Queues

1
Queues

• CS-240 CS-341
• Dick Steflik

2
Queues

• First In, First Out operation FIFO
• As items are added they are chronologically
  ordered, items are removed in their chronological
  order (oldest first)
• examples grocery store line, tube feed
  ammunition magazine, digital shift register,

3
Applications

• discrete event simulation -
• digital delay line
• task scheduling in an operation system
• staging area for data acquisition systems
• I/O buffers
• print queues
• sorting

4
front
rear
checkout
5
ADT Methods

• enqueue - add an item (at the back, move up as
  far as you can
• dequeue - return the value of the item at the
  front of the queue then delete the item and move
  all items forward one position.
• isEmpty - return false/true depending if the
  queue is empty true if it is, false otherwise
• isFull return false/ depending if the queue is
  empty true if it is, false otherwise

6
Data strategies

• use an array to hold items and use an int as an
  index for the array to indicate where the back of
  the queue is
• same as above, but use a dynamic array
• same as above but treat array as if it were
  circular and use two ints, one to indicate the
  front and the other to indicate the back
• make a type using a struct with two ints for the
  front and rear pointers and an array for the data
  part of the queue
• use a struct to define a node and add nodes
  dynamically as they are needed use one static
  pointer to a node to point at the fron node and
  another to point at the back node.

7
Static Queue Implementation
typedef struct int
front,rear int
dataMAXQSIZE queue void enqueue(queue
p , int v) if
(isFull(p)) printf(queue is
full \n) else
p-gtdatap-gtrear v
p-gtrear 1
int main() queue q1 0
enqueue(q1,50)
8
Priority Queues

• Operates like a queue except entry is at the back
  but items move up to the end of their priority
  list

new item
2
front
back
1
1
2
3
4
1
1
2
2
3
4