03 prework stacks queues dequeues #4
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robot-dreams
reviewed
Jun 9, 2020
| * [Dequeues](https://bradfieldcs.com/algos/deques/introduction/) | ||
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| ### Questions | ||
| 1. What is the difference between an abstract data type and an implementation of a data structure? |
| | | Dynamic Array | Doubly Linked List | | ||
| | --- | --- | --- | | ||
| | Push | o(1) amortized | o(1) | | ||
| | Pop | o(1) | o(1) | |
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Given the discussion from class, would you agree now that dequeueing is actually O(n)?
| self._items.push(item) | ||
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| def dequeue(self): | ||
| return self._items.pop() |
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The expected behavior of a Queue is: you get the items back in the order you enqueued them. Will this implementation give you such behavior?
| Potential performance differences | ||
| * Need to update pointers for the doubly linked list. No need to update pointers for the dynamic array. Not sure how this affects performance | ||
| * If we want to view an item that is not at the start or the end of the queue, it is faster to do so in a dynamic array since lookup is o(1) time, whereas it is o(n) time in a doubly linked list. | ||
| * Dynamic array has items in contiguous memory. There might be a limit to how large this queue can become and the limit is the largest block of contiguous memory that the disk has. This limit could be lower on a very fragmented disk (memories of Windows 98 here) Doubly linked lists (and single linked lists) can store their contents all over a disk regardless of contiguous memory, and thus the queue size limit is probably higher. |
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| Potential performance differences | ||
| * Need to update pointers for the doubly linked list. No need to update pointers for the dynamic array. Not sure how this affects performance | ||
| * If we want to view an item that is not at the start or the end of the queue, it is faster to do so in a dynamic array since lookup is o(1) time, whereas it is o(n) time in a doubly linked list. |
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Right. That being said, since your interface wouldn't even allow for viewing such items, this likely wouldn't be something you need to worry about.
| | Pop | o(1) | o(1) | | ||
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| Potential performance differences | ||
| * Need to update pointers for the doubly linked list. No need to update pointers for the dynamic array. Not sure how this affects performance |
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A pointer update can be done in constant time (it's just reassigning a variable).
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