Stacks and queues

Stacks and queues

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  Page 1 of4 Stacks and Queues An array is a random access data structure, where each element can be accessed directly and in constant time. A typical illustration of random access is a book - each page of the book can be open independently of others. Random access is critical to many algorithms, for example binary search. A linked list is a sequential access data structure, where each element can be accesed only in particular order. A typical illustration of sequential access is a roll of paper or tape - all prior material must be unrolled in order to get to data you want. Stacks A stack is a container of objects that are inserted and removed according to the last-in first-out (LIFO) principle. In the pushdown stacks only two operations are allowed: push the item into the stack, and pop the item out of the stack. A stack is a limited access data structure - elements can be added and removed from the stack only at the top. push adds an item to the top of the stack, popremoves the item from the top. A helpful analogy is to think of a stack of books; you can remove only the top book, also you can add a new book on the top. A stack is a recursive data structure. Here is a structural definition of a Stack: a stack is either empty or it consistes of a top and the rest which is a stack;
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  Page 2 of4 Applications  The simplest application of a stack is to reverse a word. You push a given word to stack - letter by letter - and then pop letters from the stack.  Another application is an "undo" mechanism in text editors; this operation is accomplished by keeping all text changes in a stack. Queues A queue is a container of objects (a linear collection) that are inserted and removed according to the first-in first-out (FIFO) principle. An excellent example of a queue is a line of students in the food court . New additions to a line made to the back of the queue, while removal (or serving) happens in the front. In the queue only two operations are allowed enqueue and dequeue. Enqueue means to insert an item into the back of the queue, dequeue means removing the front item. The picture demonstrates the FIFO access. The difference between stacks and queues is in removing. In a stack we remove the item the most recently added; in a queue, we remove the item the least recently added.
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  Page 3 of4 Adding into stack procedure add(item : items); {add item to the global stack stack; top is the current top of stack and n is its maximum size} begin if top = n then stackfull; top := top+1; stack(top) := item; end: {of add} Deletion in stack procedure delete(var item : items); {remove top element from the stack stack and put it in the item} begin if top = 0 then stackempty; item := stack(top); top := top-1; end; {of delete}
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  Page 4 of4 Addition into a queue procedure addq (item : items); {add item to the queue q} begin if rear=n then queuefull else begin rear :=rear+1; q[rear]:=item; end; end;{of addq} Deletion in a queue procedure deleteq (var item : items); {delete from the front of q and put into item} begin if front = rear then queueempty else begin front := front+1 item := q[front]; end; end; {of deleteq}