63 lines
2.6 KiB
Markdown
63 lines
2.6 KiB
Markdown
---
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tags:
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- data-structures
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---
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# Queues
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_Visualization of the queue data structure_
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## A queue is a sequential data structure and most similar to a stack
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A queue is basically a stack inverted and can be visualised as a line of people
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waiting to be served. The person that is first into the queue is served first.
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The people who join the queue behind this person will be served after them.
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As a result it can be summarised as **'first in, first out'** or FIFO.
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Just like a stack it is a sequential data structure without random access. You
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cannot access all the elements at one instant, you can only access the oldest
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element. If you wish to access the newest element, you have to move through all
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the others that are ahead of it first, at which point it becomes the oldest.
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It differs from a stack in that a stack only has one point of transaction: the
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front or 'top' of the stack. With a stack you add and remove from the top or
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front. With a queue, you have two points of transaction: the front of the queue
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for removing elements and the back of the queue for adding elements.
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We can however add a 'peek' method to see which is the next element in line to
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come out.
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As we are removing the first element added, we use an `array shift` method to
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remove items from the front of the array.
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Removing an element from the queue is called **dequeuing**. Adding an element to
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the queue is called **enqueuing**. In terms of the tail/head nomenclature, the
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end of the queue where elements are enqueued is the **tail** and front of the
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queue, where elements are removed is the **head**.
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```js
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class Queue {
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items = []; // array to store the elements comprising the queue
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enqueue = (element) => this.items.push(element); // add element to back
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dequeue = () => this.items.shift(); // remove element from the front
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// Optional helper methods:
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isEmpty = () => this.items.length === 0; // return true if the queue is empty
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clear = () => (this.items.length = 0); // empty the queue
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size = () => this.items.length; // count elements in queue
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peek = () => (!this.isEmpty() ? this.items[0] : undefined); // check which element is next in line
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}
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```
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## Use cases
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- A queue sequences data in the order that it was first received. Thus it is
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most beneficial in scenarios where receipt time is a factor. For example,
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imagine a service whereby tickets go on sale at a certain time for a limited
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period. You may want to prioritise those who sent their payment earliest over
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those who arrived later.
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- Serving requests on a single shared resource like a printer or CPU task
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scheduling.
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