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### Cs212: DataStructures

Lecture 4: Recursion

Lecture Contents

- The Concept of Recursion
- Why recursion?
- Factorial – A case study
- Content of a Recursive Method
- Recursion vs. iteration

The Concept of Recursion

- repetition can be achieved by writing loops, such as for loops and while loops.
- Another way to achieve repetition is through recursion, which occurs when a function calls itself.
- A recursive function is a function that calls itself , either directly or indirectly ( through another function).
- For example :

void myFunction( int counter)

{if(counter == 0) return;else {myFunction(--counter); return;}

}

Why recursion?

- Sometimes, the best way to solve a problem is by solving a smaller version of the exact same problem first
- Recursion is a technique that solves a problem by solving a smaller problem of the same type
- Allows very simple programs for very complex problems

=

3*2*1

=

6

Factorial – A case study- Factorial definition: the factorial of a number is product of the integral values from 1 to the number.

factorial 3

=

product [4..1]

=

product [4,3,2,1]

=

4*3*2*1

=

24

Factorial – A case study- Iteration algorithm definition (non-recursive)

1 if n = 0

Factorial(n) =

n*(n-1)*(n-2)*…*3*2*1 if n > 0

factorial 4

=

3 * factorial 2

=

3 * (2 * factorial 1)

=

3 * (2 * (1 * factorial 0))

=

3 * (2 * (1 * 1))

=

3 * (2 * 1)

=

3 * 2

=

6

Factorial – A case study7

- Recursion algorithm definition

1 if n = 0

Factorial(n) =

n * ( Factorial(n-1) ) if n > 0

factorial 3

Content of a Recursive Method

- Base case(s).
- One or more stopping conditions that can be directly evaluated for certain arguments
- there should be at least one base case.
- Recursive calls.
- One or more recursive steps, in which a current value of the method can be computed by repeated calling of the method with arguments (general case).
- The recursive procedure call must use a different argument that the original one: otherwise the procedure would always get into an infinite loop…

Content of a Recursive Method

void myFunction( int counter){if( counter == 0)…........else

{………myFunction(--counter);

}}

Use an if-else statement to distinguish between a Base case and a recursive step

Iterative factorialalgorithm

Recursive factorialAlgorithm

public static intrecursiveFactoria(intn) {

if (n == 0)

return 1; // base case

else

return n *recursiveFactorial(n-1): // recursive case

}

public static intrecursiveFactoria(int n) {

i = 1

factN= 1

for(int i=1; i<= n; i++)

factN= factN * i

i = i + 1

return factN

}

recursive implementation of the factorial function is somewhat simpler than the iterative version in this case there is no compelling reason for preferring recursion over iteration.

For some problems, however, a recursive implementation can be significantly simpler and easier to understand than an iterative implementation.

Tracing Recursive Method

- Each recursive call is made with a new arguments Previous calls are suspended
- Each entry of the trace corresponds to a recursive call.
- Each new recursive function call is indicated by an arrow to the newly called function.
- When the function returns, an arrow showing this return is drawn and the return value may be indicated with this arrow.

Recursion vs. iteration

- Iteration can be used in place of recursion
- An iterative algorithm uses a looping construct
- A recursive algorithm uses a branching structure(if-statement)
- Recursive solutions are often less efficient, in terms of both time and space, than iterative solutions
- Recursion can simplify the solution of a problem, often resulting in shorter, more easily understood source code

1 Linear Recursion

- The simplest form of recursion is linear recursion, where a method is defined so that it makes at most one recursive call each time it is invoked.

Summing the Elements of an Array Recursively

A =

Algorithm LinearSum(A , n )

input

A integer array Aand an integer n>=1, such that A has at least n elements

output

The sum of the first n integers in A

if (n = 1) then

return A[0]

else

return LinearSum(A, n - 1) + A[n - 1]

n = 5

Summation (Sigma) Notation

Algorithm sum ( n

Input

n is an integer number

o

is returned

if (n == 1)

return 1;

else

return n + sum(n-1);

End IF

Summation (Sigma) Notation con..

- For n =3

Sum (3)

return 3+Sum(2)

return 3+3

return 6

Sum(2)

return 2+ sum(1)

return 2+1

return 3

return 1

Sum(1)

return 1

Printing “Hello World” n Times Using Recursion

Algorithm printHelloWorld( n

Input

n is an integer number

Output

“Hello World” printed n times

If ( n >0 )

{

Print “Hello World”

printHelloWorld( n – 1 );

}

Printing “Hello World” n Times Using Recursion

hello world

return

printHelloWorld(3)

Print hello world

printHelloWorld(2)

hello world

printHelloWorld(2)

Print hello world

printHelloWorld(1)

return

hello world

printHelloWorld(1)

Print hello world

printHelloWorld(0)

return

return

- printHelloWorld(0)

2 Binary Recursion

- When an algorithm makes two recursive calls, we say that it uses binary recursion. These calls can, for example, be used to solve two similar halves of some problem,

Computing Fibonacci Numbers via Binary Recursion

- Let us consider the problem of computing the kth Fibonacci number.
- {0, 1, 1, 2, 3, 5, 8, 13, 21, 34,….}
- Fibonacci numbers are recursively defined as follows:
- F0 = 1
- F1 = 1
- Fi = Fi−1 + Fi−2.

Computing Fibonacci Numbers con..

Algorithm BinaryFib(k):

Input:

Nonnegative integer k

Output:

The kth Fibonacci number f

if(k< I) thenreturn k

elsereturn BinaryFib(k - 1) + BinaryFib(k -2)

Exercise

what is the output of fun(16)?

Algorithm fun(int X)

if ( x <=4 )

return 1

else

return fun(x/4) + fun (x/2)

Result = 3

3 Multiple Recursion

- Generalizing from binary recursion, we use multiple recursion when a method may make multiple recursive calls, with that number potentially being more than two.

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