Learn Ruby Queue with Examples

In Ruby Language, if we want to use a FIFO(First In First Out) Queue data structure, we can use Array as a Queue. If you want to use a queue object in a multi-threads program, then you can choose to use ruby Queue class.

Let’s learn to use them with some examples:

Use Array as a Queue

Ruby Array provides us plenty of useful methods.

If we see the last index as the head of the Queue, we can use #unshift to enqueue, #pop to dequeue, and to peek the head element, with this approach we can use #last method.

queue = Array.new # or []
queue.unshift "dog"
queue.unshift "cat"
queue.unshift "cow"

# queue is: ["cow", "cat", "dog"]

queue.last  # peek returns "dog"

queue.pop # "dog"
queue.pop # "cat"

# queue is: ["cow"]

We could see the index 0 as the head of the queue. Then we will use #push as enqueue method, #shift as dequeue method, and #first as the peek method.

queue = Array.new # or []
queue.push "dog" # or <<
queue.push "cat"
queue.push "cow"

# queue is: ["dog", "cat", "cow"]

queue.first  # peek returns "dog"

queue.shift # "dog"
queue.shift # "cat"

# queue is: ["cow"]

There are another two methods #size and #empty? used a lot for a queue. For example when we want to loop through the queue

while !queue.empty? do # or queue.size > 0
  item = queue.pop
  # do thing with item
end

Ruby Queue Class

Ruby Queue class is a thread-safe, blocking queue implementation. And we can use it in a multi-threaded program.

We can use #push, #enq and #<< methods to enqueue.

And use #pop, #deq and #shift methods to dequeue.

Those methods are more intuitive than methods in Array

The blocking feature is decided by non_block argument in #pop method, the default value for non_block is false, so when the queue is empty, the calling thread is suspended until data is pushed onto the queue.

If non_block is true, like queue.pop(true), the thread isn’t suspended, and ThreadError is raised.

I will create another post to demonstrate how to use Queue in a coding interview problem.

Ruby SizedQueue Class

SizedQueue is another Queue implementation in which you can specify the size capacity for the queue. If the capacity is full then the push operation will be blocked.

sized_queue = SizedQueue.new(3)  # the maximum size is 5

sized_queue.push("a")
sized_queue.push("b")
sized_queue.push("c")

sized_queue.push("d")  # This one is blocked.

#push method can accept another argument non_block, the default value is false, if non_block is true, the thread isn’t suspended, and ThreadError is raised.

Last Thing

There is one disadvantage of using Queue and SizedQueue classes, they both don’t have the #peek method to let you peek the head element of the queue.

I will show you how to implement a thread-safe Queue using Array in another Ruby Rate Limiter post.