Stacks and Queues

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Stacks and Queues

• Introduction to stacks
• Stack implementation
• 4 Standard operations on a stack
• A stack program
• Introduction to queues
• Queue implementation
• Basic operations on a queue
• A queue program

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Introduction to stacks 1

• A stack implements a last in first out or LIFO
  data structure. It can be visualised as a pile of
  plates. Plates are added to the top of pile and
  the plate at the top of the pile which was the
  last to be added will be the first to be removed.
• Stacks are used to keep track of a nested set of
  subroutines (otherwise referred to as functions
  or methods) internally within an executing
  program, so that when a subroutine exits and
  returns, execution will restart at the correct
  point within the program which called the
  subroutine.
• These internal stacks will typically be
  implemented using an area of memory allocated by
  the compiler for the purpose and making use of a
  special stack pointer to record the location of
  the top of the pile. Modern microprocessor
  hardware directly supports this kind of stack
  maintenance and referencing to maximise
  performance.

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An example stack application

• This requires determining which sets of copper
  items (tracks, vias, lands, polygons) on a
  printed circuit board are connected into which
  electrically conducting networks. An item with no
  netnumber is found from the list of PCB items and
  given the next available netnumber. It is then
  scanned to find other connected items which are
  given the same netnumber. Every unscanned piece
  of copper found to be connected to it is pushed
  onto the stack. On finising scanning one piece it
  is marked as scanned.
• The next item in the stack is popped and scanned
  for connected items, all of which are given the
  same netnumber if not already so allocated and
  pushed onto the stack if unscanned. This process
  repeats until the stack is empty. The next copper
  item with no netnumber is then found and scanned,
  and items found connected to it are marked,
  stacked and processed in the same way until all
  copper items have been assigned netnumbers.

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Stack implementation

• A standard method of implementing a stack within
  applications is by means of a single linked list.
  The stack can only access the head of the list
  which is known as the 'top' of the stack.

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4 Standard operations on a stack

• int isempty(TOP t)
• /return 1 if stack is empty otherwise, return
  0. /
• DATA vtop(TOP t)
• /return the value of the top element. /
• void pop(TOP t, DATA d)
• / return the value of the top element and remove
  it./
• void push(TOP t, DATA d)
• / place the value d onto the top of the stack./

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A stack program 1
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A stack program 2
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A stack program 3
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A stack program 4
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A stack program 5
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A stack program 6
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Introduction to Queues

• A queue is a first in first out structure (FIFO).
  It can be visualised as a queue of people waiting
  for a bus. The person first in the queue is the
  first on to the bus (hopefully!).
• A standard method of implementing a queue is a
  single linked list. A queue has a 'front' and a
  'rear'. New data items are inserted into the rear
  of the list (a bus passenger joining the queue)
  and are deleted from the front of the list (a
  passenger at the front of the queue getting onto
  the bus).

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An example queue application

• A network socket listening for TCP service port
  requests (on port 80) for a webserver, places
  incoming requests from clients up to a maximum
  queue length at the back of a queue, discarding
  requests if the queue is full.
• A webserver queue handling thread processes each
  new session request at the front of the queue by
  allocating the session an individual
  high-numbered port which is returned to the
  client and starts a new server thread to handle
  this client using the high-numbered port.

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4 Standard operations on a queue

• int isempty(QUEUE t)
• / return 1 if queue is empty otherwise, return
  0./
• DATA vtop(QUEUE t)
• / return the value of the first element. /
• void dequeue(QUEUE t, DATA d)
• / return the value of the first element and
  remove it. /
• void enqueue(QUEUE t, DATA d)
• /place the value d onto the rear of the queue./

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A queue scheduler program 1
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A queue scheduler program 2
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A queue scheduler program 3
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A queue scheduler program 4
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A queue scheduler program 5
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A queue scheduler program 6
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Input of queue scheduler program

• a234
• a56
• b43
• a345
• a89
• b75
• b32
• Z

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Output of queue scheduler program

• A's schedule
• JOB 1 is 234
• JOB 2 is 56
• JOB 3 is 345
• JOB 4 is 89
• B's schedule
• JOB 1 is 43
• JOB 2 is 75
• JOB 3 is 32