Recursion.ppt

Recursion
CS 308 – Data Structures

What is 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

When you turn this into a program, you end
up with functions that call themselves
(recursive functions)
int f(int x)
{
int y;
if(x==0)
return 1;
else {
y = 2 * f(x-1);
return y+1;
}
}

Problems defined recursively
• There are many problems whose solution
can be defined recursively
Example: n factorial
1 if n = 0
n!= (recursive solution)
(n-1)!*n if n > 0
1 if n = 0
n!= (closed form solution)
1*2*3*…*(n-1)*n if n > 0

Coding the factorial function
• Recursive implementation
int Factorial(int n)
{
if (n==0) // base case
return 1;
else
return n * Factorial(n-1);
}

Coding the factorial function
(cont.)
• Iterative implementation
int Factorial(int n)
{
int fact = 1;
for(int count = 2; count <= n; count++)
fact = fact * count;
return fact;
}

Another example:
n choose k (combinations)
• Given n things, how many different sets of
size k can be chosen?
n n-1 n-1
= + , 1 < k < n (recursive solution)
k k k-1
n n!
= , 1 < k < n (closed-form solution)
k k!(n-k)!
with base cases:
n n
= n (k = 1), = 1 (k = n)
1 n

int Combinations(int n, int k)
{
if(k == 1) // base case 1
return n;
else if (n == k) // base case 2
return 1;
else
return(Combinations(n-1, k) + Combinations(n-1, k-1));
}
n choose k (combinations)

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
• 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

How do I write a
recursive function?
• Determine the size factor
• Determine the base case(s)
(the one for which you know the answer)
• Determine the general case(s)
(the one where the problem is expressed as a
smaller version of itself)
• Verify the algorithm
(use the "Three-Question-Method")

Three-Question Verification Method
1. The Base-Case Question:
Is there a nonrecursive way out of the function,
and does the routine work correctly for this
"base" case?
2. The Smaller-Caller Question:
Does each recursive call to the function involve a
smaller case of the original problem, leading
inescapably to the base case?
3. The General-Case Question:
Assuming that the recursive call(s) work
correctly, does the whole function work
correctly?

Recursive binary search
• Non-recursive implementation
template<class ItemType>
void SortedType<ItemType>::RetrieveItem(ItemType& item, bool& found)
{
int midPoint;
int first = 0;
int last = length - 1;
found = false;
while( (first <= last) && !found) {
midPoint = (first + last) / 2;
if (item < info[midPoint])
last = midPoint - 1;
else if(item > info[midPoint])
first = midPoint + 1;
else {
found = true;
item = info[midPoint];
}
}

• What is the size factor?
The number of elements in (info[first] ... info[last])
• What is the base case(s)?
(1) If first > last, return false
(2) If item==info[midPoint], return true
• What is the general case?
if item < info[midPoint] search the first half
if item > info[midPoint], search the second half
Recursive binary search (cont’d)

bool BinarySearch(ItemType info[], ItemType& item, int first, int last)
{
int midPoint;
if(first > last) // base case 1
return false;
else {
midPoint = (first + last)/2;
if(item < info[midPoint])
return BinarySearch(info, item, first, midPoint-1);
else if (item == info[midPoint]) { // base case 2
item = info[midPoint];
return true;
}
else
return BinarySearch(info, item, midPoint+1, last);
}
}

void SortedType<ItemType>::RetrieveItem
(ItemType& item, bool& found)
{
found = BinarySearch(info, item, 0, length-1);
}

How is recursion implemented?
• What happens when a function gets called?
int a(int w)
{
return w+w;
}
int b(int x)
{
int z,y;
……………… // other statements
z = a(x) + y;
return z;
}

What happens when a function is
called? (cont.)
• An activation record is stored into a stack (run-
time stack)
1) The computer has to stop executing function b and
starts executing function a
2) Since it needs to come back to function b later, it
needs to store everything about function b that is
going to need (x, y, z, and the place to start executing
upon return)
3) Then, x from a is bounded to w from b
4) Control is transferred to function a

• After function a is executed, the activation
record is popped out of the run-time stack
• All the old values of the parameters and
variables in function b are restored and the
return value of function a replaces a(x) in
the assignment statement
What happens when a
function is called? (cont.)

What happens when a recursive
function is called?
• Except the fact that the calling and called functions have
the same name, there is really no difference between
recursive and nonrecursive calls
int f(int x)
{
int y;
if(x==0)
return 1;
else {
y = 2 * f(x-1);
return y+1;
}
}

=f(3)
=f(2)
=f(1)
2*f(2)
2*f(1)
2*f(1)
=f(0)

Recursive InsertItem (sorted list)
location
location
location
location

The number of elements in the current list
What is the base case(s)?
1) If the list is empty, insert item into the empty list
2) If item < location->info, insert item as the first
node in the current list
Insert(location->next, item)

template <class ItemType>
void Insert(NodeType<ItemType>* &location, ItemType item)
{
if(location == NULL) || (item < location->info)) { // base cases
NodeType<ItemType>* tempPtr = location;
location = new NodeType<ItemType>;
location->info = item;
location->next = tempPtr;
}
else
Insert(location->next, newItem); // general case
}
void SortedType<ItemType>::InsertItem(ItemType newItem)
{
Insert(listData, newItem);
}

- No "predLoc" pointer is needed for insertion
location

Recursive DeleteItem (sorted list)
location
location

The number of elements in the list
• What is the base case(s)?
If item == location->info, delete node
pointed by location
Delete(location->next, item)
(cont.)

void Delete(NodeType<ItemType>* &location, ItemType item)
{
if(item == location->info)) {
NodeType<ItemType>* tempPtr = location;
location = location->next;
delete tempPtr;
}
else
Delete(location->next, item);
}
void SortedType<ItemType>::DeleteItem(ItemType item)
{
Delete(listData, item);
}
(cont.)

Recursion can be very inefficient
is some cases
+
=
=
=
=
=
=
+ +
+ + + +
+
+ + + + +
+
+
+
+
+ + +
+ + + + + + +
+ +
+
+
+
+
+
+
+
+
+
3
3
Comb (3, 1)
2
Comb (2, 1)
1
Comb (2, 2)
Comb (3, 2)
Comb (4,2)
2
Comb (2, 1)
1
Comb (2, 2)
Comb (3, 2)
1
1
Comb (3, 3)
Comb (4, 3)
Comb (5, 3)
2
Comb (2, 1)
1
Comb (2, 2)
Comb (3, 2)
1
1
Comb (3, 3)
Comb (4, 3)
1
1
1
Comb (4, 4)
Comb (5, 4)
Comb (6,4)
15

Deciding whether to use a
recursive solution
• When the depth of recursive calls is
relatively "shallow"
• The recursive version does about the same
amount of work as the nonrecursive version
• The recursive version is shorter and simpler
than the nonrecursive solution

Recursion.ppt

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