What causes "двусторонняя очередь пуста" in krahets/hello-algo?

Guard with `deque.is_empty?` before peek_first or pop_first.

krahets/hello-algo · error · IndexError

двусторонняя очередь пуста

Error message

двусторонняя очередь пуста

What it means

Raised by ArrayDeque#peek_first (ru) when the deque is empty. Because `pop_first` is implemented as `num = peek_first; @front = index(@front + 1); @size -= 1`, the SAME raise surfaces for both `peek_first` and `pop_first` on an empty deque. Note: unlike array_queue.push, the push methods here only `puts`+`return` on full — they never raise — so this is the only IndexError in the file.

Source

Thrown at ru/codes/ruby/chapter_stack_and_queue/array_deque.rb:76

  ### Извлечение из головы очереди ###
  def pop_first
    num = peek_first
    # Указатель головы сдвигается на одну позицию назад
    @front = index(@front + 1)
    @size -= 1
    num
  end

  ### Извлечение из хвоста очереди ###
  def pop_last
    num = peek_last
    @size -= 1
    num
  end

  ### Доступ к элементу в начале очереди ###
  def peek_first
    raise IndexError, 'двусторонняя очередь пуста' if is_empty?

    @nums[@front]
  end

  ### Доступ к элементу в хвосте очереди ###
  def peek_last
    raise IndexError, 'двусторонняя очередь пуста' if is_empty?

    # Вычислить индекс хвостового элемента
    last = index(@front + size - 1)
    @nums[last]
  end

  ### Вернуть массив для вывода ###
  def to_array
    # Преобразовывать только элементы списка в пределах фактической длины
    res = []
    for i in 0...size

View on GitHub (pinned to 69932aed18)

Solutions

Guard with `deque.is_empty?` before peek_first or pop_first.
Drive drain loops with `until deque.is_empty?`.
Use to_array (empty-safe) when you only need to inspect contents.

Example fix

// before
deque.pop_first  # raises 'двусторонняя очередь пуста' when empty

// after
val = deque.is_empty? ? nil : deque.pop_first

Defensive patterns

Strategy: validation

Validate before calling

val = deque.pop_first unless deque.is_empty?

Type guard

def deque_nonempty?(d) = !d.is_empty?

Try / catch

begin
  val = deque.pop_first  # also covers peek_first
rescue IndexError
  val = nil
end

Prevention

pop_first routes through peek_first — guard covers both.
Drive drain loops with `until deque.is_empty?`.
Note push_* only puts+returns on full — it never raises, so capacity is silent here.

When it happens

Trigger: Calling `deque.peek_first` or `deque.pop_first` on an empty ArrayDeque (size 0), e.g. before any push, or after draining from both ends.

Common situations: A consumer loop that pops_first until empty then peeks; reading the front for display after a drain; mismatched push/pop counts in a driver block.

Related errors

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/2201bda92674710a. Report an issue: GitHub.