Searching Alogrithms

1
Searching Alogrithms

• with Preconditions, Postconditions, and Assertions

2
Searching Algorithms

• An algorithm is a way to solve a problem
• For most tasks (sorting an array, searching for
  an element, etc), there is more than one way to
  solve a problem
• To find an element in an array, we will look at
  three different algorithms
• Sequential Search
• Binary Search
• Hashing (we won't do today, since it requires
  discussion about the Hashtable class, which we'll
  get to later on in the year. It is really more of
  a way to store elements that makes finding them
  easier than it is a searching algorithm)

3
Iterative Algorithms

• An iterative algorithm is one that uses loops
  (aka iteration) to arrive at the solution.
• Both of the searching algorithms we will look at
  today are iterative next class, we will look at
  another possibility

4
Disclaimer

• When we talk about searching/sorting algorithms,
  we are talking about finding a value in a
  collection, like an Array or ArrayList.
• All the examples today will be with Arrays of
  primitive data types, but using an Array or
  ArrayList of Objects that implement the
  Comparable interface would work the same way
• For small collections, it really doesn't matter
  which search or sort you use, since its not going
  to be a noticeable wait anyways. Still, it's good
  to know what different algorithms are out there,
  and it gives us something to talk about when we
  discuss runtimes and Big Oh later

5
Iterative (using loops)Search Algorithms
6
Iterative Sequential Search

• IdeaIf you have a collection and you want to
  see if it contains an element, start at index 0
  and go down the line until you find it, then
  return the index. If you get to the end and
  haven't found it, return -1 to indicate "not
  found".

7
Iterative Binary Search

• IdeaIf you have a ALREADY SORTED collection and
  you want to see if it contains an element
• Start with a counter variable min set to 0, and a
  max variable set to the last index in the array
• While min is less than or equal to max
• Set a mid variable to the index in the middle of
  min and max
• If the element at mid is equal to what you're
  searching for, return mid
• Else if the element at mid is gt what you're
  searching for, change max mid - 1
• Else if the element at mid is lt what you're
  searching for, change min mid 1
• If you exit the loop, return -1 for not found.

8
Preconditions
9
Preconditions and Postconditions

• Binary Search only works if collection is already
  sorted. A big "if".
• This concept that a method only works if a
  certain case is true is called a precondition
• The idea that after a method is done, a condition
  must be true is called a postcondition
• Although when you write the kind of programs we
  do in class, we often just assume that your
  preconditions and postconditions are true, you
  can of course test for them, then throw an
  exception if they aren't

10
Preconditions and Postconditions

• On the AP test, Pre and Postconditions are
  typically written as a comment above the method
• //precondition cost gt 0 and title ! null
• //postcondition Book object created
• public Book(String title, String author, double
  cost)
• if((costlt0) (title null))
• throw new IllegalArgumentException()
• else
• ....

11
Preconditions and Postconditions

• If the precondition of a method involves an input
  parameter, you are always supposed to use control
  statements and try...catch or throws to deal with
  the error, so that the user will be aware of the
  error made so they know what not to do
• However, if the precondition involves a instance
  variable (non-public data field) or is in a
  non-public method, there is another reserved word
  in Java we can use to test pre and
  postconditions assert

12
Asserts
13
Assert

• A big addition to Java 1.4 was the assert
  reserved word
• As it sounds, assert will make sure that a
  boolean statement is true, and throw a
  AssertationError exception if it isn't.
• The syntax is as follows
• assert BooleanExpression
• You can also add a String or object that will be
  printed out to give more feedback on what went
  wrong
• assert BooleanExpression ClarifyingObjectOrExpre
  ssion

14
Assert

• Let's say we have a BankAccount class with a
  private instance variable interestRate that
  should never be lt 0, no matter what the user does
  to the account. The following method might make
  sense
• public double calculateInterest(double years)
• if(yearslt0)
• throw new IllegalArgumentException()
• else
• assert (interestRategt0) "Interest Rate
  "interestRate
• return interestRateyears

15
Assert

• Unlike with try...catch blocks and throws,
  asserts should just be used to catch things that
  should never happen.
• It should just be used as a status check when
  debugging your code, checking for errors and
  trying to verify everything is always running
  correct
• It should not be used as part of functionality in
  your code...for that, use throws
• The reason asserts are so limited is because you
  have to tell the JVM that you want to run with
  them turned on in order for them to work. They
  are not made to be used by actual users, just the
  programmers testing the code

16
Turning on Asserts

• From command line
• Instead of java FileName, type java -ea FileName
• From JCreator
• Make sure you are set to use a JDK gt 1.4. Then,
  before you execute, go to Configure Options
  Select Tool Type Run Application, click
  "Default", click edit, click Parameters tab, and
  add -ea at the beginning of the Parameters text
  field.
• From Eclipse
• Window Preferences Java Compiler and set the
  Compiler Compliance Level to 1.4 or later (5.0,
  6.0, etc). Also check Use Default compliance
  settings. Then go to Run Run Configurations,
  click Java Application, select the file name, go
  to Arguments tab type -ea in the VM arguments box

17
The Arrays/Collections classes in the API

• The Arrays class is filled with static methods
  you can use with arrays
• //precondition arr is sortedpublic static int
  binarySearch(double arr, double key)
• The Collections class is similarly filled with
  static methods you can use with ArrayLists
• //precondition list is sortedpublic static int
  binarySearch(List list, Object key)

18
Assignment

• Inside of a new class called SearchMethods, make
  the following methods that work on either a
  Comparable or aArrayListsltComparablegt (your
  choice)
• an iterative Sequential Search
• an iterative Binary Search