with `while stack.size > 0`.
Linked-list stack has no capacity limit — only the empty precondition raises.

When it happens

Trigger: Calling `stack.peek` or `stack.pop` on an empty LinkedListStack (never pushed, or fully popped). Any pop loop that overshoots triggers it.

Common situations: Unwinding then over-popping; mismatched push/pop counts; an algorithm that pops-to-empty then peeks to verify.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/f7113c134dba7338. Report an issue: GitHub.