Iterators

1
Iterators

• Why use iterators?

Tom
M

• How many female employees?
• Output those employees younger than 35.
• Raise 10K employees salary that is lt 100K.
• We only discuss List iterators today.

2
Iterators

• Allow the programmer to traverse the list and
  modify the list during iteration .

Linked list
current
List iterator 1
current
List iterator 2
3
Implement singly linked list iterator

• class ListIterator //(Textbook P.237)
• private Link current // current link
• private Link previous // previous link
• private LinkList ourList // parent list
• public ListIterator( LinkList list)
  //constructor
• outList list
• current list.getFirst()
• public Link getCurrent() // returns current
  link
• public void nextLink() // go to next link
• public boolean atEnd() // true if last link
• public void deleteCurrent() //delete item at
  current
• public void insertAfter( long dd) // insert
  after current link

Exercise atEnd, deleteCurrent
4
Generate an iterator from a list
class LinkList Link first . public
ListIterator getIterator() // return an iterator
referring to this list return new
ListIterator(this)
5
Use iterators
// Increase wages of all employees who are paid
minimum by amount void increaseWages(LinkList
empolyees, long minimum, long amount)
6
Project 2.3 GPSR

• Greedy Perimeter Stateless Routing (GPSR)
• Implementation public String GPSR(String s,
  String t)
• Idea
• path the path created from GPSR.
• len length of the path.
• cur the current vertex being checked.
• local_min current perimeter.
• preVertex the vertex added to path before cur.
• Initialization add s to path. cur s local_min
  dist(s, t) Initial mode is greedy mode.
  preVertex null
• while (cur is NOT the destination t), do
• Traverse curs neighbors for a vertex
  min_vertex closest to t. Suppose dmin
  dist(min_vertex, t).
• If dmin lt local_min, then
• Enter greedy mode.
• Add min_vertex to path. Local_min dmin. Set
  cur min_vertex.
• Else
• Enter perimeter mode.
• If last mode is greedy mode
• Traverse curs neighbors for a vertex u with
  minimum counterclockwise angle

Correction made on Nov 15th!
7
How to compute ?(t, x, u) in 0,2p)
(0, 2 p
?(t, x, u)
p
?(t, x, v)

• Counter clockwise angle ?(t, x, u)
• cosAngle cosine(t, x, u) // As in project2.2
  solution
• theta Math.acos(cosAngle) //Here theta is in
  0, p, if theta 0.0, set theta 2 p.
• Check if direction t gt x gt u is counter
  clockwise. If so, adjust theta 2Math.PI
  theta.
• If ( ) gt 0, then t gt x gt u is counter
  clockwire
• Return theta.

cosine(x, t, u)
(posxtposyx-posxxposyt posxxposyu-
posxuposyx posxuposyt-posxtposyu)
2signed area of triangle(txu)