krahets/hello-algo · error · IndexError

Queue is empty

Error message

Queue is empty

What it means

Raised by ArrayQueue#peek (en/codes/ruby/chapter_stack_and_queue/array_queue.rb:52) when the queue holds zero elements. ArrayQueue is a hand-written circular-array queue from the hello-algo teaching code; peek guards the front-access precondition because reading @nums[@front] is undefined when size == 0. Because pop calls peek first (line 43), any dequeue on an empty queue surfaces the same message.

Source

Thrown at en/codes/ruby/chapter_stack_and_queue/array_queue.rb:52

    # Add num to the rear of the queue
    rear = (@front + size) % capacity
    # Front pointer moves one position backward
    @nums[rear] = num
    @size += 1
  end

  ### Dequeue ###
  def pop
    num = peek
    # Move front pointer backward by one position, if it passes the tail, return to array head
    @front = (@front + 1) % capacity
    @size -= 1
    num
  end

  ### Access front element ###
  def peek
    raise IndexError, 'Queue is empty' if is_empty?

    @nums[@front]
  end

  ### Return list for printing ###
  def to_array
    res = Array.new(size, 0)
    j = @front

    for i in 0...size
      res[i] = @nums[j % capacity]
      j += 1
    end

    res
  end
end

View on GitHub (pinned to 69932aed18)

Solutions

1. Guard the call: only peek/pop when queue.is_empty? returns false (or queue.size > 0).
2. Track the count of successful pushes and never pop more times than you have pushed.
3. Wrap the dequeue in a rescue IndexError block to supply a sentinel/nil when the queue legitimately runs dry.

Example fix

# before
queue = ArrayQueue.new(10)
queue.pop # raises IndexError, "Queue is empty"

# after
queue.pop unless queue.is_empty?

queue.pop if queue.size > 0 && !queue.is_empty?

# ArrayQueue exposes size and is_empty?; no type guard needed.
def dequeable?(q); q.respond_to?(:is_empty?) && !q.is_empty?; end

begin
  queue.pop
rescue IndexError => e
  nil # queue legitimately empty
end

When it happens

Trigger: Calling queue.peek or queue.pop when queue.size == 0. Happens on a freshly constructed ArrayQueue.new(10) before any push, or after draining every pushed element via successive pop calls (e.g. the driver's enqueue+dequeue loop overshooting count).

Common situations: Draining a queue in a producer/consumer loop without an emptiness check; running the driver's circular-array test loop one iteration too many; reusing a queue instance across test cases assuming it was repopulated.

Related errors

• キューがいっぱいです
• キューは空です
• Queue is empty
• 両端キューは空です
• очередь пуста

AI-assisted analysis of krahets/hello-algo@69932aed18 (2026-08-13). Data as JSON: /api/errors/6bdab46f5d82eeab. Report an issue: GitHub.