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## Recursion

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### Recursive Algorithm for Calculating xn. Chapter 7: Recursion. 13. Recursion Versus Iteration ... that is presented as a two-dimensional array of color values ... – PowerPoint PPT presentation

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Title: Recursion

1
Recursion
• Chapter 7

2
Chapter Objectives
• To understand how to think recursively
• To learn how to trace a recursive method
• To learn how to write recursive algorithms and
methods for searching arrays
• To learn about recursive data structures and
recursive methods for a LinkedList class
• To understand how to use recursion to solve the
Towers of Hanoi problem

3
Chapter Objectives (continued)
• To understand how to use recursion to process
two-dimensional images
• To learn how to apply backtracking to solve
search problems such as finding a path through a
maze

4
Recursive Thinking
• Recursion is a problem-solving approach that can
be used to generate simple solutions to certain
kinds of problems that would be difficult to
solve in other ways
• Recursion splits a problem into one or more
simpler versions of itself

5
Recursive Thinking
6
Steps to Design a Recursive Algorithm
• There must be at least one case (the base case),
for a small value of n, that can be solved
directly
• A problem of a given size n can be split into one
or more smaller versions of the same problem
(recursive case)
• Recognize the base case and provide a solution to
it
• Devise a strategy to split the problem into
smaller versions of itself while making progress
toward the base case
• Combine the solutions of the smaller problems in
such a way as to solve the larger problem

7
String Length Algorithm
8
Proving that a Recursive Method is Correct
• Proof by induction
• Prove the theorem is true for the base case
• Show that if the theorem is assumed true for n,
then it must be true for n1
• Recursive proof is similar to induction
• Verify the base case is recognized and solved
correctly
• Verify that each recursive case makes progress
towards the base case
• Verify that if all smaller problems are solved
correctly, then the original problem is also
solved correctly

9
Tracing a Recursive Method
10
Recursive Definitions of Mathematical Formulas
• Mathematicians often use recursive definitions of
formulas that lead very naturally to recursive
algorithms
• Examples include
• Factorial
• Powers
• Greatest common divisor
• If a recursive function never reaches its base
case, a stack overflow error occurs

11
Recursive Factorial Method
12
Recursive Algorithm for Calculating xn
13
Recursion Versus Iteration
• There are similarities between recursion and
iteration
• In iteration, a loop repetition condition
determines whether to repeat the loop body or
exit from the loop
• In recursion, the condition usually tests for a
base case
• You can always write an iterative solution to a
problem that is solvable by recursion
• Recursive code may be simpler than an iterative
algorithm and thus easier to write, read, and
debug

14
Iterative Factorial Method
15
Efficiency of Recursion
• Recursive methods often have slower execution
times when compared to their iterative
counterparts
• The overhead for loop repetition is smaller than
the overhead for a method call and return
• If it is easier to conceptualize an algorithm
using recursion, then you should code it as a
recursive method
• The reduction in efficiency does not outweigh the

16
An Exponential Recursive Fibonacci Method
17
An O(n) Recursive Fibonacci Method
18
Efficiency of Recursion Exponential Fibonacci
Inefficient
19
Efficiency of Recursion O(n) Fibonacci
Efficient
20
Recursive Array Search
• Searching an array can be accomplished using
recursion
• Simplest way to search is a linear search
• Examine one element at a time starting with the
first element and ending with the last
• Base case for recursive search is an empty array
• Result is negative one
• Another base case would be when the array element
being examined matches the target
• Recursive step is to search the rest of the
array, excluding the element just examined

21
Algorithm for Recursive Linear Array Search
22
Implementation of Recursive Linear Search
23
Design of a Binary Search Algorithm
• Binary search can be performed only on an array
that has been sorted
• Stop cases
• The array is empty
• Element being examined matches the target
• Checks the middle element for a match with the
target
• Throw away the half of the array that the target
cannot lie within

24
Design of a Binary Search Algorithm (continued)
25
Design of a Binary Search Algorithm (continued)
26
Implementation of a Binary Search Algorithm
27
Implementation of a Binary Search Algorithm
(continued)
28
Efficiency of Binary Search and the Comparable
Interface
• At each recursive call we eliminate half the
array elements from consideration
• O(log2n)
• Classes that implement the Comparable interface
must define a compareTo method that enables its
objects to be compared in a standard way
• CompareTo allows one to define the ordering of
elements for their own classes

29
Method Arrays.binarySearch
• Java API class Arrays contains a binarySearch
method
• Can be called with sorted arrays of primitive
types or with sorted arrays of objects
• If the objects in the array are not mutually
comparable or if the array is not sorted, the
results are undefined
• If there are multiple copies of the target value
in the array, there is no guarantee which one
will be found
• Throws ClassCastException if the target is not
comparable to the array elements

30
Recursive Data Structures
• Computer scientists often encounter data
structures that are defined recursively
• Trees (Chapter 8) are defined recursively
• Linked list can be described as a recursive data
structure
• Recursive methods provide a very natural
mechanism for processing recursive data
structures
• The first language developed for artificial
intelligence research was a recursive language
called LISP

31
Recursive Definition of a Linked List
• A non-empty linked list is a collection of nodes
such that each node references another linked
list consisting of the nodes that follow it in
the list
• The last node references an empty list
• A linked list is empty, or it contains a node,
called the list head, that stores data and a

32
Recursive Size Method
33
Recursive toString Method
34
Recursive Replace Method
35
36
Recursive Remove Method
37
Recursive Remove Method (continued)
38
Problem Solving with Recursion
• Will look at two problems
• Towers of Hanoi
• Counting cells in a blob

39
Towers of Hanoi
40
Towers of Hanoi (continued)
41
Algorithm for Towers of Hanoi
42
Algorithm for Towers of Hanoi (continued)
43
Algorithm for Towers of Hanoi (continued)
44
Recursive Algorithm for Towers of Hanoi
45
Implementation of Recursive Towers of Hanoi
46
Counting Cells in a Blob
• Consider how we might process an image that is
presented as a two-dimensional array of color
values
• Information in the image may come from
• X-Ray
• MRI
• Satellite imagery
• Etc.
• Goal is to determine the size of any area in the
image that is considered abnormal because of its
color values

47
Counting Cells in a Blob (continued)
48
Counting Cells in a Blob (continued)
49
Implementation
50
Implementation (continued)
51
Counting Cells in a Blob (continued)
52
Backtracking
• Backtracking is an approach to implementing
systematic trial and error in a search for a
solution
• An example is finding a path through a maze
• If you are attempting to walk through a maze, you
will probably walk down a path as far as you can
go
• Eventually, you will reach your destination or
you wont be able to go any farther
• If you cant go any farther, you will need to
• Backtracking is a systematic approach to trying
alternative paths and eliminating them if they
dont work

53
Backtracking (continued)
• Never try the exact same path more than once, and
you will eventually find a solution path if one
exists
• Problems that are solved by backtracking can be
described as a set of choices made by some method
• Recursion allows us to implement backtracking in
a relatively straightforward manner
• Each activation frame is used to remember the
choice that was made at that particular decision
point
• A program that plays chess may involve some kind
of backtracking algorithm

54
Backtracking (continued)
55
Recursive Algorithm for Finding Maze Path
56
Implementation
57
Implementation (continued)
58
Implementation (continued)
59
Chapter Review
• A recursive method has a standard form
• To prove that a recursive algorithm is correct,
you must
• Verify that the base case is recognized and
solved correctly
• Verify that each recursive case makes progress
toward the base case
• Verify that if all smaller problems are solved
correctly, then the original problem must also be
solved correctly
• The run-time stack uses activation frames to keep
track of argument values and return points during
recursive method calls

60
Chapter Review (continued)
• Mathematical Sequences and formulas that are
defined recursively can be implemented naturally
as recursive methods
• Recursive data structures are data structures
that have a component that is the same data
structure
• Towers of Hanoi and counting cells in a blob can
both be solved with recursion
• Backtracking is a technique that enables you to
write programs that can be used to explore
different alternative paths in a search for a
solution