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The Queue Operations Front Rear A queue is like a line of people waiting for a bank teller. The queue has a front and a rear. $ $ When you think of a computer science queue, you can imagine a line of people waiting for a teller in a bank. The line has a front (the next person to be served) and a rear (the last person to arrive. Front Rear

The Queue Operations Front Rear New people must enter the queue at the rear. The C++ queue class calls this a push, although it is usually called an enqueue operation. $ $ Don’t ask me why the C++ STL used the name push. It only confuses matters with a stack. In any case, when a new item enters a queue, it does so at the rear. Front Rear

The Queue Operations Front Rear When an item is taken from the queue, it always comes from the front. The C++ queue calls this a pop, although it is usually called a dequeue operation. $ $ When an item is removed from a queue, the removal occurs at the front. Front Rear

The Queue Class The C++ standard template library has a queue template class. The template parameter is the type of the items that can be put in the queue. template <class Item> class queue<Item> { public: queue( ); void push(const Item& entry); void pop( ); bool empty( ) const; Item front( ) const; … }; These are the four most common queue operations. The empty function tells you whether the queue has any items at the moment. The front operation returns the item at the front of the queue (without removing it from the queue).

Array Implementation A queue can be implemented with an array, as shown here. For example, this queue contains the integers 4 (at the front), 8 and 6 (at the rear). [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . Just like our stack implementation in the previous chapter, one way to implement a queue is to store the elements in an array. 4 8 6 An array of integers to implement a queue of integers We don't care what's in this part of the array.

Array Implementation size The easiest implementation also keeps track of the number of items in the queue and the index of the first element (at the front of the queue), the last element (at the rear). 3 first last 2 The easiest implementation also keeps track of three numbers. The size could be as small as zero or as large as the number of items in the array. The index of the front element is stored in the first member variable. The front item in the queue is at that index of the array. The next item is after the first one and so on until the rear of the queue that occurs at the index stored in a member variable called last. [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 4 8 6

For more detail contact us A Dequeue Operation size When an element leaves the queue, size is decremented, and first changes, too. 2 first 1 last 2 This shows how the member variables change when an item leaves the queue. [ 0 ] [1] [ 2 ] [ 3 ] [ 4 ] [ 5 ] . . . 4 8 6 For more detail contact us