Worksheet 6: Vector Deque and Queue Name:
An Active Learning Approach to Data Structures using C
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Worksheet 6: Vector Deque and Queue
In an earlier lesson you explored why one cannot use a Vector as the basis for
an efficient queue. This is because adding to or removing from the first location
(that is, index position 0) is very slow O(n).
Removing a value requires elements to slide
left.
Adding a value requires elements to slide
right.
This does not mean that a vector
-like
data structure cannot be used to implement
a queue. The key insight is that we can allow the starting location of the block of
elements to “float”, rather than being fixed at location zero. An internal integer
data field records the current starting location.
Notice that the “logical” index no longer corresponds to the physical index. The
value with logical index zero is found in this diagram at array location 2. The
value with logical index 1 is found at location 3, and so on.
With this change, it is now easy to implement additions or removal from either
front or back. To add to the front, simply decrement the starting location, and
place the new element in the new starting place. To add to the back, simply
increase the size, and place the new value in the location determined by the
addition of the starting location and size. But there is one subtle complexity. The
block of values stored in the collection can wrap around from the end back to the
beginning:
